Vacuum/volume 37/numbers 1/2/pages 3 to 20/1987 0042-207X/87S3.00+.O0 Printed in Great Britain Pergamon Journals Ltd

High resolution electron spectroscopy new achievements

recent

M i l a n V Kurepa, Faculty of Natural Sciences and Mathematics, Beograd and Institute of Physics, Beograd, PO Box 57, 11001 Beograd, Yugoslavia

Photon-electron and electron-electron high resolution spectroscopies are well-established experimental methods for analysing interactions of photons and/or electrons with atoms and molecules in gas phase, with clusters, liquids and sofids, as well as atomic species on solid surfaces. Constant efforts to improve the possibilities of these methods have been concentrated lately on parts of the experimental apparatus. (1) Instead of a thermo-electron emitter at the entrance of the monchromator, new sources of polarized electrons have been developed and successfully applied. (2) In the field of electron optics new electrostatic lenses have been constructed and their characteristics experimentally investigated. This includes lenses for energy scanning of the incident and scattered or/and ejected electrons. (3) The development of intense polarized electron beams was followed by new efforts in improving the polarization analysis of such beams. (4) A breakthrough was made in the field of single electron detection by introducing multi-array channel electron detectors able to detect simultaneously parts of the electron energy spectrum instead of a single electron energy group. In the near futur~ it is expected that most experiments with incident electron beams wil l be repeated using polarized electron beams. The same is expected to happen with electron analysing systems. They have to be extended and completed with spin-analysers of scattered and/or ejected electrons. As a result, a more thorough knowledge of photon and electron collisions with various targets wil l be gained.

1. Introduction

1.1. High resolution electron spectroscopy. By high resolution electron spectroscopy (HRES) one understands a broad class of scientific and analytical methods having in common the energy analysis of electrons scattered or ejected from atoms and molecules in binary collisions, or solid surfaces and atoms or molecules adsorbed at the solid surface.

In binary collision conditions electrons can be ejected from atoms and molecules by incident photons, or they can be scattered or ejected by incident electrons. Incident photons and/or electrons probe the atom and molecule. They can be used as a typical scientific tool for studying the basic characteristics of atomic particle interactions, the energy levels of the target atomic particle and cross-sections for processes of all possible kinds.

Incident electrons can have a broad energy distribution or can be energy selected before the scattering. Photons of various energies can be used, their source depending on the wavelength of photons needed.

Photon and electron incident beams can probe the target as an analytical tool in order to determine the qualitative or quantita- tive composition of a mixure of atomic particles, making the electron spectrometer a useful piece of laboratory equipment.

Details of various aspects of HRES have been given in a number of review papers and books ~ 7.

Investigation of solid surfaces under static and stable condi- tions is possible only in ultrahigh vacuum conditions. Methods of electron spectroscopy allow the study of the chemical and physical nature of the solid in its outermost layers.

On the other hand most technologies make use of the interation of gases and fluids with solids. The basic physical and chemical processes of heterogeneous catalysis, corrosion protection, semi- conductor and thin-film technology can be investigated under static low gas pressure conditions using methods of electron spectroscopy.

In instruments for studying the surface and adsorbates, electrons and photons of various energies will be considered as particles incident to the surface. Here also, the essential part of electron spectrometer is the electron energy analyser.

Electron spectroscopic methods have become a powerful analytical tool for studying surfaces, interfaces and thin films. Results of using HRES in the study of solid surfaces and adsorbates have been presented in a number of review papers and books ~,3,7-~ 1

2. Dispersion elements

The key element of an electron monochromator or energy analyser is the dispersion element. Its role is to select from a rather wide energy distribution of electrons obtained from the source (most frequently the thermoelectron emitter) in the case of the monochromator or to select from the physical process for the analyser, a narrow energy band.

Fundamentally there are several basic principles which have been used to achieve energy an'alysis of electrons. Three of these are listed in decreasing order of importance:

(a) the deflection of the electron beam in an electrostatic,

M V Kurepa. High resolution electron spectroscopy

magnetic or a combined field which disperses them in energy, so that a narrow energy band can be filtered by a slit or lens;

(b) the use of a retarding potential in front of the collector so that only electrons having energy higher than the potential barrier can be collected:

(c) the measurement of the time taken by the electron to travel through a distance between two points.

2.1. Character i s t ics of dispersion e lements . In this short review we will consider only energy dispersion elements of the electrostatic type with deflection of the electron beam. This deflection results in spatial separation of particles with different energies. Two particles with energy difference A E = ( E 2 E~) entering the analyser along coincident trajectories are found to be separated by a distance Ax at the exit of the analyser. All analysers have focusing properties in at least the dispersive plane ~2.

The energy resolution of an arbitrary analyser depends on many, mainly geometrical, factors. The base energy resolution AE B, the range of energies over which a monoenergetic beam produces an output in an analyser set at fixed deflection voltage, can be generally expressed as

(AE~/E) = Cw(w/L ) + C~(A0~) 3 + C,(A~) 3 + C/.~(A/~ ) 2

+ C . (h/L) 2

where: A~ is the opening angle in the dispersive plane; Aft is the opening angle in the non-dispersive plane; w is the exit slit width: L the dispersive length; h height of the exit slit. Values of coefficients in the base resolution equation differ for various dispersive elements ~ 3.

In the literature the energy resolution is often defined as (AE~/2/Eo), where AE12 is the full width at half maximum ( F W H M I. In the case of a triangular energy distribution one can write

AE1/2 = (1/2) • A E u.

Apart from the base width or half width resolution there are other equally important parameters for the choice of a particular analyser~a ~9.

relative resolution: (AE/E?--the ratio of the width of the energy distribution and the energy E of the particles emerging from the analyser: absolute resolution: A ~ t h e width of the energy distribution: resolvin.q power: (E/AE}~the inverse of relative resolution: transmision: T the ratio of the emergent flux of particles at the pass energy to the flux entering the analyser: btendu: ). product of entrance solid angle and entrance area, ~) A--). ; luminosity: A product of entrance solid angle, entrance area and transmission: A = 2 - T = f ~ A - T.

Various analysers are ranked on the basis of different criteria: luminosity vs resolution; transmission vs resolution and &endu vs resolution.

2.2. Monoenerge t i c e lectron beam current. The maximum current of monoenergetic electrons obtainable by low energy monochro- mators is a very important characteristic for HRES. It is limited by electrostatic interactions between electrons in the beam 2°'2~.

If a beam of electrons has to be focused into a point or transmitted through a small aperture then the maximum current

of the beam is given by the space charge limited condition

l~ota j = 38.5 " E~1~"29~ 2,

E 0 being the electron energy, and ~ the beam divergence half-angle. The relative resolution of the electron beam can be, in most monochromators , calculated from

AE1,2 :~/"3 Eo

which, combined with the previous relation gives approximately

1~o,, I = 82 • (AE1/2) 3'2.

Of the total electron beam current only part is transmitted through the monochromator

l:x E = I tota,(AE,/2/AEk)

where AE k is the F W H M of the beam when leaving the cathode at temperature T(AEk=2.54"k" 73. Finally the monoenergetic beam of energy width AE~,2 is given by

IAL. = 8 2 (AE1/2)5"2 A&

The I~[AE1.2) s/2 dependence 22, is shown in Figure 1. Some of the experimentally obtained monoenergetic electron beam currents23 z; are inserted in the figure to show how they follow the predicted change of this current with the energy width of the beam. The agreement is rather good, proving that one can calculate and predict what kind of monoenergetic current one can expect in a particular experiment.

The limit in the electron beam current introduces some limitation to the high resolution spectroscopy where the electron beam is used to excite the atom or molecule, or the solid surface, respectively. With the reduction of the beam energy width the intensity of the signal from the interaction decreases, necessitating improvement in the detection techniques. Single electron detec- tion is often a requirement in HRES, either by secondary electron or channel electron multipliers.

f 0

i o ~

v

I 0 "

I 0 8

I© 9

LO

t o

/ /

/ A"

. . . . . . . . ] . . . . ~ , , , ]

I0 c IO t I 0 z

F W H M ( m e V )

Figure I. Cathode current and width of the energy distribution curve by coupling thermionic cathodes with electron monochromators. The data points which are shown as solid symbols are from systems using spherical deflector monochromators, while those shown as open symbols come from systems using cylindrical deflector monochromators. The solid line follows I ~ ( E } 5!2 (ref20}. Experimental data are from: • ref20: t ref26: • ref24: I - - r e f25 : :~ ref23: A ref27.

M V Kurepa: High resolution electron spectroscopy

2.3. Analyser system operation modes. The analyser system as a whole can be used in two possible operation modes.

The first mode is the constant relative resolution mode; (AE/E) is constant. The lens system between target and analyser retards (or accelerates) the electron beam by the same factor. The advantage of this mode of operation is that the transmission of the lens system and the analyser is constant and consequently it is very suitable for quantitative measurements. It is important to note that the resolution AE changes with the pass energy E.

The second mode of operation is the so called pass energy mode. In it the lens system decelerates or accelerates the electrons from their target energy to a fixed energy, so that the pass energy of the analyser is being kept at the same value. In this case the resolution AE remains constant. However, if one wants to make quantitative measurements, a well designed lens system is required to ensure the transmission is independent of the target particle energy.

3. Electron beam transport systems for high resolution electron spectrometers

Whichever the analyser operation mode is, between the source of electrons and the monochromator and/or the physical process where electrons are liberated and the energy analyser, some form of particle transport is required. For this purpose electron lenses are used. The complexity of a lens system depends on the number of parameters one wants to control independently. Usually one wants the system to fulfill a combination of some of the following requirements:

(a) fixed object and image distance; (b) variable electron energy at the exit side of the lens for fixed

energy at the entrance side; (c) fixed energy at the exit side of the lens for variable energy at

the entrance side; (d) fixed linear or angular magnification; (e) control of the beam angle, one usually requires the beam

angle at the exit side to be zero. In the following we will limit ourselves to recent advances in the

use of cylindrically symmetric electrostatic lenses. The electrostatic lenses that have been used in the past to focus

electron beams usually consisted of either two or three electrodes, having the form of either an aperture or a cylinder. The focal properties and aberrations of such lenses have been extensively studied 28-3°.

