Download pdf - Reactions of excited atoms and molecules with atoms and molecules

Z. Physik 229, 1--13 (1969)

Reactions of Excited Atoms and Molecules , i

with Atoms and Molecules I I I . Re la t ive Cross Sect ions fo r Penn ing- a n d Assoc ia t ive I o n i z a t i o n

by H e ( 2 IS) - a nd H e ( 2 3S) -Metas tab les

H. HOTOP, A. NIEHAUS, and A. L. SCHMELTEKOPF*

Physikalisches Institut der Universit/it Freiburg, GermanY

Received April 23, 1969

From a He-beam excited by electron impact we eliminated the He(2 1S) component to better than 0.5 % by irradiating light from a He discharge. The quenching process is hv(2 1P---~2 1S)+He(2 1S)--*He(2 1P)---rile(1 1S)+hv(2 1p_ 1 1S). By measuring the ions produced in collisions of the He-metastables with various target gases in a mass spectrometer, singlet to triplet Penning-cross section ratios were obtained. These ratios are without exception close to one, which is taken as evidence for the previously proposed electron exchange mechanism of the Penning ionization. In the case that more ions are produced in the collision of He(2 1S) and He(2 3S) with a target gas, separate relative production cross sections are obtained for the two metastables. For the rare gases the measurements are performed at two temperatures of the He- beam, 320 and 90 ~ It is found that the cross section ratio of associative -- to Penning ionization increases considerably as the temperature is decreased for both, He(2 1S) and He(2 3S), the effect being much more pronounced for He(2 IS). The results of this Work are found to confirm conclusions drawn from measured energy distributions of the electrons ejected in the Penning process.

I. Introduction

Cross sections for ionizing collisions of He-a toms in the metastable states 23S and 21S with other atomic and molecular gases are of interest because of mainly two reasons, (1) because these metastables play an impor tant role in the physics of the atmospheres of stars and planets, and (2) because the knowledge of the relative cross sections for the two states might give some insight into the mechanism of Penning reactions in general.

In our preceding publication 1 we proposed that ionizing reactions of metastables in thermal collisions with other gases occur by exchange of the two participating electrons, (1) and (2):

A*(1) + B(2)-+A(2) + B + + e - (1). (1)

* Present address: E.S.S.A. Laboratories, Boulder, Colorado (U.S.A.). 1 Hotop, H., and A. Niehaus: Submitted for publication, Z. Physik and VI. I.C.P.

E.A.C., Cambridge 1969.

1 Z . P h y s i k , B d . 2 2 9

H. Hotop, A. Niehaus, and A. L. Schmeltekopf:

If this is true, it is to be expected that the cross sections for the two He metastables, which differ only in the spin state, are approximately equal (see Ref. i). On the other hand, if the ionizing reactions are predominantly radiative transitions (2)

A * ( 1 ) + B ( 2 ) - ~ A ( 1 ) + B + + e - (2), (2)

in which, in the course of the collision, the perturbed metastable emits a photon, h v (A* ~A), which in turn is absorbed by the target particle B, leading to its photoionization, B ~ B + + e - , the two cross sections should in general be very different favoring the cross section for the singlet metastable because of its shorter radiative lifetime. The latter mechanism has been proposed by several authors 2,3 for the ease that the transition A* ~ A is optically allowed.

Experimental absolute cross sections have been reported by Benton et al. 4 for the destruction of the He-metastables in collisions with various target gases, and by Sholette et al. 5 for the total ionization by the He-metastables. Benton et al. give separate values for He (21S) and He (23S), whereas the Penning ionization cross sections reported by Sholette et al. are obtained for an essentially unknown mixture of He (21S) and He (2aS). The destruction cross sections for He (2aS) are in fairly good agreement with the average Penning cross sections, indicating that the main destruction process is the Penning ionization. The singlet destruction cross sections reported are larger but very unreliable, as the authors 4 remark. In the work of Ref. 5 the average Penning cross sections are obtained by measuring, in a collision chamber, the ions produced by the He-metastables contained in a He-beam excited by electron impact. By varying the energy of the exciting electrons it was possible to change the relative numbers of singlet and triplet metastables in the beam. From the fact, that the average cross sections obtained did not depend on the composition of the beam, it was concluded that the cross sections are equal for He (21S) and He (23S) within the accuracy of the determination of 10-20 ~o. The only exception was H2, for which a singlet to triplet cross section ratio of cq/o-t~0.65 was esti- mated.

