Photoelectron spectrometry of hydrogen sulfide

Volume 5, number 9

CHEMICAL

PHOTOELECTRON

PHYSICS

15 June 1970

LETTER6

SPECTROMETRY

OF

HYDROGEN

SULFIDE

J. DELWICHE * and P. NATALIS Institut de Chink,

Universif&

de Li2ge.

Sarf-TiEman,

Lisge,

Belgium

Received 20 April 1970

The high-resolution photoelectron spectrum of HSS shows mainly three vibrationally resolved bands corresponding to 3B1, 2A1 and 3B.3states of H3Sf with adiabatic ionization energies of 10.43. 12.76 and 14.91 eV and frequencies of 2476. 1113 and 2000 cm-l respectively. A sudden loss of vibrational structure in the second band is interpreted as due to a predissociation process leading to S+ ion formation. 1. mTRODUCTION

2. RESULTS

Preliminary results on the photoelectron spectrum of H$ have already been mentioned in a recently published [l] experimental study undertaken in this laboratory on the photoelectron spectroscopy of a number of diatomic and triatomic molecules. In particular, special attention was paid to the effect on the photoelectron spectrum of using photons of different wavelengths and it was shown that in many cases under certain conditions transitions to upper ionic states through autoionization processes could occur. Detailed results on such a study in the case of H2S will be described later. A study of the photoelectron spectrum of HSS obtained with a low-resolutio_n cylindrical electrostatic analyser using 584 A (21.21 eV) photons produced in a He discharge has been made previously and critical values of ionization, energies for 5 electronic states of the H2S+-ion below 21.21 eV were reported [2]. However, since as far as we know no high-resolution photoelectoron spectrum of H2S produced with the same 584 A radiation has been published up to now, we report in this paper the result of such a study using an instrument

with a resolution

sufficient

to

resolve vibrational fine structure in a case such as 0: in its a4 state (Av’ = 0.11 eV). In addition, ‘k t e H2S molecule presents some interest because of its electronic structure similar to that of the H20 molecule, which has-been carefully studied by Turner and coworkers [3] by high-resolution photoelectron spectroscopy.

The photoelectron spectrum of H2S has been obtained by means of a retarding-potential electrostatic analyser to be described in detail elsewhere. Briefly, this analyser is a single-grid ’ spherical system similar to that described by Frost and coworkers [4]. With this instrument a resolution of 30 mV, defined as the width of

the Art peak at half height, is provided. The

source is a He microwave-powered discharge. The first derivative of the photoelectron current with respect to the retarding voltage can simultaneously be recorded on an XY recorder and fed into a multichannel analyser (Intertechnique Didac 4000). Owing to the very low signal obtained (see fig. 1) in the operating conditions (sample pressure of about 1 micron in the analysing region), it is necessary to add a number of curves in order to detect the fine structure not visible on a single run, as demonstrated in fig. 1 showing a typical individilal curve and the sum of many of these. Calibration of the energy scale is provided by the peak due to positive ions which have their thermal energy of 0.04 eV 151 as well as by the peak at 5.63 eV due to a small trace of N3 present in the spectrum. The photoelectron spectrum of H&3consists of three well-defined bands (fig. 1) the first of which shows a very strong peak together with a small one, the other fsvo being broad bands partially resolved in a number of peaks corresponding to vibrational levels. Adiabatic ionization energies associated to these photoelectron bands are 10.43, 12.76 and 14.91 eV, respectively. light

These values are in good agreement with earlier * “Charg6 de recherches” of the Belgian Fonds National de la Recherche Scientifique. i 564

results obtained by spectroscopy [6], photoionization [‘I, 81 electron impact [g-11], and by

Volume 5. number 9

CHEhIICAL PHYSICS LETTERS

15 June 1970

and shows a somewhat better defined structure around 20.8 eV. Although corresponding to a very low-cross-section

process,

these

photoekctron

bands, possibly two, are different from the background current and quite reproducible. They were mentioned previously in other studies [2,1], but in these their intensity was considerably exaggerated because of instrumental factors due to the cylindrical geometry of the analysers used. 3. DISCUSSION According

to Mulliken ji2],

the electronic

structure of H2S, similar to that of H20, can be written, omitting inner-shell electrons of the sulphur atom, 0

1

2

3

L

5

0

7

9

9

lo IhLPO!.