3.1. Three-element electrostatic lenses. Characteristics of three- electrode einzel lenses are well understood and widely used. The extension to asymmetric potentials in three-electrode lenses started a~ with aperture lenses. The result was a set of voltage ratio pairs satisfying the requirements of a fixed object-to-image distance while the energy of the beam could be varied in a considerable range. Detailed calculations of three-electrode asymmetric lenses also exist 32.

Characteristics of three-element lenses made of cylinder elements were predicted 33 and shortly afterwards verified experimentally 34. Exact calculations of three-element cylindrical lenses are available 35.

The general characteristics of three-element lenses, made either of equidiameter cylinders or equidiameter apertures can be represented by two graphs. One is the 'focal loci curve' connecting the (V2/VI) ratio to the (V3/V1) ratio. The other is the 'magnification loci curve' connecting the magnification of the lens system and the (V 3 /V~) voltage ratio. Parts of the focal loci curve

can be approximated in limited (Va/VI) voltage ratio regions by straight lines, thus, allowing simple scanning of the electron energy. In this way one can cover a (Va/V 0 range between 0.1 and l0 by a number of linear scans.

A sophisticated modification of the three-element lens has been suggested 36, It consists of short segment electrodes which can be connected between themselves in order to form three-element lenses of various element lengths, chosen so as to fulfill the requirements as best as possible. This modification can be used in combination with a computer which can choose the electrode effective lengths and appropriate potentials according to the part of the electron spectrum to be scanned.

3.2. Five-element electrostatic lenses. A further improvement in the quality of lenses for high resolution electron spectrometers was suggested 37 in the form of an afocal lens system made of five elements and with the additional conditions for voltage ratios: (Vs/Va)=(V3/VI) and (V4/Va)=(V2/VI).

Characteristics of the afocal five-element lens system have been determined experimentally 38. The 'focal loci curve' is given in Figure 2. The variation of the overall energy ratio, given by (Vs/V 1), of the electron beam can be achieved by changing the ( V 2 / V~ ) ratio. A range of ( V 5 / V 1 ) from 0.1 to 50 in one linear run, without any change of the power supplies can be covered. The magnification of the lens system changes, meanwhile, in a rather narrow range.

Experimental investigations of five-element lens systems which are not strictly afocal have started recently 39, and some prelimin- ary results do exist.

io ' • A.. _ .a.¢., l . I . . . L I t m,lp , , ~ m j - . - . - . - i . - m

¢ii~I,=¢""

~. I 0 c

• . / o I I - . ° O ' ~ ' I ' v l l / ' f n u

10 " . - , I , - - , , , I , L , , L , , , . i I 0 ' ]0 0 I 0 I0 2

(V~/V~)

Figure 2. Reduced focal loci for three different 5-element lenses 38: •__lens No 2; A--lens No 3; O--lens No 4. The unbroken line is calculated.

3.3. Four-element electrostatic lenses. A special four-element lens system of cylindrical elements using a field-flee lens was investigated in detaiP ° for application in spectrometer-type instruments. With constant voltage ratios ( V 2 / V 1 ) and ( I/4/V 3) the energy scanning of the electron beam is achieved by changing the potential ratio (1/3/V2). The result of experimental investigations arc shown in Figure 3. For a wide (V3/V2) range the (V3/V4) voltage ratio for giving a focus of the beam in the plane of the exit slit is constant and so is the beam intensity. The lens system characteristics: focal lengths, focal distances and image position of the beam have also been calculated.

Detailed calculations of four-electrode lens systems were made recently41,42.

With the accumulated knowledge of the characteristics of two-, three-, four- and five-element electrostatic lenses one can now

M V Kurepa. High resolution electron spectroscopy

(a) 6

[ J i ~ I I0 20 3'0 40 50 6'0 70

(V~/V:) (b)

9

i I I i i I I0 20 30 40 5i0 60 70

(v~/v:) Figure 3. Measured characteristics for a 4-element electrostatic lens4°: (a) (V3/V J values giving a focus in the plane of the exit aperture for a given value of tl@'Vz): (b) electron beam intensities against 11/3/V2) for the focusing conditions of ( V 3 /I/4).

calculate electron beam transport systems for any requirement. Some of them are even easy to use in long term experiments at low signal levels. Calculations of such lens systems are available 43''~4.

4. S o u r c e s o f p o l a r i z e d e l e c t r o n s

Methods of obtaining and handling beams of polarized electrons have been treated extensively in the literature. They are of interest for experiments in nuclear and atomic physics, and, lately, they have begun to be important in the field of interactions with solids and adsorbates as well. Details of methods for obtaining polarized electron beams are given in review papers 45 4~, as well as in a book 5°.

In this review we will briefly discuss only the recent achieve- ments in the field of polarized electron beam sources, especially those which seem to be most suited for low energy electron spectroscopy.

4.1. C h a r a c t e r i s t i c s o f p o l a r i z e d e l e c t r o n b e a m s o u r c e s . The source of polarized electrons is characterized by a number of parameters. For each application these parameters have to fulfill different requirements. The parameters are as follows.

(a) Polarization of the beam, defined as

P = (n t - n~)/(n~ + n~)

where n T and n+ are the number of electrons with spin-up and spin-down, respectively.

(b) The quantity (P 2 • D, where / i s the beam current, is a ligure of merit. It applies when counting statistics is the chief source of experimental uncertainty.

(c) The space position of the polarization is important too, since it is desirable to know whether the beam polarization is longitudinal or transverse. Most experiments require that the polarization of the incident beam be reversible.

(d) The time structure of both the polarization P(t) and intensity l(t) of the beam are important.

(e) The electron-optical figure of merit is (E ' A • f~), i.e. the energy-area-solid angle phase space product.

lf) The energy distribution or the energy spread of electrons in the beam is an important parameter and must be considered when specifying the polarized electron beam source.

4 .2 . P o l a r i z e d e l e c t r o n b e a m s o u r c e b a s e d on l a s e r - i n d u c e d o p t i c a l p u m p i n g . Studies 5152 have shown that spin-polarized electrons

can be obtained based on a laser-pumped flowing helium afterglow. A source of polarized electrons has been developed and tested 53 comparable in characteristics with other types of sources.

The principle of the process is as follows. A microwave discharge is used to generate He(23S) metastable atoms in a flowing afterglow. The He(23S) metastable atoms are optically pumped using circularly polarized radiation of energy corres- ponding to the transition He(23SFHe(23p) to preferentially populate either mj = + 1 or mj = - I states via the Amj = + 1 or Amj= 1 selection rule for right- and left-hand circularly polarized light, respectively. A target gas is then injected into the flowing afterglow leading to chemi-ionization reaction. The resultant electrons are spin-polarized as a result of spin-angular momentum conservation during the chemi-ionization process. These electrons are extracted from the afterglow through a differentially pumped aperture and formed into a collimated beam by a series of electron lenses. One of the flowing afterglow configurations used 53 is shown in Figure 4(a). The microwave discharge is located on a sidearm to prevent direct illumination of the optical pumping region by radiation from the discharge. The He(23S) atoms were optically oriented by absorption of circularly polarized 1.08 tLm radiation from a NaF(F + ) colour-centre laser. The laser beam was parallel to an external magnetic tield transverse to the flow tube that defines the quantization axis. The sense of circular polarization, and hence the magnetic sub-level that is populated can be simply reversed by rotating the quarter- wave plate through 9 0 , thereby reversing the extracted electron polarization.

The electron polarization obtained as a function of extracted current is shown in Figure 4(b). The highest polarization of 0.80 was achieved for a current of ~ 100 nA.

The helium pressure in the flowing afterglow region was in the range 40 150 mbar.

(0)

I CO:~ M~ ~

He/4 icrowove ccvnty

~ Pump

(b)

) 6

5

N

~s 4 £

ELectron current (A) Figure 4. (a) Schematic diagram of the polarized electron source by laser induced optical pumping 53. [b) Electron polarization as a function of extracted electron current.

M V Kurepa: High resolution electron spectroscopy

4.3. Polarized electron beam source based on the 'Fano effect'. It was predicted that photoelectrons emitted by alkali atoms which are exposed to circularly polarized uv radiation would be highly polarized s4. The production of the polarized electrons can be understood to be a spin-polarization transfer from the spin- polarized photons to the electrons produced. The mechanism of this transfer is caused by the spin-orbit interaction because, without the existence of the spin~rbi t interaction, the photon spin would reappear as orbital angular momentum and not as the spin of the photoelectrons.

In order to get polarized electrons the influence of the spin-orbit interaction had to be resolved by performing the experiment at different photon energies. It was shown that polarization has opposite signs in different wavelength regions 55. To use the Fano effect as the basis of a source for polarized electrons the atoms have to be either photo-ionized at one wavelength where the corresponding electron polarization is very high 56 using rubidium atoms and laser radiation, or, in a wide wavelength range using the undispersed spectrum of a powerful uv lamp 57'58. We will discuss here the latter method 58.

The experimental arrangement of the Fano-effect source for polarized electrons consists of three main parts:

a source of circularly polarized uv radiation; an oven system for the Cs atomic beam; photoelectron extraction.

Schematically the source of polarized electrons is shown in Figure 5. The uv source delivered about 0.2 W of circularly polarized radiation in the wavelength range 28~318 nm and in a cross-section of about 10 x 3 mm at the photo-ionization target. The atomic cesium beam oven contained 20 g of cesium and could be operated for 120 h continuously.

