Ionization and dissociation by electron impact

77 ln?ernntfonaI Journal of &fass Spectrometry

and Ion Physics,

‘Q Ekevier

Amsterdam

Scientific Publishing Company,

IONIZATION 1. H&AND

AND DISSOCIATION

BY ELECTRON

in

Tte Netherlands

IMPACT

H,S

J. D. MORRKON

Division

1 I (1973) 77-88

- Printed

of Physical

_4ND J. C. TRAEGER

Chemisrry,

(First received 3 July 1972;

in

Lo Trobe Unicersity, final form 3 October

Bundoora,

Victoria, 3083 (Australia)

1972)

AB!ZlXACr

The electron impact induced ionization and dissociation of Hz0 and H2S have been studied using a quadrupole mass spectrometer. Deconvoluted first differential ionization efhciency curves are presented and structural features of the parent total

and

fragment

differential

sponding

ion

ionization

photoionization

curves cross and

discussed section.

photoelectron

in terms

A

of

comparison

their

contribution

is made

with

to the the

corre-

spectra.

INTRODUCTION There have been numerous studies made of the ionization of Hz0 and H2S, both electrons and photons being used to induce ionization. Vacuum ultra-violet photoionization1i-‘7 and spectroscopy’* ‘, electron impact ionization3-‘*, have all been used to examine the electronic photoelectron spectroscopy1s-23 structures of these two relatively simple molecules. With the development of high resolution photoelectron spectroscopy, our knowledge of electronic states for many molecular ions has been greatly increased. Unfortunately studies are largely limited to energies less than 21.2 eV, ah-bough some work has been done at higher energies with the aid of He II radiation (40.8 eV). Mass analysis of dissociation products is an essential for a complete understanding of the behaviour of an ionized molecule, so it is-nec&sary to use either electron or photon impact ionization in conjunction with a mass spectrometer. Early workers produced fairly crude electron impact ionization efficiency (IE) clurves for various fragment ions, but because of the poor signal to noise (s/n) ratios and energy spreads in the ionizing electron beams, the only really me&ni&ful results were appearance potentials. Price and Sugden4, usingaphotoelectron imp&t

78 technique without mass analysis, and Frost and McDowel16, using the retarding potential difference (IWD) technique of-electron impact for the parent molecular ions, investigated the higher ionization energies for both Ii,0 and I&S. This work was later extended to fragmentation processes by Cottin’ and Dibeler and Rosenstock8. More recently, mass analyzed photoionization studies have been made14* 15, which, in conjunction with the corresponding photoelectron spectra, have helped to elucidate the nature of the first three electronic states for HzOi and H,S*. There is still only limited information concerning the removal of an electron from a 2ai valence shell orbital. In the present experiment a combined time averaging-deconvolution technique, described previously’“, has been used to study the Grst differentia! electron impact II5 curves of Hz0 and H$, with particular emphasis on the fragmentation processes of these relatively simple molecules.

ExPEFsMEwrAL

Several modifications have been made to the combined wmputer-mass spectrometer system described in detail elsewherez4_ These include conversion of the computer output differential amplifier to solid state (Fig. 1) and use of a well regulated and filtered, constant current supply for the rhenium iiiament (Fig. 2), in place of the lead storage battery used previously. To maintain reproducible curyes for HsS, free from hysteresis effects25 it was necessary to keep the ion source continuously heated at 250 “C during experiments, rather than just baking it out prior to data wllection. This procedure was also implemented for H,O.

COMPUTEROUTPUTAHPi_IFIER

Fig.

1.

Computeroutput diiereatial amplifier(LM302,

LM307,

National

Semiconductors).

REGULATED

FILAMENT

POWER

SUPPLY

Fig. 2. Regulated filament power supply (LMZ05, National Semiconductor; Bridge Rectifier; MBI, S.T.C. Bridge Rectifier).