In this publication we report on the comparatively accurate (1 - 5 ~ ) measurement of the quantity ~=NscrJNt(rt, where NJNt=R(Ee~ ) is the ratio of singlet to triplet metastables contained in the He-beam excited by electron impact at the energy Eel. Cross section ratios are obtained from ~ by setting cr~/at(N2)=l. 5 We took N2 as reference target for 2 Watanabe, T., and K. Katsuura: J. Chem. Phys. 47, 800 (1967). 3 Smirnov, B. M., and O. B. Firsov: J. Exptl. Theoret. Phys. 2, 478 (1965). 4 Benton, E. E., E.E. Ferguson, F.A. Matson, and W. W. Robertson: Phys. Rev.

128, 206 (1962). 5 Sholette, W. P., and E. E. Muschlitz Jr. : J. Chem. Phys. 36, 3368 (1962).

Reactions of Excited Atoms and Molecules with Atoms and Molecules. III 3

reasons outlined below. A renormalization of the as~at-values can be made, of course, if a different more reliable ajar-value is known.

In the case that more than one ion species are formed in the collision of the metastable atom with the target gas, relative cross sections for their formation by both He (21S) and He (23S) are given. In addition, for the rare gases At, Kr, Xe being the target particle, the dependence of the relative cross sections for Penning- and associative ionization on the collision energy is investigated separately for the two metastable He-states.

The measurements are facilitated by a new and simple method to produce a "pure" He(23S) beam from a beam containing an arbitrary mixture of He(2aS) and He(23S) metastables. This method consists of irradiating the beam of metastables with light from a He-discharge containing photons which correspond to the transition He(2XP~2~S). Because of the very high cross section for absorption of these photons by the He(2aS)-metastables, it is possible to eliminate the singlet component almost quantitatively (to better than 0.5 ~) by the process

hv(21P~21S)+He(21S)~He(21P)~He(11S)+h ~(584A). (3)

The corresponding quenching process for the triplet metastable does not occur, because the I-Ie(23S), if excited to the (23P)-state, returns to the (23S)-state.

II. Experimental Fig. 1 shows the main parts of the apparatus used. A He-beam formed

by passage through a multichannel tube (M) is excited by electron impact in an excitation chamber (EC) and passes through an area where charged particles are removed by potentials applied to the grids (G1 - G4), and where the light from a He-discharge lamp is irradiated. The beam finally enters a reaction chamber (RC) where it crosses a beam of target particles without hitting any metal surfaces in RC. Ions produced in collisions of the excited He-atoms with the target gas are mass analysed in a mass spectrometer, and electrons can be energy analysed in an electron spectrometer consisting of an electrostatic Einzellens and a retarding electric field, described elsewhere 1. The He-discharge lamp is a closed glass spiral (pyrex glass) surrounding the metastable beam path for ca. 3 cm and filled with 6 Torr He. It is operated at currents (i) up to 50 mA and a voltage of ca. 1 kV. Since no cooling was applied, the time of operation was limited to ca. 30 sec, the time needed for cooling down being a few minutes. Under these conditions the intensity of the (21P~21S)-photons was found to depend linearly on the current (i) up to 50 mA. The efficiency of the lamp to quench He(21S)-atoms

1"

H. Hotop, A. Niehaus, and A. L. Schmeltekopf."

lcml

"il E• chamber G1

Electron - ' ~ - - - - - G4

Spectrometer ~ ~ c t i o n chamber

" " Target Gas

Mu-Meta[ DI

~1 to Mass Si~ectrometer

Fig. 1. Schematic drawing showing the part of the apparatus used for the production of a pure He(23S)-beam

was tested in the following way: In the Penning process for each ionic state of the target, electrons of energies approximately equal to the difference between the excitation energy of the metastable and the ionization potential of the target gas are liberated 6,1. In the retarding electric field we therefore measure, for each target ion state, two electron groups separated in energy by about 800 meV, the difference between the excitation energies of He(2aS) and He(23S). The corresponding " 'steps" of the integral retarding curve are shown in Fig. 2 for the ionization of Xe. The step hights Ss and St are proportional to Nsa~ and Ntat.* If only metastables with a discrete velocity v are present in the beam, we expect that the singlet step hight S~(i) depends on the

( ') .discharge current (i) as S~(i),,~N~(O). as .exp - c v . A plot of log S~(i)

against (i) is shown in Fig. 3. The deviation from linearity is expected because (1) the metastables have a velocity distribution, (2) ~r~ depends probably on v, and (3) the angular distribution of the emitted electrons might change with v. The presently important fact however is, that N~(40 mA)/Ns(O),,~S~(40 mA)/Ss(0)<0.5%. In other words, at 40 mA we can regard the singlet metastables as completely removed from the beam.