All photoelectrons produced were extracted by an electric field. The direction of the spin-polarization vector was defined by the photon spin for this experimental arrangment. The spin polariza- tion of the photoelectrons produced was proportional to the spin polarization of the photons absorbed. The easiest way to reverse the electron polarization is the reversal of the optical polarization

Concave mlrror

Cs otomlc -~ beo m

J

LI ELectron beam

ens uor ter-wove plate

lan prism

ondenser

Ig high pressure lamp

by rotating the quarter-wave plate by 90 ° . This procedure has the main advantage that intensity, polarization and position of the photon beam and therefore all figures of merit of the polarized electron beam are stable during the sign change of the polariza- tion.

The source yielded polarization values of more than 0.80 at electron currents up to 10 nA. The time instability of the electron beam polarization is smaller than 1% in 10 h.

The pressure in the photoelectron source region is of the order of 10- 5 mbar.

4.4. Polarized electron beam source based on photoemission from GaAs. A new type of spin-polarized source is based on photo- emission from GaAs 59. Details of the process have been analyzed6O,61. Finally, a spin-polarized electron source applicable to various experiments in physics has been developed and described 62, and will be presented briefly here.

The photomission from GaAs can be treated as a three-step process: photoexcitation; transport to the surface and escape of electrons into the vacuum.

The key factor in obtaining polarized electrons is the spin-orbit splitting of the valence bands of GaAs.

At the point F the valence band maximum, the otherwise degenerate p band is split into a fourfold degenerate P3/2 level and a two-fold degenerate P3/2 level and a two fold degenerate pl/2-1evel, which is located 0.34 eV lower in energy. The origin of the spin polarization can be understood by considering the transition from the mj sub-levels. For circularly polarized light, the optical selection rules require that Arnj = + 1 for tr ÷ positive helicity light. The quantization axis is defined by the light angular momentum direction. For a ÷ light three times as many electrons go to the mj = - 1/2 as to the mj = + 1/2 state. Thus for a ÷ light it is P = -0 .50 and for tr light P=0.50. The maximum polariza- tion is obtained for photon energies less than 0.1 eV greater than the band gap energy.

The GaAs photocathodes were found to be suitable for activation to negative electron affinity 63 6 s. The negative electron affinity is obtained by activation with Cs and 0 2. The electrons are excited in a region determined by the light absorption length of the order of 1 #m. The electrons thermalize to the conduction band minimum in 10- 12 s, from where they can diffuse to the surface and can be emitted 66'67.

The polarized electron beam source is shown schematically in Figure 6. The incident light beam was obtained from a GaAs laser diode. The light beam is first linearly, then circularly polarized by a quarter-wave plate and finally focused onto the GaAs photo- cathode surface through a hole made in the outer electrode of a cylinder electrode condenser. Photoelectrons from the cathode are accelerated, energy selected by the same cylindrical analyser and then transported towards the target by a set of electrostatic lenses.

The polarization of the electron beam can be changed by the rotation of the quarter-wave plate.

The emitted electron energy width of the order of 0.13 eV has been measured 62. The GaAs photocathode works efficiently in a vacuum higher than 10 8 mbar.

Concove mlrror

Figure 5. Schematic cross-section of the uv source of polarized electrons and the electron extraction 58.

5. Electron beam polarization analysers

5.1. Mott scattering polarization analyser. The most widely used polarization analyser until very recently was the so called Mort

7

M V Kurepa. High resolution electron spectroscopy

~-- GoAs [aser diode ~ Lens

Linear pol.a rizer

X/4 plate

B,- Lens

~250+_25V~ I i 25ov i 250V 350V IO00V

J- GaAs, OV

Figure 6. Schematic drawing of the polarized electron beam source using photoemission from GaAs 6z.

scattering analyser. Its operation is based on the spin-orbit interaction of an electron scattered by a high Z atom.

The polarization effect in scattering is caused by the magnetic field which the electron experiences in its rest frame. In this frame the charge centre moves, this movement represents a current that produces a magnetic field B = E x v. If the polarization of the electron does not lie parallel or antiparallel to the magnetic field the magnetic moment connected with P experiences a torque that induces P to change its direction and to precess. Only if the P is parallel or antiparallel to the magnetic field does it retain its direction.

Two electron beams with opposite polarization are scattered differently by the scattering centre. This is because the scattering potential consists of the Coulomb and the spin-orbit part: V= V~ + ~]~. The potential V~ contains the scalar product (Is), and has different signs for electrons of the same orbit but different spin directions.

The resulting potential will be higher or lower for spin-up electrons (Q) than for spin-down electrons (eQ depending on which side of the atom they pass. The number of e T and e~ scattered in a particular direction 0 are, therefore, different from each other. Thus, the scattered beam is polarized for the non- polarized incident electron beam.

The left-right asymmetry in the case that P and n are parallel is

0 - ( 6 = 0 ° ) - - o - ( 6 = 180 °) P . S(O)

cq6 = 0 " ) + a(~b = 180 ~')

where n is the unit vector normal to the scattering plane. Since the number of electrons scattered is proport ional to the correspond- ing cross-section, this can be transformed to

N , , ~i~ = p . S(O). N~ + N r

If the Sherman function S(O) for the scattering centre is known, the left-right asymmetry can be used for measurement of the electron beam polarization.

In Figure 7 a latest version of the Mat t scattering polarization analyser is shown 68'69.

It differs from the earlier versions of the Mott detector in using lower electron energies. Usually the electron beam to be polarization analysed was accelerated to around 120 keV and

Outer chemlsphere

~ h o n n e L t r o n no I

Incident elect ran beam

/

Inner J chemlsphere ~ /

foil

\ Guard \ ring

--A_\ ~; honneLtron no 2

Figure 7. Schematic diagram of a spherical Matt analyseff ~.

then scattered by a thin gold foil. Electrons scattered at + 0 = 120 were detected giving the left and right signal.

This Mat t detector operates at electron energies in the region 2 6 4 0 keV. This is a voltage region much easier to handle in scattering experiments, making the apparatus smaller and more versatile. The Sherman function for the gold foil used as the scatterer in the Mat t detector has been measured and it has been found that its value at 0 = 120 and electron energies of 40 keV is only some 20% lower than for 120 keV, introducing a tolerable decrease of the detected electron signal.

5.2. Secondary electron emission polarization analyser. A new type of electron spin polarization detector based on the spin-dependent absorption of a polarized electron beam incident on a surface has been discussed in refs 7(~72.

When an electron beam strikes the solid surface different events can occur. Electrons can be elastically or inelastically scattered from the surface, they can eject secondary electrons or they can be absorbed into the material of the surface. The corresponding electron currents are related by

lo :/back +/at, where

Ib,ck = I~ + li.e~ + I x .

The usual secondary electron yield curve, defined as (lba~k.l o ) VS the incident electron beam energy, crosses the value of unity at an energy E 0 where the absorbed electron current is equal to zero. At lower energies this current is positive and at higher ones it is negative.

The elastic scattering of electrons from surfaces was found to be spin-dependent due to spin o rb i t interaction. As a consequence the current collected by the sample, i.e. the absorbed electron beam current has been found to be spin dependent, too. Usually one measures the net absorbed electron current l h for an unpolarized incident beam. If the incident electron beam is 100%, polarized the absorbed electron current crosses the zero value at an energy different than for the unpolarized beam. The energy difference (E 0 - E0Q between the crossing points for two opposit- ely polarized beams is defined as A. At an energy Eo; where li = 0 on the average only incident spins parallel to the quantization direction give rise to a net absorbed current. For quantitative

M V Kurepa: High resolution electron spectroscopy

determination a parameter

I y - I ~ rl= Io

is defined. For a 100% polarized incident beam at E0i. or Eojq is just the ratio of the absorbed current to the incident current and may be of the order of 1%.

The detector of electron beam polarization is characterized by the parameters A, r/ and E o. The parameters A and r/ are determined by a calibration procedure using an electron beam of known polarization. The energy E o may vary during the course of an experiment due to small changes in the work function of the detector surface or changes in electric fields at the surface caused by neighbouring electrodes. The other two parameters A and q are independent of these changes.

The polarization of the incident beam can be measured by different methods. If the measurement is made at an energy (E o - A/2) than the polarization is

p = Q + - Q - Q++Q_

where Q+ and Q_ are absorbed charges measured for two reversed beam polarizations.

If the incident beam polarization cannot be reversed with ease than the polarization has to be measured at the energy E o and it is given by

P = [ 2 Q+(Eo)]/( q Qo) where Q0 is the total incident charge.

An electron beam polarization detector based on the principle explained has been developed v3. The beam absorber was gold, since it has several desirable attributes. It is a high - Z material with large spin-orbit interaction. It is also inert, so that changes due to contamination in the vacuum system are small.

The absorbed current spin detector is best suited for measuring low energy electrons where angular resolution of about 1 ° is needed. The intensity of the incident electron beam must be large enough so that the absorbed current is in the measurement range of a sensitive electrometer. This detector is especially suitable for measuring the change in polarization when a polarized incident beam is scattered v3. The detector can be moved into the primary

. • 0 . . . . . . . . . . . . . . . . . . .

Z

I Incident energy

Figure 8. Currents absorbed over a period of time plotted vs energy for four different incident beam polarization: + 1, P, - P and - 1. This is the physical principle of a polarized electron beam analyser 72.

beam for calibration and into the scattered beam for measure- ment.

A polarization detector of this kind was further investigated TM.

It was shown that typically an incident angle of 30 ° at an incident electron beam energy of 130 eV is suitable for most experiments.

The efficiency of this type of polarization detector is equivalent to that of the Mott type and the fact that no high voltages are needed is a significant experimental simplification. On the other hand this detector needs a clean ultrahigh vacuum with pressure of the order of 10- ~o mbar for operation.

6. Multichannel detectors

6.1. Channel electron multipliers. Detection of single electrons in usually performed using secondary-electron multipliers. Of all possible varieties for most applications it has now become accepted that the continuous dynode multipliers are the best. Their features are small size, ease of operation, high and rather stable gain, narrow pulse width and minimal power requirements.