II 12 ~Voltsl

Fig. 1. He 584ii photoelectron spectrum of H2S: ‘L) single individual run. b) sum of 20 curves, c) and cl) sum of 30 curves. Peaks due to nitrogen impurity are indicated.

photoelectron spectroscopy studies made by Turner and coworkers [2] as well as in previous

work done in this laboratory using a lower-resolution instrument [l]. Data oacritical ionization energies of H,S are summarized in table 1. In addition to the three above-mentioned photoelectron bands, another fegture is observed in the region of low kinetic-energy photoelectrons, 1-3 eV. It appears as a diffuse band beginning around 18 eV (on an ionization potential scale)

Electronic Earlier spectroscoPY 10.45 I61

...

‘A1 9 to the bond-

where (yb2) and (zal) correspond

ing electrons of S-H bonds and (3xbf) to the

non-bonding lone-pair electrons located on the sulfur atom. Removal of an electron from (3&I), (zal) and (yb2) orbitals requires 5~:;; more energy and leaves H2Si ion in 2B~, I 2B2 electronic states respectively. The first photoelectron band observed shows essentially a single peak and therefore corresponds to the removal of a non-bonding eLectron from a (3xb) orbital, leading to an adiab-

atic ionization potential of 10.43 eV. A small peak is also observed at 10.74 eV which indicates of 70 (2476 2B1.

Table 1 states of H2S+ (in eV, by reference

works

photoionization

(3sa1)2~b2)2(za~)2(3xb,)2 9

a 2~’= 1 + ZI” = 0 transition. The average measurements yields a value of 0.307 eV cm-l) for the vibration frequency cf H2St, This corresponds to “1 = 2611 cm-l in the

to neutral ground state) Photoelectron

spectroscopY

electron impact

earlier ref. [2] 10.42

10.46”)

lO.G3

2Bl,

vibrat.

struct.

12.62

12.78

12.76

2Al.

vibrat.

struck

14.84

14.91

2B2.

vibrat.

struct.

18.09

@.a.0 )

10.46 [7l

10.5 ISI

10.43t101

10.451w

12.62[‘]

12.46t101

works ref. [l]

this work

10.4 WI

- 14 . 18[101 16.07[101

14.82 (18.0 20.12

)

20.8

no structure single broad peak

a) Using Ar 1048-10605 A light xi photon source,

b) Using Ne i36-744 A light as photon source.

565

Volume 5, number

CHEMICAL PHYSICS LETTERS

9

H3S ground state [13]. Complete resolution of these two peaks at 10.43 and 10.74 eV enables experimental

Franck-Condon

factors

of the 2B1

ionic state to be calculated from the present data. The FC factors are OS37 for the (0, 0,Q) c (0, 0,O) transition and 0.03 for the (1, C, 0) + (0, 0,O) transition. It is to be added that the base of the strong peak at 10.43 eV exhibits a quite reproducible asymmetry on the high energy side (on an IP scale). This possibly indicates the existence of an unresolved peak of low intensity associated to a frequency not higher than about 1000 cm-1 and would therefore correspond to v2. The second band observed in the photoelectron spectrum begins at 12.76 ev and shows interesting features. It starts by a vibrational sequence of 5 partially resolved peaks followed by a sudden loss of structure. The broad shape of the band indicates the remoylal of a bonding electron leading to the first excited state of H2S+ which is 2A1 if the ejected electron comes from a (zal) orbikl. The mean spacing between the vibrational levels

is 0.138

ev

(1113

cm-l).