* There is a small electron background of presumably Auger electrons. This fact has been taken into consideration in the course of the evaluation of the quenching efficiency.

6 ~ermhk, V.: J. Chem. Phys. 44, 3774 (1966).


Xe~(2p3/2) He(2sS) I He(2'S)

I

75 8 0 8.5eV Time Electron energy

Fig. 2. Stopping curve for electrons resulting from Penning ionization of Xe by He(21S)- and He(23S)-metastables. Also shown is the decrease of the singlet Signal S s and of the total signal S~+ S t caused by irradiation from the He-discharge lamp

100

50

20

10

5

2

1

Q5

0.2

10 20 ) ~ I I l I I

o

o

30 40 i I I

He(2'S) ~Xe

10 20 3 4 Discharge current i [rnA]

Fig, 3. Plot showing the dependence of the intensity of electrons arising from Penning ionization of Xe by He(21S) on the discharge current in the He lamp

For the determinat ion of ratios Ns~Ts/Ntt7 t it is essential to establish that the number of triplet metastables reaching the reaction chamber is not influenced by the light f r o m the discharge. A part ial quenching is possible in principle if He(23S) is excited to a higher s tate of the triplet system f rom where a transit ion to the singlet system by spin

H. Hotop, A. Niehaus, and A. L. Schmeltekopf:

orbit coupling can occur. From Fig. 2 it can be seen that such a quenching does not occur to a measurable extent: the reduction AS(E1) of the singlet signal Ss by irradiation of the discharge light is equal to the reduction A S(E2) of the sum of singlet and triplet signal, S~ + St. From this a quenching of the triplet can be excluded to better than 3 700-

The conditions under which the measurements described in the next section were carried out may be summarized as follows:

1. The exciting electron current was 1 mA at 50 eV. In order to keep Ns/Nt constant, these conditions were not changed. In addition, to correct for small variations of N~/Nt, the ratio N~aJN~cr t for N2 was measured periodically. We chose N 2 as a normal because as/a t for N 2 is very insensitive to pressure and temperature variations, mostly due to the fact that no associative ionization with He metastables O c c u r s .

2. A contribution of photons, emitted by excited He-atoms in the excitation chamber, to the ion production in the reaction chamber, can be excluded to better than 1 70 since no photoelectrons could be detected in the electron spectrometer (see Ref. 1).

3. A small contribution to the signal may arise from ionization of the target gas by He atoms in highly excited long-lived Rydberg states. As was tested, He atoms in such states are not influenced by the He lamp. It was found that their number density is so low (mainly because of the long distance from EC to RC and the electric fields in between) that their contribution to the signal does very probably not exceed 1 ~o.

4. Since no differential pumping was applied, the total pressure in the mass spectrometer during the measurements had to be kept low enough as to avoid scattering of the ions as well as collisional dissociation of molecular ions. Especially in the case of molecular ions produced by associative ionization, for instance HeAr +, collisional dissociation was important down to very low pressures ( ~ 10 -6 Torr) in the mass spectrometer. In each case the pressure was chosen in such a way that the effects mentioned did not influence the measurements.

5. In general the energy distribution of the excited He-atoms is different from the distribution of the ground state atoms due to the exciting electron impact. Although mainly He-atoms with small-angle deflections remain in the beam, the corresponding change in energy may be rather high. In the case that the He beam and the electron beam are perpendicular to each other, the energy may be increased or decreased. As an example, the effect of the momentum transfer of exciting electrons of 50 eV on He atoms of 40 meV kinetic energy shall be outlined: the deflection angle of the excited atoms ranges from 5 to 38 ~ For deflections into these limiting angles the change in energy is 2.7 meV. The


extreme energies possible for excited He atoms are 23 and 79 meV at the middle deflection angle of 21.5 ~ .

The general effect of the momentum transfer will be that the energy distribution for the metastables is broader than the one for the He beam from the multichannel jet M. The average energy of the metastable beam, however, is surely considerably lowered when cooling of M down to about 100 ~ is applied. What is important in the present study is to obtain qualitative ideas as to what happens when the relative energy of the collision system is changed.