The single channel electron multiplier consists of a glass channel coated with resistive material, usually vanadium glass or metallic lead. The end contacts of the channel provide a standing current along the walls which sets up a field to accelerate secondary electrons from one impact to the next. The typical channeltron has a sufficient gain for single-pulse counting: 106-108 .

The efficiency of a channel electron multiplier and the change of this efficiency with the incident electron energy has been investigated by a number of authors. The efficiency was found to be close to 0.90 and almost constant in the incident electron energy range between 100 and 1000 eV.

A channel plate consists of a closely packed array of thousands of micro-channels, having 0.5-2 mm length and 20-50/~m dia. Due the shorter length the gain of the channel-plate is lower: 103-104. Thus, for single electron detection a sandwich of two channel-plates is usually employed.

Until recently, in electron spectrometers only channel electron multipliers were used, in which case the scanning of the electron energy spectrum was performed by the electron lens system. Recently, channel-plates have been introduced in which parts of the spectrum only are detected instead of a single electron energy.

6.2. Analogue position-sensitive detectors. The application of a multichannel plate for detection of electron spectra in an analogue mode can be presented as in Figure 9. The dispersed electron image at the exit of the analyser is amplified by a multichannel plate electron multiplier and the intensified image is accelerated onto a phosphor screen 75 77.

The visible line image of the electron spectrum can be further processed in different ways. The phosphor screen can be coated directly on a fibre optic window 78. This window guides the light image out of the vacuum system to a combination of two lenses, and this projects the original line spectrum on a sensitive area of a vidicon camera tube which is a part of the optical multichannel analyser. The image stored at the vidicon target is resolved into 500 lines by the scanning electron beam. The video signals from each of these lines are intergrated and stored in digital form. The contents of a given channel are displayed digitally on the display panel of the OMA console. The complete image can be viewed on a cathode ray tube and can be read-out with an X-Y recorder.

The optical image of the phosphor screen can be focused onto a charge coupler device 77. This device has a linear array of 256

M V Kurepa. High resolution electron spectroscopy

2 3 4 5 6 7

Energy " onotyzed Vidlcon electron ~ comero becrn

)-

muLt ichonnel anaLyzer

C R T

Figure 9. Schematic diagram of the analogue position senstive detector: (1) exit slit; (2) CEMA input; (3) CEMA output; (41 phosphor screen: (5i fibre optics; (6) and (7) photo-objective lenses.

photosensitive elements. Photons entering the array generate hole~electron pairs and the electrons are allowed to accumulate under the electrode called the photo-gate. This is maintained at a positive potential for a short interval of time called the integration period. At the end of the integration period the pattern of the charges under the photo-gate corresponds exactly to the distribu- tion of light intensity in the image produced on the phosphor screen. The 256 charge packets are then transferred in parallel into a pair of charge coupled device (CCD) analogue shift registers running along either side of the line of photosensitive elements. The analogue signals obtained from the CCD are digitized and stored in a data system. This version has an active area of the multi-detector corresponding to an energy range of about 12% of the mean energy of electrons passing through the analyser. A spectrum covering a wider range of energies could be scanned across the detector by changing the potential difference between the electron analyser system and the target region.

6.3. Single event position sensitive detector. Systems with simulta- neous single event detection and position detection are in the developing phase 79 s2. They require an on-line computer for the digital accumulation of the positional distribution. The most simple version would be to construct a multi-anode to collect the charges from the channel plates. Each anode element has to be connected to a preamplifier and a counter. But, since the count rate of the channel plate is of the order of 10 kHz cm 2 it is unlikely that more than one anode is active at a time. Therefore, simpler electronic methods have been used for the positional information of the pulse.

A method using discrete anodes is shown in Figure 10. Here all anodes are connected by means of capacitors. The charge pulses reaching the ends of the chain of capacitors depends on the position of the excitation.

This method allows also a two-dimensional sensitive detection. In this case two signals in the X direction and two in the Y direction have to be processed in order to locate the position of the electron beam excitation.

A highly-resistive carbon-coated silica filament can he. used as the detector of the intensified signal 79. The pulse of electrons flows to the ground through two ends of the resistive wire in a ratio determined by the fractional resistance from the excitation point. Charge sensitive preamplifiers at the ends of the wire integrate the

ELectron ctoud

[7

[7

~ Fielder11 p@rticte

/

[ , , / / / ! % • , ~ ~ , / , / , , 1 L \ ' . \ ' . ' . " " , ' , ' , ' , ' , 1

r - l _ \ . . . . . . .

F

ChanneLpLatet

CoLLector system

C~3pocltor network

I Q i <0H

Figure 10. Schematic of a single event position sensitive delector with discrete anodes.

current and give an output voltage proportional to the charges at two ends. These charges are further processed in order to determine the point of excitation,

7. High resolution electron spectroscopy of atoms and molecules

High resolution electron spectroscopy of atoms and molecules includes many variations of experimental apparatus+ The com- mon property of all of them is that the scattering or ejection of electrons from atoms or molecules is achieved m binary conditions, usually in a geometric conliguration where the electron or photon beam collides with free atoms or molecules either as a gaseous target or as a beam.

Incident photons or electrons of fixed or variable energies can be used, depending on the method for exciting or ionizing the target particle.

Various dispersing elements have been used so fiir for the energy analysis of electrons ejected from the target atoms or molecules. We will not concentrate to the type of analysers used, and for drawing convenience a spherical deflection analyser of 180 will be shown m all figures, representing, in fact, any possible type of analyser used in particular experiments.

A simplification will be used for presenting the electron beam detection as well. Various methods have been used for detecting electrons at the exit of the energy analyser. In all drawings the sketch of the detector is "spiraltron" like, again for drawing convenience.

7.1. Photon-polarized electron spectroscopy. A general schematic drawing of a photon-polarized electron spectrometer is shown m Figure 11. It is presented as being made of a source of photons, a photon beam intensity detector, an electron energy' analyser and electron polarization detector.

With an instrument of this configuration Ar and Kr atoms have been investigated ~3 in order to prove the theoretical prediction 8+'~s that with an unpolarized photon beam apart from

10

M V Kurepa. High reso lu t ion e lect ron spec t roscopy

Energy \ / ~ ~ ' ~ ' ~ n a t y z e r

AIO ~'\\

L5 / " 'd/ / o ", / % z

hu

@ Target PBD ( / / PoLarization Photon~) ~ [ _ _ . ~ _ _ ~ 0 _ _ __ ~ J _ e _ __ ~[ / / ~ A O 0

source L~ Photon 12 beam ~ onatyzer monochro- matic

Figure 11. Schematic drawing of a photon-polarized electron high resolution electron spectrometer.

"the photo-ionization cross-section one can determine the follow- ing two parameters:

(a) the asymmetry parameter describing the angular distribu- tion of the differential cross-sections,

(b) the spin polarization perpendicular to the reaction plane. Results of these experiments for Kr atoms are shown in

Figure 12. The graphs are of polarization of photoelectrons with the ions in two possible states. The energy analyser was tuned to pass only electrons of known energy, ejected to an angle of 54 ° 44' (magic angle), with a subsequent electron spin polarization measurement. As one can see photo-ionization processes have a strong polarization dependence vs the incident photon energy. The polarization of photoelectrons associated with the 2P~/2 and 2P3/2 states of the residual ions differ in sign. Polarization of the order of 0.30 has been detected.

Some preliminary measurements do exist obtained with instruments as in Figure 13. In Figure 14 photoelectron intensit- ies and photoelectron polarizations of the xenon atom are shown 86. They were obtained with synchrotron radiation as a source of photons. The radiation was wavelength selected and, afterwards, linearly and circularly polarized. The polarization of the ejected electrons was measured by a Mott detector.

Structures in photoelectron spectra in krypton are due to auto- ionizing states excited by the incident photon beam. The striking information is that the maxima and minima of the polarization curve do not coincide with maxima and minima in the photoelec- tron intensity spectrum. Polarizations of the order of 0.80 were detected for the ejected electron beam, proving that some excitation processes are strongly polarization dependent.

nergy n o t y z e r

Photon [ - r Photon ~2 beam / . . . . . . . . . . Target PBD / / y source monochro - ~ / / / A O 0

motor

. / / / PoLarization / analyzer

Figure 13. Schematic drawing of a polarized photon-polarized photon high resolution electron spectrometer.

7.2. Polarized photon-electron spectroscopy. The ejection of polarized electron by photo-ionization of alkali atoms has been predicted s4 and it is already used as a source of polarized electron beams. The behaviour of other atoms, especially those with closed electron shells in photo-ionization processes by circularly polar- ized photon beams is yet to be investigated.

(a) 02

01

( o

- 0 1 ~

- 0 . 2 I 0 0

I 8O

Kr*(2~/2) 0 20

0 I0

0

- - 0 1 0

I I - o zo 60 40 20

WaveLength ( nm )

ct:)

0._

(a) e=

~z ~ > . ( 9 + _

roz ,oo 98 96 94 (b) 0 7 e "

4~ 0

~ o 2 ~ - o) N o : - o ~ 0 ~ ~'~

o. -025

-05C-- WaveLength (nm)

Figure 14. Photo-ionization of Xe atoms in the auto-ionization range a6 (a) cross-section, photoelectron intensity; (b) spin polarization of photoelectrons.

(b) 04 Kr +(2P/2 ) 0 40

~¢ 02 020 ~

O-- 0 n

- 0 20 - o 2 - I I

I00 80 60 40 20

WaveLength (rim)

Figure 12. Experimental results of photoelectron polarization produced by unpolarized radiation incident to krypton atoms: (a) the 2P3/2 ionic state of the Kr + (b) the 2P~z ionic state 83.

7.3. Electron-ejected electron spectroscopy. The ejected electron- spectrometer arrangement shown in Figure 15 includes only the energy analysis of electrons scattered or ejected from atoms. The excitation and ionization of the target atom is carried out using a rather simple electron gun.

This arrangement was extensively used for the investigation of auto-ionizing states of atoms, as well as for Auger transitions following the ionization of an inner shell electron 8v'88.

