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Translational energy distribution of excited hydrogen atoms produced by 0–100 eV electron impact dissociation of SiH4 and GeH4
Volume 112, number 1
TRANSLATIONAL
CHEMICAL
ENERGY
PHYSICS LETTERS
DISTRIBUTION
OF
23 November 1984
EXCITED HYDROGEN ATOMS
PRODUCED BY O-100 eV ELECTRON IMPACT DISSOCIATION OF SiH4 AND GeH4 J. PERRIN and J.P.M. SCHMITT Equipe Synrhese de Couches Minces pour I’Energetique. Ecole Polyrechnfque. 91128 Palaiseau Cedex. France
Received 31 July 1984; in final form 7 September 1984
Translational energy distributions of H(n = 4) produced io e-S%& and e-GeH, collisions have been obtained by differentiating the Doppler profile of Balmer Ho emission Line. Results are similar for both molecules in contrast with CHJ. Two energy components centered at about 0.4 eV (respectively 0.3 eV) and 2.5 eV (respectively 2.0 eV) are attributed to diiferent dissociative escitation processes of the parent molecule SiH4 (respectively GeIG). A third broad component at higher energy is also evidenced.
I_ introduction
During the past few years we have reported several studies on electron impact dissociation processes of silane (SiH4), disilane (SizHs) and germane (GeH4) in the O-100 eV energy range [l--4] _ In particular, we analyzed UV-visible photon emission from fragments created in short-lived electronically excited states [ 1,3 ] _Emission cross sections for H Balmer lines, Si, Si+ (respectively Ce, Ge+) lines and SiH, SiH* (respectively GeH) bands were measured and successive appearance potentials (AI’) of excited fragments in the exkitation functions were related to dissociation processes of the parent molecules via their excited, superexcited or ionized states (the term superexcited refers to neutral states lying above the first adiabatic ionization limit [S-7] ). Moreover excited molecular fragments SiH(A ‘A) and Si.H+(A Ill) created by electron impact on SiH4 or Si2H, were found to have large rovibrational excitations. However this excess energy cannot account for the few eV energy difference between APs of fragments and calculated dissociation limits [ 1,3]. Most of the excess energy is indeed imparted to fragments as kinetic (translational energy) and the heaviest SiHk fragrnents are expected to separate with a much smaller velocity than hydrogen atoms. As a matter of fact, it has been already mentioned that the Doppler broaden-
0 009-2614/84/S (North-Holland
03-00 0 Elsevier Science Publishers B.V. Physics Publishing Division)
ing of H Balmer emission lines, measured with a moderate resolution [ 1.31, indicates that H(rr > 3) atoms have average kinetic energies of the order of 1 eV, whereas a recent study of ground-state SiH(X’Il) by laser-induced fluorescence [4] has shown that SiH(X ‘ll) receives only 0.07 eV kinetic ener-7 in addition to rovibrational excitation. In the present Letter we report a detailed investigation of the translational energy distribution (TED) of excited H(fz = 4) atoms, obtained by measuring the high-resolution spectrum of the Balmer /3 line and differentiating the Doppler lineshape. During the last decade several groups have used this method or simply measured average H(n 2 3) kinetic energies from *tiewidths, in order to get some insight in the dissociative excitation mechanisms of small hydrogen containing molecules such as Hz [8--131, HCl (DCl) [14,15), HF [16], H20(D,0) [17-19];H,S [ZO], HCN [21], NH, [22,23] and CH, [8,22.24]_ A critical review of these earlier studies has been recently given by Hatano [225]_
2. Experimental SiH4 or GeH4 gas (9999% from L’Air Liquide) w&introduced in the multipole dc discharge apparatus described earlier [l-4]. Only the salient features of this device are reviewed. 69
Volume 112, number 1
CHEMICAL PHYSICS LETI-ERS
The discharge is generated by primary electrons emitted from hot tungsten filaments negatively polarized with respect to the surrounding cylindrical wall. Permanent magnets located on the wall create a multipole magnetic field which induces multiple reflections of primary electrons. This restdts in a homogeneous. and isotropic excitation of the gas in the reaction chamber, and allows to initiate a discharge at very low pressure, where the primary electron geometrical mean free path before collection by the wall (50-60 cm [2]) is larger than the electron-molecule inelastic collision mean free path:
241. The Ho line spectrum centered at 486.1 run was recorded with a low wavelength scanning velocity (0.0045 A s-l)_ The wavelength scale on the recorder was calibrated with known Ar lines in the 480490 _.nrn region, emitted from a low-pressure Ar discharge. The instrumental resolution, estimated from the Ar+ line profde shown in fig. la, is better than 0.085 A (fwhm). For each gas, H, line spectra were recorded at several electron energies: 27,32,37,42,47,58,69 and 1lOeV. 70
23 November 1984 HP_ 4861 A
Fig. 1. (a) Profile of an AI+ line, emitted from a low-pressure Ar multipole discharge, giving the instrumental wavelength resolution (b) HP line spectra obtained from CH4, SiH4 and CcH4 multipole discharges generated with 110 eV primary electrons.
