8/6/2019 Auger Electron- Ion Coincidence

1/5

Auger electron-ion coincidence experiment on nitrogen molecule excitedby electron impact

Ettore Fainelli, Francesco Maracci, and Rosario PlataniaInstituto di Metodologie Avanzate lnorganiche del CNR, Area della Ricerca di Roma, Via Salaria Km 29.3CP 10, 1-00016 Monterotondo Scalo Roma Italy

(Received 17 May 1994; accepted 17 June 1994)

The first Auger electron-ion coincidence experiment on the nitrogen molecule excited by electronimpact is described. The kinetic energy releases of the N++ and the N+ fragments in the 43-72 eVbinding energy range have been measured. The experimental results are compared with previousexperimental data obtained by using synchrotron radiation and theoretical predictions.

I. INTRODUCTION

The study of doubly charged ions, or dications in chemistry language, is important for both fundamental and applicative aspects. The physical and chemical information available is scarce if compared to neutral and singly chargedmolecules. The shape of the dication potential curves is characterized by a potential barrier with an energy minimum generally located above the dissociation limit of the dication.This makes the dications interesting as energy storage systems. Several experimental techniques can provide information on the potential curves and on the dissociation dynamicsfor these species. However, the initial dicationic state mustbe well defined to associate it with a definite fragmentationpattern. A state-to-state study of the fragmentation process ispossible via an Auger electron-ion coincidence experimentwhere one observes, for a selected Auger transition, Le., for aselected initial state of the dication, the products of the dissociation. Such an experiment has been first performed on N2by Eberhardt et al. 2 by using pulsed synchrotron radiation asexciting source. The basic idea of the present work is torepeat that experiment but using a continuous electron beamas ionizing source. The goal is to make the Auger electronion coincidence more accessible. Indeed an electron beamgenerated by a commercial electron gun is easily available inseveral laboratories and cheaper to use with respect to a synchrotron radiation source. The Auger electron spectra (AES)excited by several keY electrons are expected to be almostidentical to the AES excited by white light. 3 On the otherhand, electrons are able to produce autoionization satellitelines (ASL) in the AES while a monochromatized light can

produce ASL only for the proper selected values of the incident energy. However, the contribution of ASL is, in general,a minor contribution to the overall intensity of AES. A maindifference in using electrons or photons to produce core ionization is that when photons are used, the number of dications originated by Auger decay is relatively large comparedto the number of total ions originated by direct ionization.When, instead, electrons are used this ratio becomes lessfavorable. In other words, in an Auger electron-ion coincidence experiment by electron impact, a small Auger electroncurrent corresponds to a detected total ion current. Assumingthat the direct produced ions are mainly Nt ions, we candesume the ratio between the integrated oscillator strengthfor the formation of Nt ions and the production of 1 S - I

core ionized ions to be about 150 from experimental resultsof electron energy loss spectroscopy (EELS) at 8 keV. 45

Consequently, in our case, to obtain true coincidence ratesand statistics comparable to the ones achieved by using photons, we had to use a cylindrical mirror analyzer (CMA) witha large solid angle accepted to detect the Auger electrons. Tocompensate the angular term contribution to the overall electron energy resolution, the length of the same CMA had to bechosen proportionally large. A minor difference between thetwo techniques is due to the continuous nature of the electronbeam that results in a background of false coincidences notaffected by the timing structure of the incident beam.

II. EXPERIMENT

The experimental setup of the Auger electr on-ion coincidence spectrometer is shown in Fig. 1. The spectrometer iscontained in a cylindrical amagnetic stainless steel vacuumchamber which is evacuated by two oil diffusion pumps(1000 j?ls total pumping speed). The limiting pressure is3XIO- 7 mbar and increases up to 2XIO- 6 mbar when theeffusive gas beam is operated. The earth's magnetic field isshielded by three square pairs Helmholtz coils disposed onthe sides of a cube (L=200 cm). The residual magnetic fieldin the scattering zone never exceeds 5 mGauss. The electron beam of energy continuously tunable from 0.1 to 5 keYand intensity from 0.1 nA up to 50 /LA is supplied by aLeybold-Heraeus EQ 22/35 electron gun. At 4 keY energyand 0.1 nA intensity, the diameter of the electron beam at thescattering center is about 0.1 mm. This corresponds to anindetermination of 1.5 e V in the Auger electron energy when