The advantage of this technique is that, since it uses the decay process of an excited auto-ionizing state, the ejected electron energy does not depend on the incident electron energy with which the excitation is achieved. The incident electron energy influences only the cross-section for the excitation. Thus, the

11

M V Kurepa: H i g h r e s o l u t i o n e l e c t r o n s p e c t r o s c o p y

Energy otyzer

beam ~ D ~ , /

F i g u r e 15. Schematic drawing of an electron-ejected electron high resolution electron spectrometer.

s tructures in the ejected-electron spectra are very sharp and their width depends upon the resolut ion of the energy analyser and the natural width of the excited line. In Figure 16 results obta ined for thal l ium a toms 89 are shown. The resolved structures are num- bered, each cor responding to a detected energy state decaying into a free electron and an ion. In the energy range shown these levels are due to the exci tat ion of the ou te rmos t closed subshell electron into the con t inuum of the first ionizat ion limit.

7.4. Electron energy-loss spectroscopy. A typical a r rangement for the high resolut ion energy-loss spectroscopy is shown in Figure 17. There are many varieties for these spectrometers, the par t icular choice of elements depending on the problem studied.

Three par t icular types of experiments can be performed with these spectrometers. They are:

measurements of energy loss spectra, at a fixed incident electron energy and fixed scattering angle, in order to determine the energy values of different states;

angular dis t r ibut ion measurements of electrons at a fixed scattered electron energy and fixed incident electron energy, with the aim of de termining the differential cross-sections of par t icular processes:

scanning of the pr imary electron beam energy at fixed scattering angle and fixed energy loss, in order to determine the differential excitat ion cross-sections curve, to search for reso- nances etc.

The first example is the result for differential cross-section measurements of electrons using a xenon a tom 9°. The experiment was performed with an electron spectrometer with spherical dispersion elements in bo th the electron m o n o c h r o m a t o r and

==

to

m g c

i 2 7

I I

Y I I

2

I~ i9 24 ~ /03 - I I I

i 4

Elected etectron energy (eV)

Figure 16. E jec ted-e lec t ron s p e c t r u m of t ha l l i um v a p o u r a t o m s in the r ange 1 8.2 eV obse rved a t 75 "~ wi th respect to an inc ident e lec t ron b e a m of 50 eV kinet ic ene rgy 89.

~ Aoo

A [ O ~

- - - - r - - - . - ~ ' ~ ' ~ . ~

I k I Etectron t ~ ~ monochromator

Figure 17. Schematic drawing of an electron energy-loss high resolution electron spectrometer.

energy analyser. The scat ter ing angle of inelastically scattered electrons could be measured from 30: to + 150 . The overall energy resolut ion was a round 40 meV.

Energy loss spectra were measured at various scattering angles and incident electron energies. Relative intensities of maxima due to the excitat ion process to a given state are used to obta in differential scat ter ing dependences shown in Figure 18.

Special cal ibrat ion procedures have to be under taken to bring the relative intensities to an absolute differential cross-section scale. But, even without absolute values for differential cross- sections the angular var ia t ion of elastically and inelastically scattered electrons can be used to test theoretical models for the scattering of electrons by atoms.

As a second example we present two recent searches for excited states in the a m m o n i a molecule. This was investigated using a

C)

b~

u~ i

,j

x o~

q~ d~

Xe 6s 3/2)',i

i o :

io o

o o

\ ~ % , E,,= 8 0 ( e V '

~ < + / " i , ,! l I I , ,

30 60 90 120 '50 Scatter ing angle ( ° )

Figure 18. Differential cross-sections for inelastic scattering of electrons by xenon atoms by exciting the 6s 3,/20 state. Incident electron energies varied from 15 to 80 eV 9°.

12

M V Kurepa." High resolution electron spectroscopy

commercial electron spectrometer (VG LEELS 400) modified for the study of gases 91. Both the monochromator and the energy analyser had 150 ° hemispherical dispersion elements. The overall energy resolution of the spectrometer was 20 meV. For most of the spectra taken, the energy scale was calibrated to within 5 meV. The spectra obtained are better resolved than in any previously published electron impact study. The series of maxima detected between 5.728 and 7.347 eV belong to vibrational levels of the ,~ state, which was studied previously both by optical absorption and electron impact methods. Its excitation from the ground state is optically allowed and the two types of spectroscopies give data in goo.d accordance.

The same molecule was investigated by the so-called 'threshold electron spectrometer '92. In it the incident electron beam is obtained by a hemispherical monochromator. The scattered electrons after exciting the molecule are energy selected by a combination of an electrostatic trap and small cylindrical mirror analyser.

The result obtained with the threshold electron spectrometer is presented in Figure 19. In the energy range from 4.8 to 6.5 eVa set of vibrational levels was detected as 15 well-resolved maxima.

I I I I l I I I t O.~|L 4 6 ~, 8 I0 12 14

[ V ~ I J ~ t I nl+ 21 C 4 I

£ lJ ~ I ' ~ 0 2 4 B 6 8

n+ I ~ I I I I I ~

I i I l 5 6 7 8 Incident electron energy (eV)

Figure 19. Threshold electron spectrum of the ammonia molecule, as measured with the threshold high resolution electron spectrometer 92.

But, the corresponding electronic state of the NH 3 molecule is lower than the ,~ state by almost 0.8 eV, although similar in shape. It was neither detected by optical absorption methods nor by electron impact methods with high incident electron energies. The authors concluded that it can only be due to a state whose excitation is optically forbidden from the ground state, and can be excited by low energy electrons, especially in the narrow energy range above the threshold, as occurs in the threshold electron spectrometer.

7.5. P o l a r i z e d e l e c t r o n - p o l a r i z e d e l e c t r o n s p e c t r o s c o p y . A break- through in the field of a 'complete' experiment in electron scattering has been achieved during the last few years with the development of efficient sources of polarized electrons. A schematic drawing of such an experiment in shown in Figure 20.

We will present recent experimental results of scattering of polarized electrons by xenon atoms 93. The elastic scattering of electrons from spinless atoms can be described completely by the direct scattering amplitude

f = Ifl exp(i 71)

sPo%°r%°~ ~ ~ Energy L, ~ ~ ,~~~oL~z,~

))1

~Potorizotion SIO ~,/ onotyzer

~ Pholocothode Figure 20. Schematic drawing of the polarized electron-polarized electron high resolution electron spectrometer.

and the spin-flip amplitude

g = Ig] " exp( i 72)

which includes the Coulomb and the spin-orbit interaction between the continuum electron and the atom.

The observable quantities which can be measured in elastic scattering experiments are connected with the scattering ampli- tudes as follows:

a = ]fl 2 + ]g]2 differential cross-section;

S= 21fllg] s in (71-72) Sherman function; Isl 2 + Igl

T = Isl -Igl isl +lgl polarization parameter;

v = 21Sllgl cos(7,- 2) if l 2 ÷ igl polarization parameter.

The above equations are interconnected by

S 2 ÷ T a ÷ U 2 = l .

Four observable quantities have to be measured in order to determine unambiguously the quantities If I, ]gl and the relative phase shift (7 ~ - 72)- If the primary beam polarization is parallel to the scattering plane, i.e. P, =0 and P=Pp, than the scattered beam polarization is given by

P ' = S . rJ+ T . P + U(~ × P)

where fi is the unit vector normal to the scattering plane. The change of the polarization vector in the scattering process can be described as follows.

(a) Normal to the scattering plane a component S ' ri arises which is independent of the primary beam polarization P.

This can be interpreted as a rotation of the polarization vector out of the scattering plane.

(b) In the direction of the original polarization vector P a component T" P remains.

(c) In the scattering plane the polarization vector is rotated as a new component U(fi × P) arises.

In this particular experiment a Mott detector was used for the polarization measurement. It contained two pairs of detectors in mutually perpendicular planes," allowing simultaneous measure- ments of two tranverse polarization components P ' - r i and

13

M V Kurepa. High reso lu t ion e lect ron spec t roscopy

' 5 •,

t

i,)c 20 ) 3 0 0

~r : ,:2 q: , ' :

T o 5

'S,C, ?'OC 30C,

u

U

7 [, c.: • g• :" " (0 , ,~ , '

'OC 2%]C' 300

E (eV)

E;

E] S L]

t3 ? []

Cb

o,' C )5 •R

(,

O 0 ?OC, 300

V U iJ Q

[; i I

I00 ~'h} "X)O

o -~ 560

,L ~ '8 I

I ( ,b >..

I O0 200 300

E(eV)

Figure 21. Results for the parameters S, r and b at a scattering angle 0 = 80 for xenon, as ,,,,,ell as evaluated scattering amplitudes Ill, Iol and

., )'~3 relative phase shifts 1/1 ,2 -

P;(k2 x n). When the Wien filter, incorporated in the scattered electron analyser system, is turned on, the two polarization components perpendicular to the magnetic field B are rotated through 9 0 so that the originally longitudinal component P1 - ,(e is converted to a transverse component.

The measured values for S, Tand U and the corresponding [I1, I,#1 and (;'~ -72 ) values are shown in Figure 21.

8. High resolution electron spectroscopy of solid surfaces and adsorbed atoms or molecules

As in the case of free atoms and molecules the high resolution electron spectroscopy of solid surfaces and adsorbed atoms or molecules includes many variations of experimental apparatus• The important difference is that experiments with solid surfaces have to be conducted in ultrahigh vacuum. 8'°.

The differences between the consequences caused by incident photons or electrons to the investigated surfaces are considerable, so, generally, the photon electron and e lec t ron~lect ron spec- troscopies are separately treated.

Pho ton~ lec t ron spectroscopy allows a considerable number of experimental variables and observable parameters. One can measure the number and energy of emitted electrons as a function of two angles, polar and azimuthal, and their spin. For the incident light the energy, angle of incidence, and the polarization may be varied. The sample may be prepared in various ways, single or polycrystalline, thin or thick. And, finally, various adsorbates may be brought to the surface.