5OFBIL, International

Contrary to the observations of other worker?‘, we have found conditioning rhenium Laments essential for stable emission currents. As well, much lower filament currents, and hence, filament temperatures can be achieved. By passing benzene at a pressure of 10-j torr over a new filament in operation, for approximately 30minutes, areduction of the filament current from 3.OA to 2.2A is possible, the total electron emission remaking at approximately 20 JU%_It was found that after several hours of exposure to H,S, the filament showed signs of being poisoned. Fortunately, in most instances, reconditioning of the iilament with benzene was successful. The water sample, obtained from the laboratory distilled water supply and the hydrogen sulphide, obtained from the Matheson Corporation (stated purity 99.6 %) was used without further purification. No detectable impurities were present in the mass spectra recorded at 40 eV. The IE curves were processed in several ways using a PDP9 digital computer. Firstly, the differential IE curves were deconvoIuted24 to nullify the effect of the electron energy spread and appearance potentials were deduced from the ionization onsets, xenon and krypton being used for *he electron ener,y scale calibration. To investigate the fragment ion contributions to the total ionization, all IE curves were scaled for relative intensities. Correction for the isotope abundances of sulphur (32S = 95.0 %, 33S = 0.86 %, 3’S = 4.2 %)” were then made to the HSi and HzSi curves. It was assumed that the isotopic contributions.of 2H, I70 and “0 were negligible. Each curve as measured comprised 1024 values of the ion current over the energy range scanned. Since this was more than were needed, a least squares smoothing and differentiation as described by Savitzky and Gola)i28 was performed, the convoluting integers corresponding to an eleven point fit of a quintic curve.

80

At this stage every fourth point of all ionization curves was plotted using a Hewlett-Packard 7004B XY recorder, interfaced to the computer. With this processing, the experimental scatter in the first differential curve near threshold is negligible. At the high energy end of a scan, the scatter is estimated lo be still less than & of fu!: scale, except for the fragment ions of low abundance. The first differential IE curves were then added sequentially, commencing with the lowest mass fragment, so that a curve representative of total ionization was constructed. The relative contributions of each ion to the total differential cross section, with the exception of H’ which was not studied, were then calculated and plotted out as a clastogram on the XY recorder_

RESULTS

AND DISCUSSION

Deconvoluted f%rstdifferential IE curves for H,O+, OHi and O+ are shown in Figs. 3-5. These are the result of time-averaging 2000 individual curves over a period of 3 hours as previously described 24. Appearance potentials are indicated on each figure. The relative ion intensities are shown on the undeconvoluted curves of Fig. 6. The curve for H20+ (Fig. 3) shows appreciable structure preceding the 2B2 state at 17.2 eV19 which tends to interfere with the lower energy *Al state at 13.7eV (ref. i9). This structure was reproduced in curves run over different electron energy ranges and using a different digital-to-analogue converter to generate the ramp voltage from the computer. Both excited states are not clearly defined, as we have observed for other atoms*’ and molecules2’. Two particularly pronounced peaks

F&3.

Dceonvohted

lint different& IE curve for I&O+.

Fig. 4. Deconvoluted

6rs.t dierential

IE curve for OH+‘.

Fig. 5. Deconvoluted

fizst differential

IE curve for Oi.

occur at 15.5 eV and 16.0 eV. Earlier workers4* ‘9 ’ assigned structure in this region to the ‘B2 state. However, photoelectron spectroscopy has since shown that this state is somewhat higher in energy with no evidence for-any significant process in the vicinity of 16 eV. Sjogrcn’ ’ suggested tbat the break at 16.2 eV observed in electron impact experiments could be due to either ion-pair formation or. to preionization; He assigned the broad peak at 16 eY in the electron .energy Ipss s@cctrum of H20 obtained by Skerbefe and J-settle9 & the excitation of a lb2 electron to the 4& orbital, and, following an electron and ion impact study of the water molecule, concluded that preionization was a more likely process than ion-&r forinaton. The peak-that we observe at 16.0 eV would be cons@tcnt with this reasoning

Fig. 6. First differential

IE curves for H20i,

OH+ and O+ showing

the relative ion intensities.