III. Measurements and Results

The quantity ~=Ns/N t x ajar for a certain ion is obtained in the following way: (1) The ion intensity I at the output of the mass spectrometer is measured with the He discharge lamp off (i=0). For the ion intensity I(0) we then have the relation I (0)= (N s a s + Nt at). C, where C is a constant. (2) The ion intensity is measured at i=4 0 mA. Since at this current the singlet metastables are essentially all quenched (Fig. 3) while the triplets are not influenced, we have I ( 4 0 ) = N t a t . C, where C, of course, is identical with the constant C for the foregoing measurement. For ~ it follows

1(0) -1(40) Ns as = (4)

I(40) Nt at

Using the result of Ref. 5 that as/at= 1 for the total ionization of Na, cross section ratios are obtained from the measured quantities ~. The results are summarized in Table 1 (for the rare gases, see Table 3).

If ~ is measured as a function of the energy, (E~I), of the exciting electron beam, the energy dependence of Ns/Nt(Er ) =R(E~1) is obtained. R(E~i ) measured in this way is compared with curves reported in the literature in Fig. 4. The curves of Dugan et al. 7 and of Holt et al. s are obtained by assuming that the efficiency for Auger ejection of electrons from surfaces is equal for the two metastables. ~erm~ik's 6 ratios are obtained from step hights of electron stopping curves for Ar, measured in a Lozier tube, and therefore can be expressed as N sas/Nt at. C, where C takes into account a possible difference of the collection efficiency for electrons from processes with the two metastables due to different angular intensity distributions.

Relative cross sections a k, a~ and a k, a~ for the production of different ions k, l in collisions of He (2 1S) and He (2 3S) with a certain target gas can be measured as follows: (1) the relative ion currents 1 k, 1l

7 Dugan, J. L. G., H. L. Richards, and E. E. Muschlitz Jr. : J. Chem. Phys. 46, 346 (1966).

8 Holt, Helen K., and R. Krotkov: Phys. Rev. 144, 82 (1966).

i H. Hotop, A. Niehaus, and A. L. Schmeltekopf:

Table 1. Cross section ratios for the produetion of various ions in collisions with He (21S)- and He ( 23S).metastables

Target as/a t gas

N~

CO

NO

02

H2

D2

HD

H20

co2

CH 4

C2H2

I0n ~

Muschlitz This et al. work

CO +

NO +

o: O +

n + Hel l +

D+ HeD +

HI) + } He l l § HeD + H e H D §

I-I20 + OH +

co + CO +

1 a i1

la 1.05 (2)

-- 0.69 (1)

0.93 b 0.75 (1) 2.2 b 2.95 (10)

0.65 ~ 0.57 (3) 0.80 (6)

-- 0.59 (1) - - 0.80 (2)

-- 0.53 (1) -- 0.73 (5) - 0 .82 (5) - - 1.20 (10)

-- 0.54 (1) -- 0.49 (1)

-- 0.74 (1) - - 1.35 (5)

0 § . . R

C l i f f 0.81 e CH~" !,16 e CH + 1.29 e

CzH2+ -- C2 H +

0.73 (1)

0.67 (2) 1.20 (3) 1.50 (10)

0.73 (1) 0.91 (1)

Target Ion as/a t gas mass This work

Propane 44 0.93 (2) 43 0.92 (2) 42 0.90 (3) 41 1.06 (2)

" 40 0.95 (4) 39 1.00 (2) 29 0.82 (1) 28 0.91 (2) 27 0.90 (2) 26 0.76 (6)

a Reference 5) b Reference 9) e Reference 10)

a re m e a s u r e d i n t h e m a s s s p e c t r o m e t e r w i t h t h e d i s c h a r g e l a m p of f : Ik l (0) ~ (N~- k t , as' + N t �9 ak ' t ) ; (2) f o r e a c h i o n ekl, = N s as'kt/N t trk, l is

d e t e r m i n e d in t h e a l r e a d y d e s c r i b e d way . T h e c ross s e c t i o n r a t i o s a~/a,k z a n d ak/a~ c a n t h e n b e e x p r e s s e d i n t e r m s ofct k, cd a n d f lk t=Ik(O)/I t (O) as

: 1 + 1

k / l ,~k 1 ~ l trs/tr s = p a n d k/ l okt 1 + C~ ~ �9 at/at = p �9 - - ( 5 )