The photon energy is the most important parameter in photoemission spectroscopy. It can be varied over several decades and encompasses methods that use different experimental and

theoretical techniques and are applied in different fields of science. Threshold spectroscopy, using photon energies around 5 eV is

usually performed without energy analysis. It probes the yield in a region where it is dominated by the emission threshold function. At higher photon energies, in the 5-20 eV range, energy-resolved spectra show rich structure due to k-conservation selection rules or tinal-state modulation. Ultraviolet photoemission spectro- scopy is commonly understood as excited by resonance light sources in the 1641 eV range. A short electron mean free path in the sample makes those spectra surface sensitive. The energy probing depth is suffÉcient to cover the valence band of most solids, Photon energies in the 0.1 5 keV range can excite core electrons, giving rise to sharp characteristic lines in the spectra.

8.1. Photon-electron spectroscopy of solids and adsorbates. In the past most experiments kept the photon energy constant during a scan of photoelectrons. Synchrotron sources permit the photon energy to be scanned while observing through a narrow energy window, which may be either kept fixed (constant tinal state spectroscopy) or moved together with the photon energy (constant initial state spectroscopy) {Figure 221.

Photon 0 ~ Energy . . . . . . ) ~ L / , , ~ ~ a n a [ y z e r

P O,OO \ \ rnon°chromat°r Y ~ ) )

h~ ~ / ~ / / % /

Target / AOO

Figure 22. Schematic drawing of a photon clectron high resolution electron spectrometer of solids.

At sufficiently high photon energy the core levels of atoms in the bulk become available. Core levels can be localized and the photoelectron emission characteristics are atom-like, so the HRES can be applied to study core level cross-sections.

It was predicted 9"*'9s that minima do exist m the atomic photoemission cross-section if the initial state wave function exhibits a radial node. In such cases the cross-section, and as a consequence the photoemission current, may vary strongly over small energy range. Experimentally it has been investigated whether such effects can be observed in photoemission from valence bands in solids ~,. The study included the d-band intensity in Cu (3d, no radial node), Ag (4d, one radial node) and Au (Sd. two radial nodes) in the range 40 eV < hv < 250 eV. The obtained results are shown in Figure 23. The energy dependence of the d- band intensity is significantly different for the three metals• The measured d-band intensities should be approximately propor- tional to the photo-ionization cross-sections of the corresponding atom. The selection rules for electric dipole transitions [AI = + 1 ) connect a d-initial state to p- and f-partial-wave final states. The d-+fchannel is dominant at high energies. Above threshold the d- band intensities first exhibit maxima. At higher energies the d- band intensity decrease steeply to a minimum, called the Cooper minimum• The minimum will appear at the energy for which the sum of the squares of the p and f channel radial matrix elements vanishes.

Angle resolved spectroscopy is a new type of photoemission

14

M V Kurepa." High resolution electron spectroscopy

I O ° ~

c ~ I02

iO I

o to i

5 I01 _

noO I I I I I I 40 80 120 160 200 240

Photon energy (eV)

Figure 23. Relative d-band intensity of Cu, Ag and Au asa function of photon energy 96

measurement. It requires a high-intensity light source, since the observation is made on a small fraction of the total number of photoemitted electrons. The measurement may be performed by scanning an energy distribution spectrum for fixed values of the emission angle, or by fixing the energy window at an interesting feature and varying the polar or azimuthal angle.

The emission direction of the electron is related to the moments of exciting photon and excited electron. The angular distribution of photoemitted electrons might provide information on the orbital quantum number of locally bound electrons or on the wave vector of delocalized electrons in solids. An experiment that measures both the energy and emission angle of a photoelectron simultaneously observes all quantities conserved within the one- electron picture of the photoelectric process: the final state energy and the two k-vector components parallel to the emitter surface.

If observed with good energy resolution, the hole created by the photo-ionization process, may spread over a large number of atoms during the measurement time. All these atoms will appear to emit coherently, so sharp interference effects are observed, which are identical to the k-conservation selection rule known from solid state optics. This situation is characteristic for low excitation energies in the band structure regime. The photoemis- sion spectrum in this energy range will reflect the energy density of joined density of states.

The type of spectra obtained by angular resolution photoelec- tron spectroscopy 9v is shown in Figure 24. The result that can be evaluated from angle resolved spectroscopy is the electronic band structure, as for G a P shown in Figure 25. In this particular experiment electrons emitted normal to the surface have been detected so that the momentum of the electron inside the metal can be determined from the relation

2 Ek(kz)=E° + 2m (k ins )2 -hv

E e being the bot tom energy of the muffin tin (E o<0) . The

2 c

o

<

-iO 5 0

Energy above VBM (eV]

Figure 24. Photoemission spectra at normal emission with photon energies from 31 to 85 eV. Dispersive structures are denoted 1 3. Non- dispersive structures are denoted with respect to the critical points X 3, Em~ . and X507.

.:.. . : ; i ×5. . . t . "%" • >~ • I b l g • •

• • X s KI •

> ~ •%• . . , - e ° 22 K,

o -e u x ~ -IC L 3 . o•••• • c~ ' • ELI -12 • • • •

l e o o 0 0 I J

A r A X U K ~: F

Figure 25. Electronic band structure of GaP by normal and non-normal photoemission processes. Only the dispersive structures are shown 9~

experimental results are in very good agreement with theoretical band structure calculations.

Photoemission is a useful tool for the study of adsorbed species on surfaces since it almost does not destroy the adsorbed layer and the surface underneath. The cross-sections of photodesorption and photodissociation ofadsorbates are relatively small, too. This allows the observation of the undisturbed system: surface- adsorbates. The figures obtained allow one to a clean solid surface and an atom in free space with a filled electronic state of binding energy Eat. After adsorption the atomic energy might be shifted (or split) and broadened, forming a resonance level Ead s. A

15

M V Kurepa. High resolut ion electron spectroscopy

photoemission experiment may detect such a resonance level as a distinct change in the local density of states near the surface after adsorption. Comparison of the energy levels of the same atom before and after adsorption can provide information on the electronic changes of the one chemisorption bond acting in the adsorbate system.

An example of the adsorbate studies is the photoelectron spectroscopy of clean platinum (111) and carbon monoxide exposed platinum 9~'.

Spectra for the two conditions of the photon illuminated surfaces have been measured for different incident energies. The difference between the two is due to the presence of the CO at the surface. The common features in the spectra are two peaks at ~8.7 eV and ~ 11.8 eV attributed to the (5a+ l~z) and 4~ CO molecular orbitals respectively.

The adsorption of a molecule on a metal surface affects the bonding orbital of both the adsorbate and substrate. Comparing photoemission spectra of the adsorbate and the substrate alone the bonding mechanism can be studied. In Figure 26 spectra are given for two energies of the Pt(l 11 ) and CO covered Pt. One can see that the valence band peak nearest to the Fermi energy decreases in intensity relative to the other peaks in the Pt valence band. This indicates in the case of Pt + CO that the states nearest to the E v donate electrons to the CO molecule as it chemisorbs.

Pt 5d CO CO bends 5o- + I Tr 4G

f ~ m

i~\

-..Jl , E F 3 6 9 P2

Binding energy (eV)

Figure 26. Photoemission spectra of carbon monoxide adsorbed on Pt( l l l ) at 70 and 150 eV photon energy ~.

8.2. Polarized photon-electron spectroscopy of solids. The use of polarized photons for HRES started recently since a high intensity source of photons is needed. In the low energy range laser beams can be used while in the high energy range synchrotron radiation is applicable. An HRES with polarized photon incident beam is shown schematically in Figure 27. During the experiment the incident photon energy and impact angle can be varied, as well as the analyser viewing angle.

In Figure 28 photoelectron spectra taken around the L point of Cu(111) for photon energies between 59 and 84 eV are given 98. The result shows that the intensity of the peak corresponding to the upper sp band suddenly vanishes around 70 eV photon energy. The other effect is that the sp band is strongly perturbed by the surface.

In order to explain these results the electronic structure has

x

Photon ~:~ source J L,

Photon ~ ~ E n e r g y monochromotor~</ L, _ _ Ale ~ > ~ ~ ~ - - " ~ nol'yzer

plote ,~F~e D~~A~O///

Figure 27. Schematic drawing of a polarized photon electron high resolution electron spectrometer.

/

, ~ \ Surface

---1 C,--

o

2 I EF

Bmding energy (eV)

Figure 28. Normal emission distr ibut ion curves obtained from Cu l l 1 l ) with p-polarized light for photon energies between 59 and 84 eV ~.

been calculated by a linear chain model including only s and p orbitals, with the assumption that only orbitals in nearest- neighbour atoms interact. The result of the calculation showed that the surface state is split from the top of the valence band, i.e. there is charge transfer between the sp band and the surface.

16

M V Kurepa. High reso lu t i on e lec t ron spec t r oscopy

8.3. Electron-electron spectroscopy of solids and adsorbates. A HRES of the electron-electron type can be shown schematically as in Figure 15, but instead of an atomic beam as a target the solid surface takes its place• In general, the electron beam is formed by an electron gun. The beam energy distribution is the same as it is from thermoelectronic emission• The incident beam angle to the surface can be changed. The ejected electrons are energy analysed by a analyser looking at the metal surface at any desired angle.

The overall spectrum of electrons emitted from a solid subject to irradiation by a 2 keV electron beam is rather complicated• Electrons of different origin are observed in the four energy ranges. The relative intensity of various contributions to the spectrum depends on primary energy, angle of incidence and angle of emission. A narrow peak is made of elastically diffracted electrons which contain the structural information of the bulk and surface. Energy losses due to phonon excitation are resolved only by more sophisticated analysers. They are followed by character- istic energy losses due to electronic excitation and ionization losses. Most secondaries, at low energy, result from cascade process which were of little analytical use until recently.

As an example of the HRES of metals we will give results of the Auger electron investigation.