3. Results 3. I. Spectral liizeshape Experimental He line spectra obtained in discharges of SiH4 and GeH4 with 110 eV primary electrons are presented in fig. lb. For the sake of comparison, the H, line spectrum obtained in a discharge of CH4, with the same electron energy, is also given. The excellent agreement between the present measurements on methane and those obtained by Ogawa et al. [24] in an electron beam apparatus, not only at ~100 eV but also at lower energies, confirms that the multipole discharge operated at low pressure is well suited to the study of electron-molecule collision processes_ The resolution used appears good enough to induce negligible instrumental distortion of the lineshapes, and no parasitic contribution of other lines are expected nor observed. In order to perform differentiation of the lineshape (see below), each recorded spectrum was converted into a discrete set of points
13 November 1984
CHEMICAL PHYSICS LETTERS
Volume 112, number 1
F(hk) where A&= kbkis the distance in waveleR~h
Wavelength
(A)
from the line center and the wavelength step &A= 0.0235
A_ Intensity measurements
alon& the spectrum
have been made after noise averaging and subtraction
of the baseline. The We_center was adjtisted by checking the symmetry so that F(k&X) = F(-k6A): The resulting smoothed lineshapes, normalized to the Mensity maximum; are presented in fig. 2a for SiH4 and Energy leV) 2 4 6
Wavelength [A) 0 9.5 -0.5
Electron
Impact Energy
(b)
Fig 3. (a) Normalized Ho lineshapes obtained from lowpressure germane discharges at various primary electron enerpies. The accuracy, linited by the noise averaging and the subtraction of the baseline on the esperimf+a.l spectra, is indicated by the enor bars at the maximum and on the wings. (b) Tmnslational energy distniution of H(rz= 4) atoms produced by electron impact dissociation of GeH4. 4
2
faf
Ekctron
Impact Energy
6
W
Fig. 2, (a) Normaiiz&i HP lines&apesobtained from Iowpressure silane discharges at variotis primary election ener. gies. The accuracy, liqited by $be _~oiseaveragingand _tie sub-action of the basetie on the experimental spectra, is indlca
fig. 3a for Gel&. The accuracy of these measurement% Limited by the signal-to-noise ratio on the spectrum is indicated by the error.bars at the maximum and on the wings.
The observed bro~de~g
of the IAsline is ascribed 71
Volume 112, number 1
CHEMICAL PHYSICS LETTERS
to the Doppler effect and all the other causes of broadening (Stark or pressure broadenings) are neghgible in the present experimental conditions. Assuming an isotropic velocity distribution function for the excited H atoms, their TED R(E) is obtained by differentiating the lineshape F(h).according to the relation Pl,l4] ll(,Y) = - dt;‘(h)/dh
,
with
where c is the light velocity, IIZ the atom mass, and X, the wavelength at the center of the line. The assumption of an isotropic velocity distribution of the fragments is not justified a priori in an electron beam experiment, Kouchi et al. 1271 pointed out the importance of the angular ~st~bution and the polarization effect arising from magnetic subIevc1 populations of H(U) on simulated Doppler profiies. Indeed, in the case of CH4, Ogawa et al. 1241 did detect a small polarization effect of the Hl(rz = 4) fragment emission but con&de to a negligible effect of this anisotropy on the derivation of the TED within experimental errors. Anyhow, in our multipole apparatus, the primary electron trajectories have no preferential orientation with respect to the opficai dctection axis so that the light emission from the excitation region is perfectly isotropic and unpola~zed due to statistical averaging.