5 1 + 1 + ~k

9 Herce, J .A. , K .D. Forster, and E.E. Muschlitz: Bull. Am. Phys, Soc. 13, 206 (1968):

10 Herce, J. A., J. R. Penton, R. J. Cross, and E .E . Muschlitz Jr. : J. Chem. Phys. 49, 958 (1968).


Table 2. Mass spectra arising f rom collisions with singlet and triplet metastables o f He, aks and akt, and singlet to triplet cross section ratios f o r the total ionization, S s / Z t

Target gas Ion ask at r Z s / Z t

CH 4 CH + 1 1.49 CH + 1.70 1.42 0.95 CH + 0.21 0.14

C2H 2 C2 H + 1 1.37 0.78 CzH + 0.47 0.52

CO z CO~" 1 1.36 CO§ 0.16 0.12 0.77 O + 0.27 0.37

Table 3. Results obtained f o r the ionization o f the rare gases by singlet and triplet He-metastables, at two different temperatures, cr (a) and a (p) are the cross sections

f o r associative and Penning ionization. Z is the cross section f o r total ionization

Target Reference a s (a) a t (a) ~s(P) a s(a) ~r t (p) a t (a) Z s

gas ~s(P) at(P) S t [%] [%]

Ar this work The= 320 ~ 21.2 15.5 100 21.2 91 14 1 .15 90 ~ 44.3 16.7 100 44.3 112 18.7 1.10

Ar Muschlitz et al. 9.3 14.8 1

Kr this work 320 ~ 13.0 17.0 100 13 65 11.1 1.48 90 ~ 45.5 23.8 100 45.5 58 13.8 2.03

Kr Muschlitz et al. 8.0 12.9 1

Xe this work 320 ~ 2.2 11.0 100 2.2 58 6.4 1.6 90 ~ 7.2 17.6 100 7.2 32.3 5.7 2.8

Xe Muschlitz et al. < 5.7 < 5.7 . . . . . 1

Note Added in Proof. The temperature in the experiment of Muschlitz et al was about 420 ~ for the He beam 11 concerning the results in the first two columns. The numbers for the last columns were taken from Ref. s (He-beam temperature in that experiment was about 340 ~

It should be noted that the ratios (5) are independent of R = N s l N t, but, on the other hand, for energetic fragment ions an error might arise from discrimination effects in the mass spectrometer. In Table 2 the quantities

11 Muschlitz, E. E. : Private communication.

10 H. Hotop, A. Niehaus, and A. L. Schmeltekopf:

a] and a~, normalized to the singlet cross section for the parent ion asP= 1, are given for the target gases CH4, C2H1, and CO2.

For the rare gases, measurements of c~ and fl have been carried out at two temperatures of the He-beam source M, 320 and 90 ~ to study the effect of relative kinetic energy on the formation of molecular ions. The results are given and compared with data of Muschlitz etal. 9'11 in Table 3.

IV. Discussion

a) Determination of Ns/N t

The physically meaningful ratio of the excitation cross sections for He (2 iS) and He (2 3S) by electron impact is in general not identical with the ratio R = Ns/Nt of singlet and triplet metastables contained in a He beam excited by electron impact, despite the fact that the life times are large enough. This is because the original ratio of metastables produced is changed by the interference of various effects for which He (2 1S) and He(2 3S) have different cross sections, such as elastic scattering, deactivation, superelastic collisions with slow electrons, and the popu- lation by resonance radiation. The influence of these effects on the measured R(Ee~ ) can be tested, however, and from the experimental conditions, under which the determination of R(Eel) has been made by the different authors, it seems unlikely that they are responsible for the discrepancies shown in Fig. 4. On the other hand, there exists one more effect which possibly could lead to different results and which seems to

2.0

1.5

1.o Z

0.5

o

It / a a----a Dugan et al I / . . . . . Holt et al If~.. [ 3 / ~ this work f I

I i I

s'o Etec.tron.energy [eV]

2.0 �84

1.5

1.0

0.5

60

Fig. 4. Plot showing the ratio of the numbers of singlet to triplet metastables as a function of the energy of the exciting electrons


have been overlooked: if the angular intensity distribution of the He (2 IS) and He(2 3S) - atoms, formed in the crossing point of the electron with the He-beam, is different for these metastables, then NJNt is angular dependent and only equal to the ratio of the excitation cross sections if all metastables produced are collected. This could lead to discrepancies especially between a measurement in which a directed He-beam is used (Ref.6, 7, and this work) and a measurement with diffuse He-gas in the excitation chamber (Ref.S). But also different degrees of collimation of the metastable beam could be important. Measurements of the angular dependence of direct electron excitation to the 2 1S and 2 3S states in He by Simpson et al. ~2 indicate that such an effect will occur.