The

most

reason-

able assignment of this frequency is the bending mode v2 which has a value of 1290 cm-1 in the H2S ground state [13]. As mentioned above, the vibrational structure suddenly disappears around 13.4 eV. This seems to have to be interpreted by a predissociation process of the 2A1 state. Supporting evidence of this is given by the experimental results from electron impact [ll] and photoionization [8] studies which show that the appearance potential

of S+ ion occurs at 13.4 eV. A possible process for the fragmentation H2S - H2 + S+ at 13.4 eV may thyefore proceed through a predissociation of Al excited state of H#+. The mechanism of formation of S+ ions has also been examined by Fiquet-Fayard

and coworkers

1141. On the

basis of symmetry rules these authors conclude that S+ ions can be formed only by predissociation and that, considering the appearance potential value of S+ ions, upper vibrational levels of the ‘A1 electronic state are probably predis-

sociated. This conclusion finds a strong experimental support in the present photoelectron data. A similar behaviour had already been observed in the photoionization of SF6 [15]. The photoelectron spectrum of SFe exhibits an ex-

tensive vibrational progression in the fifth pho-

toelectron band. This structure, starting at 19.32 eV consists of 10 well-resolved peaks, but the band extends further on the high-energy side without any vibrational structure over more 566

15 June 1970

than half a volt. This was interpreted as due to a predissociation of SF; ions in that particular electronic

state.

The third photoelectron band starts at 14.91 eV, extends almost over 2 eV and consists of 7 peaks separated by about 0.25 eV (ca 2000 cm-I). The broadness of the band clearly indicates that the transition corresponds to the remov‘al of a bonding electron, presumably one from a (yb ) orbital which would leave the H2S+

ion in the BB2 electronic state. It is to be noted that the peaks are neither sharp nor symmetric which may indicate the possibility of two unresolved components corresponding to similar vibrational frequencies. The value_Tf the mean , is to be comvibrational spacing, ca. 2000cm pared to the stretching modes ~1 and ~3, 2611

and 2684 cm-1 respectively, in the neutral molecule 1131, Further details on the photoelectron spectra of H2S and related molecules using photons of various wavelengths will be published later. We

gratefully

acknowledge

the Fonda

National

de la Recherche Scientifique and the For& de la Recherche Fondamentale Collective for financial support. REFERENCES [I] P. Natalis and J. E. CoIlin, J. Chim. Phys. 67 (l9iO)

69.

[2] AI. I. Al-Joboury and D. W. Turner, J. Chem. Sot. (1964) 4434. [3] C. R.Brundle and D. W. Turner, 307 (l9G8) 27.

Proc. Roy. Sot.

[4] D. C. Frost. J. S. Sandhu and D.A. Vroom, Nature ‘212 (1966) 604. [5] J. E. Collin and P. Nat&s, Intern. J. Mass Spectry. lon Phys. 1 (1968) 483. [6] W. C.Price, J. Chem. Phys. 4 (1936) ‘147.

[7] K. Watanabe, T. Nakayama and J. Mottl. J. Quant.

Spectry. Radiative Transfer 2 (1962) 369. 181 . . F’.H.Dibeler and S.K.Liston. J. Chem. Phvs. 49 (1968) 482. [9] VI. C. Price and T. RI. Sugden, Trans. Faraday Sot. 44 (1948) 108. [lo] D. d. Frost and CA. McDowell, Can. J. Chem. 36 (1958) 39. [ll] 1’. H. Dibeler and H. BI. Rosenstock, J. Chem. Phys. 39 (1963) 3106. [12] R.S.Mulliken, J. Chem. Phys. 3 (1935) 506. 1131 G. Herzberg, Electronic spectra of potyntomic

molecules (Van Nostrand. Princeton, 1968).

[14] F. Fiquet-Fayard (1966) 17.

[15] J.Delwiche. (1969) 215.

and P.M. Guyon. Mol. Phys.

&l.Classe

Sci.Acad.Roy.Belg:

11

45