a dc extraction electric field of 150 V/cm is applied to theinteraction region. The current is monitored by a cylindricalFaraday cup (10 mm i.d. and 50 mm long) placed on the axisof the electron gun at 30 mm from the scattering center. Theeffusive gaseous beam is let into the chamber via a needle(0.3 mm Ld. and 20 mm long) orthogonal to the electronbeam path. The needle tip is placed 1.5 mm below the electron beam. The gas density is kept constant by a manual leakvalve that regulates the flux of the gas from an external vessel at a fixed pressure of 1200 Torr. The gas density in theinteraction region is about 200 times larger than the residualdensity in the chamber. The interaction region is placed inthe center of the first stage of a Wiley-McLaren time-offlight mass spectrometer (TOFMS)6 designed and con-

J. Chem. Phys. 101 (8), 15 October 1994 0021-9606/94/101 (8)/6565/5/$6.00 1994 American Institute of PhYSics 6565

ownloaded 13 Sep 2010 to 220.227.97.99. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissions

8/6/2019 Auger Electron- Ion Coincidence

2/5

6566 Fainelli, Maracci, and Platania: Auger electron-ion coincidence

10 .. .

f---!

~ I ~r:____ 3____ w : : , . . . . L - - - ~ sIt L : iii IL- - _ _ _ - - - J

ITMFIG. I. Schematic representation of the apparatus (I ) anode, (2) MCP, (3)TOFMS, (4) electron gun, (5) gaseous beam inlet, (6) CMA, (7) channeltron, (8) Faraday cup.

structed in-house. To obtain the best alignment in the scattering zone, a suitable system (shown in Fig. 2) was realized.It consists of a ion repeller plate (A) with a central circular

aperture (o.d.= 13 mm) shielded by a gold mesh (90% transmission) that constitutes the input slit of the CMA as well.The electron gun (B), the Faraday cup (C), and the needle(D), mechanically aligned, are fixed to the ion repeller plate.Four hollow self-centering cylinders (F) allow the insertionof the accelerating stage (E) of the TOFMS. The ions enterthe accelerating region of the TOFMS through a slit plate 1mm thick with an aperture 13 mm o.d. To avoid field penetration into the scattering zone, gold meshes are attached toboth surfaces of the slit plate. 7 The repeller plate-slit platedistance is 10 mm. The accelerating zone is 52 mm long, asystem of five circular apertures (0.d.=40 mm) connected by

resistors maintains the accelerating field homogeneous. Thedrift zone (L=544 mm, 0.d.=40 mm) is limited by gold

E

FIG. 2. Schematic view of the scattering zone (A) CMA, (B) electron gun,(C)

Faraday cup, (D) hypodermic needle,(E)

accelerating zoneof

theTOFMS (F) selfcentering hollow cylinders.

100

>-uzUJ

~U.U.UJ

Z~t

Ua:t-xUJ

16

KINETIC ENERGY leV)

FIG. 3. Extraction efficiency (%) vs the K.E. of the ionic fragments when150 V cm extraction electric field is used.

meshes at both its ends. Ions are detected by two 40 mmmicrochannel plates (MCP) assembled in a chevron configuration. The operating voltage of the front and the back plateof the MCP and of the anode were - 1800, 0, and 50, Vrespectively. The determined resolution tJ.M I M at 150 V cmelectric dc extraction field is about 110. This corresponds toa full-width-half-maximum (FWHM) of 18 ns for the Nt+peak. In this conditions, the typical transit times of a thermalNt + ion in the source region and in the accelerating zone ofthe TOFMS are about 0.3 and 0.6 }/"S, respectively. The efficiency of the ion extraction depends on the strength of theextraction field and on the kinetic energy of the ions. A typical calculated curve for an extraction field of 150 V/cm over