The differentiation is approxhnsted by
of the experimental
lineshapes
E& = fmL;L [(I&,, 1 + x, - ~~~)~~~~I 2 Due to the limited precision of a given set ofpoints F&j, the fi(&) computed curves have a “noisy”appcarance. By allowing slight changes of the 4X,) values within their respective error bars. one can obtain an estimate of the envelopes of this noise. It has been chosen to represent these envelopes instead of the result of one computation with an arbitrary set of values for P’&). The results are given in fig. 2b for Sill, and fig. 3b for GeH4. For both molecules, three energy components of the H(rr = 4) fragments can be identi~ed as the elec-
23 November 1984
tron impact energy increases from the HP emission threshold up to 110 eV_ At electron energies 27 and 32 eV, the TED seems to consist of a single component and its peak lies at about 0.4 eV for SiH,+ and 0.3 eV for GeHd. At higher electron energies a second component appears with a maximum at about 2.5 eV for SiH4 and 2 eV for GeH4. Finally a third broad component in the 3-6 eV region is unambiguously present in the TED obtained with 69 and 110 eV electrons. These three components are denoted as components 1,2 and 3 respectively.
4. Discussion The successive appearance of components I,2 and 3 in the TED of I-Z(~Z = 4) as a function of e?ectron impact energy on the parent molecule, can be related to the onsets observed in the HP excitation functions [3]_ For SiH, (respectively GeH,) three onsets have been found at 19 * 0.5 eV (respectively 18.5 k 0.5 ev), 22.5 F 1.0 eV (respectively 22.0 * 0.5 ev) and 40 i: 3 eV (respectively 38 f 1 ev). Each onset corresponds to the AI? of a new dissociation process which could be characterized by a specific traI~slation~~ energy of the excited hydrogen fragments, depending on the potential energy surfaces of the dissociative superexcited or ionized states of the parent molecule 17, 251. However it does not seem possible to establish a one to one reIations~p between the three APs in the HP excitation function and the three components in the H(u = 4) TED. In fact, component 2 appears only at 37 i 2 eV electron energy and is probably related to the third AP of HP for both molecules. Then it follows that component 1 involves the undistinguished cont~butio~ of dissociation processes associated with both the first and second APs of HP: Finally component 3, although not correIated with any onset in the HP excitation function must originate from high energy dissociation processes in the 50-70 eV region. In ref. [3] it had been shown that, at the first AP of HP at least part of the formation of H(n = 4) may proceed via dissociative superexcited Rydberg states converging on the (3al)-l (respectively (4al)-I) ionized states of Silil, (respectively GeH4) [28]. In the case of silane, one can assign to these states the dissociation into SiH,(X) f H(lr = 4) for which the cat cufated dissociation limit (for fragments at rest) is
CHEMICAL
Volume 112, number 1
PHYSICS
16.8 +-0.1 eV [3]. The difference between the first AP of HP and this first dissociation limit represents an excess energy of 2.2 + 0.6 eV imparted to the fragments, which is higher than the peak energy of component 1 but compatible with its energy distribution. However dissociation into SiH(X) + Hz + H(rz = 4) with a dissociation limit of 18.3, i 0.1 eV [3] can also be correlated with the first AP of HP since the excess ener,T is 0.8 t- 0.6 eV in this case. The formation of H(rr = 4) at the second AP of HP proceeds also via dissociative superexcited states to which several dissociation processes can be assigned: SiHz(X)+H(~z=1)+H(n=4)
19_4eV,
SiH-JA) + H(JZ= 1) + H(IZ= 4)
21.1 eV,
SiH(A) + H,(X) + H(rz = 4)
21.2 eV _
The origin of component 2 at the third AI’ of HP may be the dissociative excitation of a series of doul bly excited states converging on a third ionized state of the parent molecule (possibly the doubly ionized states (2t2)-2 (respectively (3t-Jm2) of SiH, (respectively GeH,)). The existence of this series of states has been suggested by the analysis of the isotope effect in silane [3] _Many dissociation processes, where H(rr = 4) can have an ionized fragmentation partner, could be assigned to these states. It is striking to notice that while the HP lineshapes and consequently the H(n = 4) TEDS obtained from the dissociation of SiH4 and GeH4 Iook quite similar, they are very different from those obtained from the dissociation of CH, (see fig_ lb and ref. [24]). This is another aspect of the particular behaviour of CH4 in the series of isovalent Xl-l, molecules, as &eady illustrated in comparative studies of photoelectron spectra [28] and ionization mass spectra [3,29], or comparative ab initio computations [30], where it appears that the structure and stability of C@ are very different from those of the otherMb ions. In fact the parallel between ionization and TED of excited H fragments is not fortuitous according to the ion core model [31] if the superexcited states leading to the dissociation into H(/z> 3) are essentially high Rydberg states converging to the various ionized states of the parent molecules. The Rydberg character of these superexcited states is supported by several experimental evidences including the close analogy between @, H metastable, and H(tz = 4) TEDS in the case of CH,
LETl-ERS
23 November 1984
[24) and also the power law dependence of H Balmer emission cross sections as a function of the principal quantum number [3] for the series of XII4 molecules.