but no satisfactory assignment is then possible for the sharp peak at 15.5 eV or the broad band at 14.7 eV. The 3pa, and 3pb, Rydberg levels would appear as a relatively broad feature but are expected to occur between 15.0 eV and 15.5 eV”‘. It should be noted that Sjogren based his calculations on an energy of 18.0 eV for the lb, electron whereas recent high resolution photoelectron spectra’9323 place the verticai ionization energy at 18.5 eV. There is a noticeable decrease in the H1O+ first differential IE curve at the onset of OH+ (18.2 eV), whereas the total ion curve (Fig. 7) exhibits an increase at this ener,T. A similar effect has been observed in photoionization curves for

Fig. 1. First differential 1E cqrve for HzOi-+ lwtions of each ioh to tlxi total curve.

OX+ f 0’.

The shaded areas represent the contri-

83 these two ionsf4, “. In an earlier paper3’ we suggested that the OHi ions formed from Hz0 at about- 18 eV probabIy arise by predissociation from the *Bz state, but that this predissociation may occur by some kind of interaction with the ‘B, or ‘A, states, rather than simply by way of the 4A” repulsive state as discussed by Fiquet-Fayard and Guyon ” . only in this way could we find a possible exphmation for the marked decrease in the first differential IE curve of the parent ion at 18.2 eV. No attempt has been made here to explain other structural features present in the deconvoluted curve for HzO’. Apart from the overall. change in gradient at 29.5 eV, preceded by a broad unresolved band commencing at 21 eV, we carr find no evidence for the ‘A, state of H20i at 32.2 eV as observed by photoelectron spectroscopy 23* 32- A recent photoionization study” has also failed to detect this state. The curve for OH* does not show any pronounced structure and is similar in shape to the corresponding photoionization curve”, with the exception that the latter shows a general cross section fah off at photon energies in excess of 19 eV. There is some evidence of the ‘A and ‘Xi states of OH’ but the ‘II state is obscured by a relatively large peak in the region of 21.7 eV. Known states3’ are indicated on Fig. 4. As for H,Oj no significant structure is present at 32.2 eV. The O+ ion produced from H20i is in a “S ground state, which has been suggested by Hughes and Tieman 33 does not form by predissociation of H20+ (‘B,) via a ‘A, repulsive state31. They argue that predissociation, although energetically possible, is in competition with the dissociation leading to OH+ (‘C-J the latter being strongly favoured. As a result, the dissociation yielding excited O+ proceeds from some molecular-ion state of higher energy which is weakiy populated by electron impact. Because the fourth electronic state Of-Hz0 is known to be in the region of 32 eV, the identity of this ionic state is unciear. The major ionization process yielding 0’ from Hz0 has an onset at 25.8 eV which is the energy for the process H,O(rA,)+e+ O’(‘D)+ZH(*S)-f-Ze-, (ref. 34), assuming that the dissociation products have negligible kinetic ener,?. A further increase in the 0’ curve can be attributed to the production of O’(‘P) at 28.5 eV34. The lower energy dissociation processes, being less sign&ant, cannot be distinguished because of the extremely low level signal. There is little evidence from the O+ curve that the fourth electrc iic state of H20’ has a vertical ionization energy of 32.2 eV. An interesting feature of the O+ curve is theextended horizontal region between 36 and 44 eV [Fig. 6), unlike most first differential IE curves which tend to fall off soon after reaching a maximum. Figure 8 represents the percentage contribution of each ion to the total first differential IE curve. The contribution of Oj is negligible until about 29 eV after which there is a continuous increase. At this particular electron energy the OH+ contribution shows a change to an almost constant value whereas H,O+ does not appear to be affected. The dissociation of Hz0 to O+ seems to he proceeding at the expense of OH +.