the 0.03-15 eV ion kinetic energy is shown in Fig. 3. Theejected Auger electrons are analyzed by the CMA (i.d.=88mm, o.d.= 196.8 mm and length 326 mm) mounted with itsaxis orthogonal with respect to both the electron and gasbeams. To generate a homogeneous electric dc field betweenthe two cylinders a fringing field corrector formed by fourrings at different voltages has been placed at each end of theCMA. Two gold meshes (90% transmission) were put onboth the apertures of the inner cylinder to reduce field penetration. In the present experiment, the inner cylinder washeld at the same potential as the repeller plate. A slit plate(o.d.= 1.5 mm) at the repeller potential was placed in front of

a channeltron used as electron detector. The CMA accepts7 around 42.7. This results in a geometrical solid angleof tJ.f1= 1.53 sterad. The electron energy resolution was determined by measuring the elastic scattered electrons at several incident electron energies. A value of tJ.EI E about 1.1 %was determined.

The Auger electron-ion coincidence spectra were obtained by using standard coincidence counting electronics.Briefly, the pulses of the channeltron mounted at the exit slitof the CMA provides the start signal for a time-to-amplitudeconverter TAC, after being preamplified, discriminated, anddelayed. The stop signal is furnished by MCP pulses. TheTAC outputs are sent to a multichannel analyzer and thenprocessed by a personal computer to obtain the electron-ion

J. Chern. Phys., Vol. 101, No.8, 15 October 1994

wnloaded 13 Sep 2010 to 220.227.97.99. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissions

8/6/2019 Auger Electron- Ion Coincidence

3/5

Fainelli, Maracci, and Platania: Auger electron-ion coincidence 6567

'0 0

w>..

a:

'".......3ou

100 55 10

BINDING ENERGY (eV)

FIG. 4. Auger electron spectrum of molecular nitrogen induced by electronimpact. The electric extraction field in the source was 150 V/cm. The arrowsand the numbers in brackets show the B.E. central values selected to collectthe corre,ponding Auger electron-ion coincidence spectra.

coincidence spectra. To avoid statistical distortion due to thefact that faster ions have a larger probability to produce astop signal, the total ion counting was maintained below 100kHz that corresponds to 0.1 nA electron beam current and to2 X 10 - 6 mbar of working pressure with 150 V cm extractionfield.

III. RESULTS

The Auger spectrum of molecular nitrogen obtained at 4ke V electron incident energy is shown in Fig. 4. I t was re

corded with 0.1 nA of electron beam current, 150 V/cm ofextraction field and a working pressure of 2X 10 - 6 mbar. Thebinding energy (B.E.) associated with the double hole configuration of the dication, i.e., the difference between thekinetic energy (K.E.) of the Auger electrons and the N2 1 Score ionization potential is reported on the abscissa. The Auger electron-ion coincidence spectra, measured in the sameexperimental conditions, at eight different B.E. values in them / q range 7-14 a.m.u. are shown in Fig. 5. The typicalcount rates of Auger electrons and ions were 10 and 10 5 Hz,respectively. The true-to-false coincidence ratio and the truecoincidence rate for Nt + ions were 9: I and 0.1 Hz, respectively. The spectra exhibit several structures, centered atm/q=7 and m/q=14, whose variable shape and relative intensities depend on the selected B.E. value. The signal ofeach fragment ion is split into two peaks because the transmission of the TOFMS depends on component of the ionvelocity perpendicular to the instrument's axis.

At 43 e V, the dominating peak corresponds to the undissociated N ; + ion. On both sides of this peak, the peak of theN+ fragment occurs. The relative intensity of N;+ peak decreases as B.E. increases until it disappears completely at 64eV. From 55 eV up, the spectra exhibit the peak of the N++fragment as well.