Another difficulty concerns the way R is obtained from the actually measured quantities, a = R as/a t (in the case of Ref. 6 and this work), and 6 =R?J?t , where 7~, Yt are efficiency factors for Auger ejection of electrons from metal surfaces by the metastables (in the case of Ref. 7 and 8). From the quantity 6, R is obtained by setting ?~/?t = 1, which, to our knowledge, has never been shown experimentally. McLennan a3 reports that "~s and ?t are equal within the experimental error" of 20 ~ . However, he obtained the separate yields 7~ and Yt from the measured yield ~ of a mixture of metastables - produced at 28 eV of the exciting electrons - by using the result of Dugan et aL 7 that N~/N, (28 eV) =0.40, which, as mentioned above, was obtained under the assumption of

?s=?t . We obtained R by using the experimental result s as/at(N2)=l. It

should be noted, however, that to our understanding the results obtained in Ref. s also involve the assumption ?~/~t = 1.

In view of all these difficulties the agreement of the curves R(Er is rather good, but no definite conclusions as to the correct ratio of the excitation functions can be drawn from these curves.

b) Cross Section Ratios

As can be seen from Table 1 the singlet to triplet cross section ratios are indeed without exception close to one, indicating that the ionization process is controlled by the exchange process (6) rather than by the radiative process (7)

A*(1) +B(2) ~ A ( 2 ) + B + + e- (1), (6)

A*(1)+ B(2) - .A(1)+ B + +e- (2). (7)

12 Simpson, J.A., M.G. Menendez, and S. R. Mielczarek: Phys. Rev. 150, 76 (1966).

13 McLennan, D. A.: Phys. Rev. 148, 218 (1966).

12 H. Hotop, A. Niehaus, and A. L. Schmeltekopf:

The rare gas data at 90 and at 320 ~ (Table 3) seem to disagree with the data of Herce et al. 9, who used an inhomogenious magnetic field to select singlet and triplet metastables. However, if we extrapolate from our results linearly to higher temperatures, fairly good agreement is obtained at 450 ~ In the ionization process

A* +B (A +B +) + e-

k i ~ A + B + (8)

k 2 ~ A B §

the channel leading to the formation of AB + becomes more important at low collision energies as expected. In addition we see from Table 3 that the increase of k2/k 1 =~(,)/~(p) - when going from 320 to 90 ~ - is much greater for the singlet metastable. This result is also reflected by the energy distributions, N(Eei ), of the electrons ejected in reaction (8) at the two different temperatures. These distributions are reported elsewhere 1. As an example the distributions at 90 and 320 ~ for Ar are shown in Fig. 5. We see that for the.singlet metastable the distribution does not change within the experimental error, while the triplet distribution becomes broader (at 320 ~ towards higher energies, showing that part of the collision energy is transferred to electronic energy. Since the ratio kz lk i is controlled by the internal energy Ein t of

l dI , " I He(2~S)IAr+(ZP3/z) ~-~ [a.u.] / , , . . - , , - - -

, , / ~ ". 320 K i !/ \ " ,

I 50 100 150 ~E(rneV)

I l - \ He(21S)/Ar+( iP3/2) 1 I \

, / A ~ 320~ i I ~x

6 so loo is"o 2E(r.,V) Fig. 5. Penning electron distributions for the ionization of Ar by Hr - and (23S) - metastab]es, measured at two temperatures, 90 and 320 ~ and showing the different dependence of sing]r and triplet distributions on the kinetic energy of the co1|ision


the intermediate system in (8), and since for the change in collision energy AE~ we have the relation AEk=AE~+AEint, it is clear that for a certain AE k the resulting AEin t is the greater the smaller dee1. Therefore the measured distributions predict a greater change of k2/kl, in the singlet case, as is actually found.

The result that a,~/a v =k2/kl decreases when a heavier rare gas target a tom is used is in accord with the finding that the corresponding distributions N(Er ) are shifted towards lower electron energies.

H. Hotop, Dipl.-Phys. A. Niehaus, Dr. rer. nat. Phys. Inst. d. Univ. Freiburg 7800 Freiburg, H.-Herder-Str. 3

A. L. Schmeltekopf, Ph. D. ESSA Research Laboratories Boulder, Colorado 80302, U.S.A.