Acknowledgement We are grateful to Dr. E. Fabre for lending us his high-resolution monochromator for this experiment.
References [ 1) J. Pen-in and J.P.M. Schmitt, Chem. Phys. 67 (1982) 167. [2] J. Perrin, J-PM. Schmitt, G. de Rosny, 8. DreviIIon, J. Iiuc and A. Llorct. Chem Phys. 73 (1982) 383. [3] J. Perrin and J.F.M. Aarts. Chem. Phys 80 (1983) 35 1. [4] J-P-M. Schmitt, P. Gressier, M. Krishnan, G. de Rosny and J. Perrin. Chem. Phys. 84 (1984) 281. [5 1 R.L. Platzmann, The Vanes 23 (1962) 372. [6] F_J_ de Heer, Intern J. Radiat Phyr Chem 7 (1975) 137. [7] FJ_ de Heer, H-A. van Sprans and G.R. Mohhnann, J. Chim. Phys. 77 (1980) 773. [ 8 1 IL Ito, N. Oda, Y. Hatano and T_ Tsuboi, Cbem. Phyr 17 (1976) 35: 21(1977) 203. [9] R-S. Freund, J.A. Schiavone and D.F. Brader. J. Chem. Phys. 64 (1976) 1122. Polyakova, V-F_ Erko, A.I. Ranyuk and O.S. Pavlichenko, Soviet Phyr JEEP 44 (1976) 921. ill T. Ogawa and hi. Higo. Chem. Phys Letters 65 (1979) 610; Chem. Phys. 52 (1980) 55;56 (1981) 15. [I2 1 hf. Higo, S. Kamata and T. Ogawa, Chem. Phys. 66 (1982) 243; 73 (1982) 99. 113 J. Kurawaki and T_ Ogawa, Chem. Phyr Letters 98 (1963) 274. [ 141 hi. Higo and T. Og3w3_ Chem. Phys. 44 (1979) 279. [I5 ] T. Ogawa, F_ Masumoto and N_ Ishiiashi, Chem Letters (1976) 207. [ 161 hi. Ohno, N. Mouchi, K. Ito, N. Oda and Y. Hatano, Chem. Phys. 58 (1981) 45. [ 17 ] N. Kouchi, K. Ito, Y. Hatano, N. Oda and T. Tsuboi, Chem. Phys. 36 (1979) 239. [ 181 hi_ Higo, T. Ogawa and N. Ishibashi, Chem. Letters ( 1977) 7 39. [ 191 T. Ogawa and J. Kurawaki, Chcm. Phys. Letters 95 (1983) 274. [20] N. Kouchi, M. Ohno. K. Ito, N. Oda and Y. Hatano. Proceedings of the XIth ICPEAC, Kyoto, 1979, p_ 366. [2 1] I. Nishiyarna, T. Kondow and K. Kuchitsu, Chem. Phys. Lerters 68 (1979) 333. [22 ] N. Kouchi, M. Ohno, K. Ito, N. Oda and Y. Hatano, Chem. Phys. 67 (1982) 287.
w II G.N.
73
Volume 112, number 1
CHEMICAL PHYSICS LElTERS
f23] J. Kurawaki and T. Ogawa, Chem. Phys. 86 (1984) 295. 1241 T_ Ogawa, J. Kurawaki and M. H&o, C&em. Phys. 61 (1981) 181. [25] Y. Hatano, Comments At. Mel Phys. 13 (1983) 259. (261 G,R. ~fohima~, F.J. de Heer and J. LOS, Chem. Phys. 25 (1977) 103. f271 N. Kouchi. I;. Ito, Y. Hatano and N. Oda, Chem. Phys. 70 (1982) 105.