The K.E. release of the various fragments has been cal

culated considering the so-called tum-around time, i.e., theTO F difference between two identical fragments originated

N++2

UJ ~c:::J

.0 I...0

"--"

w~ 0:::

wUZ WCl

U' z

0 U

2 3 .5 5

FLIGHT TIME (u sec)

FIG. 5. Time-of-flight mass spectra of nitrogen in coincidence with theAuger electrons. The values in brackets represent the B.E. central value ineV.

with the same initial K.E. at the same point on the axis of theTOFSM but in opposite direction, by the formula 7

KE(eV) = (M 2 . q 2 . E 2 ) / (8.323 m) . (1)In Eq. (1) M(f-tS) is the difference between the full

width of the time-of-flight peak and the same quantity of thethermal ion with the same m/q, q(a.u.) is the ion charge,E(V/cm) is the extraction electric field and m(a.m.u.) is theion mass.

IV. DISCUSSION

Our data of Fig. 5 are discussed taking into account theN2 dication potential curves calculated by Wetmore and

Boyd 8 and referring to the experimental Auger spectra ofnitrogen molecule shown in Fig. 4. The dissociation energyof nitrogen molecule is 9.759 eV,9 and the first and the second ionization energies of nitrogen atom are 14.548 and29.611 eV, respectively,1O therefore the Nt+ and Ni+ adiabatic threshold energies E T . H . for the dissociative channels,leading to fragments in their fundamental state are

N ;+ -+N++N+ E T .H . = 38.86 eV, (1')

N ;+ -+N+++N E T .H .=53.92 eV, (2')

Ni+ -+N++N++ E T . H .= 68.4 7 eV. (3')

The E T .H . of processes involving excited fragments are increased with the corresponding excitationenergies. Consid-

J. Chern. Phys., Vol. 101, No.8, 15 October 1994wnloaded 13 Sep 2010 to 220.227.97.99. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissions

8/6/2019 Auger Electron- Ion Coincidence

4/5

6568 Fainelli, Maracci, and Platania: Auger electron-ion coincidence

TABLE I. Energies (eV) of selected N i+ states in the Franck-Condon region calculated by Wetmore et al.(Ref. 13). The dissociative states are labeled with (*). The energy ranges delimited by the arrows are the B.E.ranges selected when the electron energy analyzer is set at 43, 46, and 50 eV.

1BE=46 eV

1* * *

lL.;+

9

31 1u

3L.;+u

' 1 1u

3L.;-

9

31 1

9

1/:1

9

lL.;+9

'1 1

9

lL.;+u

3L.;+u

3/:1u

42.9 43.5 44.5 45.1 45.4 46.8 46.8 46.5 48.7 51.2 50.9 51.843.3 44.7 44.7 46.3 47.1 47.7 48.4 48.5 50.3 51.7 52.7 53.7

rL--_E_=4_3 _V --.Jf rL--_ _ E = _ 5 0 _ e V _ - - 1 r

ering our electron energy resolution (FWHM:!:2 eV at 150V/cm of extraction field) and referring to the assignmentsand the corresponding potential curves calculated by Wetmore and Boyd,S the dicationic states which may contribute,

with different intensity, to the three coincidence spectra centered at 43, 46, and 50 eV of B.E. are shown in Table I.Wetmore and BoydS calculated the potential energy curve for12 dicationic states of Ni+ in the B.E. range 43-54 eV.Eight of them are metastable states, the other four are dissociative states. We can conclude that, in all the three cases,metastable states as well dissociative states, with dissociationlimit N+ ep) +N+ ep), are simultaneously present. This explains the appearance of both Ni + and N+ ions in eachspectrum. Due to our experimental extraction efficiency weare not able to detect all the discrete KE. releases of a specific fragment but only the maximum KE . release KErn. Wecan determine the B.E. of the corresponding dicationic dissociative state using the formula for homonuclear diatomicmolecules

(2)

where Eth is the adiabatic threshold energy of the consideredfragmentation channel.