Recently, considerable interest has been directed towards the role of relaxation energy contribution to observed kinetic energies of photo- and Auger electrons. Due to the rearrangement of atomic and extra-atomic charge distribution around the core hole(s) the kinetic energy of the emitted electron is increased. Especially in the case of metals, the extra-atomic charge can move easily towards the atom with one or two holes• Because the Auger process produces two holes in the final state of the parent atom, the effects of extra-atomic relaxation are expected to be strongest for the Auger spectra of metals. In Figure 29, M4,sN4,sN4,5 Auger spectra measured for cadmium are shown 99. The upper spectrum has been taken with a cadmium atom beam, the lower one with cadmium in the solid state, both with the same spectrometer. As one can see, the most important solid state effects on Auger spectra are the increase in the kinetic energy and the broadening of the line components. In addition the solid state has effects on energy splitting and relative intensities of the line components. The energy shift of the Auger spectra on going from free metal atom to solid metal is, in this example, around 12 eV which is rather easy to measure. The determination of the line broadening is a rather difficult problem to handle. It was shown that for a proper comparison Auger electron spectra for free atoms and their solid have to be measured with the same spectrometer.

8.4. Electron energy loss spectroscopy of adsorbates. Vibrations of adsorbed atoms or molecules on solid surfaces are well known from infra-red spectroscopy. HRES energy loss spectroscopy can be applied to study the way molecules are oriented while adsorbed at the surface. As in infra-red spectroscopy there is a selection rule for the excitation of vibrational states: only those vibrations can be excited which create dipoles with a component perpendicular to the surface. This rule is important in giving help in the interpretation of the energy loss spectra.

Experiments of this kind are performed with instruments as shown in Figure 17 where, instead of the atomic particle beam, the target is the investigated surface•

As an example we will give the recent result obtained with a new instrument with the Cu(100) surface at 80 K ~°°. The spectrum obtained is shown in Figure 30. The lower trace is for a clean surface and the upper curve shows the effect of a 1/2 monolayer

30

2o o~

>o ~o

355

Cd MsN4 '5 Na'5 vopour

, - i i " ' . /

360

"', ....... ,.,-.J"

I 365

Kinetic energy (eV)

M4N4,5 N4,5

...~

I 370

3C

>.

2C

Cd sol.id

i

I 370

MsN4,5N4,5

. , . . ..,.

- ! ".. -

t .: I : l 'k.-

I I 375 380

Kinetic energy (eV)

M4N4,sN4,5

2...

\ j - -

I 3 8 5

Figure 29. The M4,sN4.sN4,5 Auger spectrum form cadmium vapour and from cadmium solid 99.

263

g e 2

x 333

g

I I 0 10(3 200 300

Energy Loss (meV)

Figure 30. High resolution electron-electron energy Joss spectra of clean Cu(100) (lower curve) and with (1/2) monolayer of carbon monoxide adsorbed at 80 K (upper curve) l°°

coverage of adsorbed CO molecules. The single loss peak at 262 meV is due to the C O stretch of top-bonded molecules, and it is in good agreement with some earlier measurements•

8.5. Electron-polarized electron spectroscopy of solids. The recent development of spin analysis of low energy electrons has made possible a new type of spectroscopy, schematically similar to the spectrometer shown in Figure 20 where, instead of the atomic

17

M V Kurepa. High resolution electron spectroscopy

particle beam, the solid surface is the target. Although this field of research is rather new a considerable amount of results already exist ~°~. Of all of them only one example will be given here, the experiment with spin-and-energy analysis of secondary electrons from a ferromagnet performed in order to test the hypothesis that the spin polarization of the secondary cascade is closely related to the net spin density near the surface of the material ~ o2

Energetic primary electrons are incident on a solid, they lose energy and cause valence electrons to be excited to unfilled conduction states by direct excitation of valence electrons through the screened Coulomb potential, by Auger processes or by plasmon excitation. These excited electrons can excite still other valence electrons in a casade process leading to the familiar low- energy peak in the energy distribution of secondary electrons.

The target was a Fe-based glass Fe81.sB14.sSi,,. The results of the measurements are shown in Figure 31. Graph (a) shows the secondary electrons as a function of kinetic energy. Graph (b) is the spin polarization of the secondary electrons. The polarization is of the order of 0.2(~4).25 at low electron energies, decreases with increasing energy and levels out to a value of approximately 0.10 above 10 eV. The energy dependence of the secondary-electron spin polarization is not yet understood. Depolarization of electrons in the emission process is unlikely. The variation of the net spin density through the valence band might be looked to as a source of energy dependence of the secondary polarization. The most likely cause of the energy dependence of the spin polariza- tion is the energy dependence of the secondary electron escape depth. This can cause a polarization energy dependence if the spin density at the surface is different from that in the interior of the sample. Secondary electrons of 0-1 eV kinetic energy have mean free paths 3 5 times as long as electrons with 5 10 eV kinetic energy. Thus, high-energy electrons are more sensitive either to an

g ~D

O g

"6

E D 2"

(o)

0 I0 20 30

0 25

0 20

g 015

g 2 o O I0

005

iI(b)

'11 II I I Iii I

I I i

0 I 0 20 3 0

ELectron kinetic energy(eV)

Figure 31. Electron-polarized electron spectroscopy of a Fe-based glass ~ °2: (a) the energy distribution of secondary electron emission; (b) the spin polarization of the secondary electrons as a function of kinetic energy.

intrinsic decrease in the magnetization at the surface or to contamination at the surface which decreases the magnetization or acts as a source of unpolarized secondaries.

8 . 6 . P o l a r i z e d e l e c t r o n - p o l a r i z e d e l e c t r o n s p e c t r o s c o p y . A very new line in high resolution spectroscopy is the polarized electron- polarized electron analysis that can be schematically shown as m Figure 32.

An experiment of this sort has been performed with emitted electrons crudely energy analysed and not polarization analysed. but instead the incident beam polarization altered )°3. The target was a gold monocrystal. The spin polarized experiment was limited to the scattering plane, the P being perpendicular to it. The Au crystal was oriented by the L E E D reflection patterns on a big cylindrical screen. A movable "mini' L E E D system consisting of retarding grids and Faraday cup enabled the measurement of the intensities It and 11 of a single L E E D pattern while modulating the

~ L Photon source

L @ P h o t o n monochromator PoLarizer ~ . h

Quarter-wave plate ~

\ ~ ~ ~ Energy ,+ .\onoL.e

" .... Soo /2" ' , P,o,o.°,ho°e "X-q ,U>" / / ,,. ©J

Target ~ AO0

/ / PoLarization /" analyze r

Figure 32. Schematic drawing of a polarized electron-polarized electron high resolution electron spectrometer.

( degrees )

_4. 6o, '~c'. ,zcl 5.,I b. • I

b ( a I

, i , , ,%." ,

i

.~'e%,-~S" A . " ?

j , I I "~ 60 90 120 50

Scattering angle (degrees)

Figure 33. The (00) reflection spectrum for normal incidence of electrons to the Au(1101 crystal face: (at intensity profile: (bt asymmetry profile; (c) polarization by scattering of unpolarized electrons at the same surface ~°~.

18

M V Kurepa: High resolution electron spectroscopy

p r imary e lec t ron b e a m spin po la r iza t ion . The tota l in tensi ty

I (E, O, 49 )=( I /2 ) [ IT (E , O, ~ ) + I~(E, O, 49)]

and the a s y m m e t r y

IT(E, O, 49)-- I~(E, O, 49) A ( E , O, 49)= IT(E ' O, 49)+I+(E, O, 49)

were measu r e d at c o n s t a n t p r ima ry e lec t ron energy E as a func t ion of sca t te r ing angle 0.

A result o f the sca t te r ing p lane in m i r ro r s y m m e t r y of the crystal for the a s y m m e t r y p a r a m e t e r A is given in F igure 33. As a c o m p a r i s o n , a result for po la r i za t ion ob ta ined by scat ter ing of unpo la r i zed e lec t rons at the surface is also given ~°¢.

References

i K Siegbahn, C Nordling, A Fahlman, R Nordberg, K Hamlin, J Hedman, G Johansson, T Bergmark, S E Karlsson, I Lindren and B Lendgren, Nova Acta R Soc Scient upsM, Ser, IV, 20, (1967). 2 D A Shieley (ed), Electron Spectroscopy, North Holland, Amsterdam (1972). 3 D Chattarji, The Theory of Auger Transitions, Academic Press, London (1976). * A A Bakke, H W Chen and W L Jolly, J Electron Spectrosc Relat Phenom, 20, 333 (1980). s C R Brundle and A D Baker (eds), Electron Spectroscopy, Academic Press, London, Vol 1 (1977), Vol 2 (1978), Vol 3 (1979). 6 j H Eland (ed), Photoelectron Spectroscopy, Butterworths, London (1974). 7 Proc Confon Electron Spectroscopy, Uppsala, 1977, Phys Scripta, 16, (1977). s V V Nemogkalenko and V G Alegin, Elektronnaja spektroskopija kristalov Izd Naukova Dumka, Kijev (1976). 9H Ibach (ed), Electron Spectroscopy for Surface Analysis, Springer, Berlin (1977). 10 N V Richardson and D A King, Phys Bull, 36, 114 (1985). 11 T A Carlson, Photoelectron and Auger Spectroscopy, Plenum Press, New York (1975). 12 D Roy and J D Carette, in Electron Spectroscopy for Surface Analysis (Edited by H Ibach), Springer, Berlin (1977). 13 E H A Granneman and M J van der Wiel, in Handbook of Synchrotron Radiation (Edited by E E Koch), Vol 1, North Holland, Amsterdam (1983). 14 H Z Sar-E1, Rev Sci Instrum, 41, 561 (1970). is D W O Heddle, J Phys E: Sci Instrum, 4, 589 (1971). 16 D Roy and J D Carette, Can J Phys, 49, 2138 (1971). 1~ W Schmitz and W Mehlhorn, J Phys E: Sci Instrum, 5, 64 (1972). 18 W Stechelmacher, J Phys E: Sci Instrum, 6, 1061 (1973). 19 M V61ker and W Sanders, J Phys E: Sci lnstrum, 16, 456 (1983). 20 C E Kuyatt and J A Simpson, Rev Sci lnstrum, 38, 103 (1967). z 1 F H Read, J Comer, R E Imfoh, J N H Brunt and E Harting, J Electron Spectrosc Relat Phenom, 4, 293 (1974). 22 C S Feigerle, D T Pirce, A Leiler and R J Celotta, Appl Phys Letts, 44, 866 (1984). 23 H Ibach and D L Mills, Electron Energy Loss Spectroscopy and Surface Analysis, Academic Press, New York (1982). 24 j E Demuth, as reported by H Ibach and D L Mills ibid. 25 N Avery, as reported by H Ibach and D L Mills ibid. z6 N Sanson, Data obtained on NBS model D (private communication) (1980). 27 L L Kesmodel, J Vac Sci Technol, A1, 1456 (1983). 2 8 0 Klemperer and M E Barnett, Electron Optics, 2nd edition, Cambridge University Press, Cambridge (1971 ). 29 p Grivet, Elecron Optics, Pergamon Press, London (1972). 3o E Harting and F H Read, Electrostatic Lenses, Elsevier, Amsterdam (1976). 31 R E Imhof and F H Read, J Sci Instrum, 2, 859 (1968). 32 F H Read, J Phys E: Sci lnstrum, 3, 127 (1970). 33 D W O Heddle, J Phys E: Sci Instrum, 2, 1046 (1969). 34 D W O Heddle and M V Kurepa, J Phys E: Sci. Instrum, 3, 552 (1970). 35 A Adams and F H Read, J Phys E: Sci Instrum, 5, 156 (1972).