Fig. 8. Percentagecontributions of H,O+,

Deconvoluted

OH+ and C?+ to the total fist differential IE curve.

first c!ifXerentizl IE curves for H,S+,

I%*

and S’

are shown

in Figs. 9-l 1. These are also the result of time-averaging 2000 individual IE curves over a period of 3 hours. The electron energy sweep range was less than that used for H20. Appearance potentials are indicated on each figure whilst the relative ion intensities are shown on the undeconvoluted curves of Fig. 12. The curve for HtS* shows quite an abrupt change in gradient at the onset of S+ and a similar, but less distinct change at the appearance of HS’. ThiPis in good agreement with the corresponding photoionization curvesI and is probably analogous to the interaction discussed above for H,O+. Some indication of the

Fig. 9. Deconvoluted first differential IE c&e

for H+S+.

Fig. IO. Deconvoluted

first differen+

IE curve for HS+.

Fig. 11_ Deconvoluted

first differential IE curve for S+.

*A, state of HzS+ can be seen at 12.9 eV although there is interference from probable autoionizing processes. From photoelectron spectraz3 the adiabatic and vertical ionization energies of the 2a, orbital for H,S are found to be 12.78 eV and 13.33 eV respectively. There is a broad band between 17 and 22 eV which precedes a gradient change at 22.6 eV. This may be evidence for the fourth electronic state of H,S’ which is found at 22.2 eV by photoelectron spcctrosc~py*~. It should be noted that the overali faILoff of the curve for H2S+ is greater than in the case for HzO+ over the same electron energy range. As observed for OH*, the HS+ curve does not show any pronounced features. Known states for HS’ (ref. 31) are indicated on Fig. 10 but there is little correlation with them. No structure present at 22.2 eV can be attributed to the fourth electronic state of Ha!!++.

Fig. 12. First differential

IE curves

for I&S+,

HS+ and Si showing

the relative ion intensities.

It has been suggested3’ that HSi is initially formed by predissociation from HzS* (‘A,) and rhat when it is energetically possible, from HzSi (‘B,). There is little evidence for this changeover from our HS+ curve, although the threshold region is somewhat broader than that of H2St and Si. The dissociation of H,S’ to S+ and H2 is quite significant, unlike the analogous case for Hz0 +. There is also good agreement with the known spectroscopic states of’Si (ref. 34) indicated on Fig. 1 I ; it is assumed that S i- has negligible kinetic energy. Production of both S’ (ZD)+2H (‘S) and S’ (‘P)+-2H (‘S) cause the S’ curve to exhibit a second maximum at 24.5 eV instead of slowly decreasing as is commoniy observed with first differential IE curves. Both these processes appear to play a major role in the dissociation of H2S+ and H,O’ to S: and O+ respectively. However, they are more apparent in the case of O+ because of the relatively insignificant lower energy processes_ Thereis quiteextensive, reproducible structure in the curve for S+ which is probably due to autoionization of the parent molecule, followed by dissociation. Figure 13 represents the total first differential IE curve due to S*, HSi and HzS+. The onsets of the 2A1 states are both not very clear. However, the 232 state, which has been found to have an adiabatic ionization ener,y of 14.8 eVZ3* 36 is quite a prominent feature. This state is not stable as H2S+ but predissociates to both HS+ and S+, giving rise to diffuse vibrational structure in the photoelectron spectrum. Examination of the curve envelope created by sequential addition of both HS’ and S- curves (Fig. 13) reveals an interesting feature. It appears that the fall-off .in Si is compensated by an increase in HSC so that the envelope is relatively smooth. A similar but less noticeable effecr was observed with Oi and Z-I+. The p&cenhze contribution of each ion to the total first differential IE curve

87

Fig. 13. First differential IE curve for HzS+-!-HS+ butions of each ion to the total curve.

Fig. 14. Percentage

contributions

of HzS +, HS’

+S +_ The shaded areas represent

and Si

to the total first differential

the contri-

IE curve.

is represented in Fig. 24. At the onset of HS+ only Si appears to be affected, the H2Si curve showing no marked cbaq;e at this electron energy. However, at about 21.5 eV, where the contribution of S+ to the total curve increases rapidly, it appears to be at the expense of H,Si.

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1 2 3 4

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