At 43 eV, N+ fragments with KE rn =4.3:!:0.6 eV are detected. This results in a corresponding B.E. of 47:!: 1 ey'Thepossible dissociative states involved might be 3IIg and/orl.:1

g

At 46 eV, the KErn of N+ fragments becomes 4.5:!:0.6eV, the calculated B.E. is 48:!: 1 eV that likely corresponds to

the I I Ig dicationic dissociative state.In the last spectrum at 50 eV, we found N+ fragments

with KErn =6.1 :!:0.7 eV that corresponds to 51:!: 1 eV of B.E.and to the 3.:1 u dicationic state. Up to this value of B.E., there

is a considerable amount of experimental works so we areable to compare our data with previous reported results. Asummary of K.E. release values obtained by various authorsin recent years is reported in Table II. We note that, despite

the different ionization sources and detection methods, overall agreement among the experimental data is good. A detailed interpretation of the coincidence spectra in the 55-72e V region is more difficult. Previous assignments becomerare 3,11-13 and no calculated potential curves are available.Moreover, the unfavorable signal-to-background ratio makesuncertain the determination of the KE. of the fragments. Thesimultaneous presence, even if with variable intensity ratios,of the ml q = 7 and 14 fragments seems to indicate that all the(1 ' )-{3') fragmentation channels are open. The 55 and 59 e Vcoincidence spectra correspond to a zone of the nitrogen Auger spectrum interpreted by several authors?11-13 Moddeman et at. I I concluded that the most relevant contribution inthis zone of the Auger spectrum is coming from transitionsof the type

IS- I - I - 3N V -+ v (1")

leading to a N ~ +final state. Sambe et al. 3 confirmed the existence of these transitions and supposed them to be responsible for the featureless background underlying the contributions due to 1 S.v 11 '7Tg - + 2 T ~ I v - I I 7Tg transitions. Edwardsand Wood,14 using a time-energy spectroscopy technique, didnot observe the dissociative channel (3') even when theyused 1 Me V He + incident energy in order to enhance the

N ~ +yield in the Auger region where the its contribution wasexpected to be present. We think that the N+ + fragments inthe 55 and 59 e V coincidence spectra come from the dissociation of the N ~ +ion resulting from transitions of the type

TABLE II. Total K.E. (eV) released in the N i+ -+N+ +N+ fragmentation process measured by several Authorsin the 43-53 eV B.E. range.

State B.E. (eV)' Ref. 2 Ref. 14 Ref. IS Ref. 16 Ref. 17 Ref. 18 Ref. 19

3IIg ,lag 47:!:1 9.0:!: 1.2 7.8:!:0.2 8.1 :!:0.3 7.8:!:0.2 7.0 7.7 8.6:!: 1.2II I 48:!:1 11.0:!: 1.6 1O.2:!:0.2 9.1:!:0.3 12 1O.6:!:0.2 9.7 10.0 9.0:!: 1.23 g

51:!: I 14.8:!:0.6 14.2:!:0.5 16 15.0:!:0.6 14.0 15.2 12.2:!: 1.4u

'Present work.

J. Chern. Phys., Vol. 101, No.8, 15 October 1994

wnloaded 13 Sep 2010 to 220.227.97.99. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissions

8/6/2019 Auger Electron- Ion Coincidence

5/5

Fainelli, Maracci, and Platania: Auger electron-ion coincidence 6569

(I"). In fact, if the N++ fragments were originated from thechannel (2'), they should have a too low K.E. because theB.E. of the initial N ; + states (55-59 eV) is very close to theEth (53.92 eV) of that process. On the other hand, the pattern(3') is based on initial N ~ +states with about 29 eV excessenergy. II Therefore, in the coincidence spectrum at an apparent B.E. of 59 eV but at a true B.E. of 88 eV (59 plus 29 eV),the fragments N+ and N+ + are expected to have K.E. ofabout 10 eV. This is in good agreement with the observedK.E. of 93 eV and 92 for N++ and N+, respectively.Moreover, Agren 12 and Liegener l3 attributed a significant intensity, in the same region of the Auger spectrum, to transitions of the kind I S ; ; ; I - t V- 3 1 7 T g or v - 3 3