36 F H Read, J Phys E: Sci Instrum, 16, 636 (1983). 37 D W O Heddle, J Phys E: Sci Instrum, 4, 981 (1971). 38 D W O Heddle and N Papadovassilakis, J Phys E: Sci Instrum, 17, 599 (1984). 39 D W O Heddle et al, Private commenication (1984). a0 M V Kurepa, M D Tasi/: and J M Kurepa, J Phys E: Sci lnstrum, 7, 940 (1974). 41 G Martinez and M Sancho, J Phys E: Sci Instrum, 16, 625 (1983). 4z G Martinez, M Sancho and F H Read, J Phys E: Sci lnstrum, 16, 631 (1983). 43 A Chutjian, Rev Sci Instrum, 50, 347 (1969). a¢ p G Cowell, J Phys E: Sci lnstrum, 15, 994 (1982). 4s L D Schearer, in Atomic Physics 2 (Edited by O K Woodgate and P G H Sanders), p 87, Plenum Press, London (1971). ¢6 WRaith, in Atomic Physics 3 (Edited by B Bederson, V W Cohen and F M J Pichanick), p 389, Plenum Press, New York (1973). ,,7 M S Lubell, in Atomic Physics 5 (Edited by R Marrus, M Prior and H Shugart), p 325, Plenum Press, New York (1977). 48 R J Celotta and D T Pierce, in Advances in Atomic and Molecular Physics, Vol 14, Academic Press, New York (1980). 49 WRai th , in Electronic and Atomic Collisions (Edited by J Eichler, I V Hertel and N Stoeterfoht), North-Holland, Amsterdam (1984). so j Kessler, Polarized Electrons, Springer, Berlin (1976). s i p j Keliher, F B Dunning, M R O'Neill, R D Rundel and G K Waiters, Phys Rev, A4, 1271 (1975). s2 L A Hodge, F B Dunning and G K Waiters, Rev Sci Instrum, 50, 1 (1979). 53 L G Gray, K W Giberson, Chu Cheng, R S Jeiffer, F B Dunning and G K Waiters, Rev Sci Instrum, 54, 271 (1983). s¢ U Fano, Phys Rev, 178, 131 (1969). 55 U Heinzmann, J Kessler and J Lorenz, Z Phys, 240, 42 (1979). 56 W V Drachenfels, U T Koch, Th M Muller and H R Schaefer, Phys Letts, 51A, 445 (1975). sT p F Wainwright, M J Alguard, G Baum and M S Lubell, Rev Sci Instrum, 49, 571 (1978). 58 R M611enkamp and U Heinzmann, J Phys E: Sci Instrum, 15, 692 (1982). s9 E Garwin, F Meier, D T Pierce, K Sattler and H C Siegmann, Nucl Instrum Meth, 120, 483 (1974). 6o D T Pierce, E Meier and P Zfircher, Phys Letts, A51, 465 (1975). 6x D T Pierce and F Meier, Phys Rev, B13, 5484 (1976). 62 D T Pierce, R J Celotta, G C Wang, W N Unerti, S Galejs, C E Kuyatt and S R Mielczarek, Rev Sci Instrum, 51, 478 (1980). 63 j j Scheer and J van Laar, Solid State Communs, 3, 189 (1965). 6,, R L Bell, Negative Electron Affinity Devices, Clarendon Press, Oxford (1973). 65 W E Spicer, Appl Phys, 12, 115 (1977). 66 B Reihl, M Erbudak and D M Campbell, Phys Rev, Bll9 , 6358 0979). 67 C S Feigerle, D T Pierce, A Seiler and R J Celotta, Appl Phys Letts, 44, 866 (1984). 68 L G Gray, M W Hart, F B Dunning and G K Waiters, Rev Sci Instrum, 55, 88 (1984). 69 D M Campbell, C Hermann, G Lampel and R Owen, J Phys E: Sci Instrum, 18, 664 (1985). 7o H C Siegmann, D T Pierce and R J Celotta, Phys Rev Letts, 46, 452 (1981). 71 M Erbudak and N Mfiller, Appl Phys Letts, 38, 575 (1981). 72 R J Celotta, D T Pierce, H C Siegmann and J Unguris, Appl Phys Letts, 38, 577 (1981). 73 D T Pierce, S M Girvin, J Unguris and R J Celotta, Rev Sci Instrum, 52, 1437 (1981). 7¢ M Erhudak and N Mfiller, J Phys E: Sci Instrum, 17, 951 (1984). 75 p j Hicks, S Davies, B Wallbank and J Comer J Phys E: Sci. Instrum, 13, 713 (1980). 76 p j Hicks, S Davies, B Wallbank and J Comer, Nucl lnstr Meth, 195, 323 (1982). 77 T Reddish, B Wallbank and J Comer, J Phys E: Sci Instrum, 17, 100 (1984). 78 H H Tuithof, A J H Boerboom and H L C Meuzelaar, Int J Mass Spectrom Ion Phys, 17, 299 (1975). 79 G V Hansson, B Goldberg and R Z Bachrach, Rev Sci lnstrum, 52, 517 (1981). 8o H A Engelhardt, A Zarther and D Menzel, Rev Sci lnstrum, 52, 1161 (1981). 81 H Fellner-Feldegg, U Gelius a Wannberg, A G Nilsson, E Basilier and K Siegbahn, J Electron Spectrosc, 5, 643 (1974).

19

M V Kurepa. High resolution electron spectroscopy

~2 R W Wijnaendts van Resant and J Los, in Physics ~l' Electronic and Atomic Collisions (Edited by N Oda and K Takanayagi), North Holland, Amsterdam (1979). 83 U Heinzmann, G Sch6nhense and J Kessler, J Phys B: At Mol Phys, 13, L153 (1980). 8,~ N A Cherepkov, Zh ~ksp teorJiz, 65, 933 (1973). s5 C N Lee, Phys Rev, AI0, 1598 (1974). so U Heinzmann, Appl Optics, 19, 4087 (1980). sv K J Ross, T W Ottley, V Pej6ev and D Rassi, J Phys B: At Mol Phys, 9, 3237 {1976). ~s V Pej6ev, C G Back, K J Ross and M Wilson, J Phvs B: At Mol Phys, 14, 4649 (1981). s9 C Back, V Pej6ev, K J Ross and M Wilson, J Phys B: At Mol Phvs, 16, 2413 (1983). ,~o B Marinkovil], V Pej~ev, D Filipovi,5 and L Vugkovi~:, XIII ICPEA('. Berlin, 1983, Proc oJ Contributed Papers, p 85 (1983). '~] M Furlan, M J Hubin-Franskin, J Delwiche, D Roy and J E Collin, J Chem Phys, 82, 1797 (1985). ,~2 D Cvejanovi,5, J Jureta and S Cvejanovi(:, IX SPIG. Duhrornik, 1978, Contributed Papers, p 15 (1978).

,3 R M611enkamp, W Wubker, O Berger, K Jost and J Kessler, .I Phys B: At Mol Phys, 17, 1107 (1984). ,;4j W Cooper, Phys Rel3 Letts, 13, 762 11964). ,5 I I Fano and J W Cooper, Rev Mod Phys, 40, 441 (19681. % D A Shirley, K St6hr, P S Wehner, R S Williams and G Apai, Phvs Scripta, 16, 398 119771. ,7 F Solal, G Jezequel, F Houzay, A Farski and R Pinchaux, Solid State Communs, 52, 37 (1984). ~8 Y Petroff and P Thiry, Appl Optics, 19, 3957 11980). "'~ J Vayrynen, S Askela and H Askela, Phys Scripta, 16, 452 (1977). i~.~ S D Kevan and E H Dubois, Rev Sci Instrum, 55, 1604 (I984). ]~1 D T Pierce and R J Celotta, in Advances in Electronics attd Electron Physics, Vol 56, p 219, Academic Press, New York (198X). io2 j Unguris, D T Pierce, A Galejs and R J Celotta, Phys Rev Letts, 49, 72 11982). ~,3 B Heihl, PhD Thesis, Eidgen6ssische Technische Hochschule, Ziirish ( 198O ). I(~4N Muller, IPP 923 Max-Planck lnstitui, Garching ([9791.

20