74
23 November
1984
[28 J A-W. Potts and W.C. Price, Proc. Roy. Sot. A326 (1972) I6.S. [ZS] J.D. Nor&on and ;.C, iraeger, intern. J. lliass Spectrqm, Ion Phys. ll(l973) 289. 1301 M-S_ Cordon, C&m_ Phys. Letters59 (1978) 410. [?I J J-A. Schkvone, R.C. Smyth and R.S. Freund, J. Chem. Phys 63 (1975) 1043.
TRANSLATIONAL
CHEMICAL
ENERGY
PHYSICS LETTERS
DISTRIBUTION
OF
23 November 1984
EXCITED HYDROGEN ATOMS
PRODUCED BY O-100 eV ELECTRON IMPACT DISSOCIATION OF SiH4 AND GeH4 J. PERRIN and J.P.M. SCHMITT Equipe Synrhese de Couches Minces pour I’Energetique. Ecole Polyrechnfque. 91128 Palaiseau Cedex. France
Received 31 July 1984; in final form 7 September 1984
Translational energy distributions of H(n = 4) produced io e-S%& and e-GeH, collisions have been obtained by differentiating the Doppler profile of Balmer Ho emission Line. Results are similar for both molecules in contrast with CHJ. Two energy components centered at about 0.4 eV (respectively 0.3 eV) and 2.5 eV (respectively 2.0 eV) are attributed to diiferent dissociative escitation processes of the parent molecule SiH4 (respectively GeIG). A third broad component at higher energy is also evidenced.
I_ introduction
During the past few years we have reported several studies on electron impact dissociation processes of silane (SiH4), disilane (SizHs) and germane (GeH4) in the O-100 eV energy range [l--4] _ In particular, we analyzed UV-visible photon emission from fragments created in short-lived electronically excited states [ 1,3 ] _Emission cross sections for H Balmer lines, Si, Si+ (respectively Ce, Ge+) lines and SiH, SiH* (respectively GeH) bands were measured and successive appearance potentials (AI’) of excited fragments in the exkitation functions were related to dissociation processes of the parent molecules via their excited, superexcited or ionized states (the term superexcited refers to neutral states lying above the first adiabatic ionization limit [S-7] ). Moreover excited molecular fragments SiH(A ‘A) and Si.H+(A Ill) created by electron impact on SiH4 or Si2H, were found to have large rovibrational excitations. However this excess energy cannot account for the few eV energy difference between APs of fragments and calculated dissociation limits [ 1,3]. Most of the excess energy is indeed imparted to fragments as kinetic (translational energy) and the heaviest SiHk fragrnents are expected to separate with a much smaller velocity than hydrogen atoms. As a matter of fact, it has been already mentioned that the Doppler broaden-
0 009-2614/84/S (North-Holland
03-00 0 Elsevier Science Publishers B.V. Physics Publishing Division)
ing of H Balmer emission lines, measured with a moderate resolution [ 1.31, indicates that H(rr > 3) atoms have average kinetic energies of the order of 1 eV, whereas a recent study of ground-state SiH(X’Il) by laser-induced fluorescence [4] has shown that SiH(X ‘ll) receives only 0.07 eV kinetic ener-7 in addition to rovibrational excitation. In the present Letter we report a detailed investigation of the translational energy distribution (TED) of excited H(fz = 4) atoms, obtained by measuring the high-resolution spectrum of the Balmer /3 line and differentiating the Doppler lineshape. During the last decade several groups have used this method or simply measured average H(n 2 3) kinetic energies from *tiewidths, in order to get some insight in the dissociative excitation mechanisms of small hydrogen containing molecules such as Hz [8--131, HCl (DCl) [14,15), HF [16], H20(D,0) [17-19];H,S [ZO], HCN [21], NH, [22,23] and CH, [8,22.24]_ A critical review of these earlier studies has been recently given by Hatano [225]_
2. Experimental SiH4 or GeH4 gas (9999% from L’Air Liquide) w&introduced in the multipole dc discharge apparatus described earlier [l-4]. Only the salient features of this device are reviewed. 69
Volume 112, number 1
CHEMICAL PHYSICS LETI-ERS
The discharge is generated by primary electrons emitted from hot tungsten filaments negatively polarized with respect to the surrounding cylindrical wall. Permanent magnets located on the wall create a multipole magnetic field which induces multiple reflections of primary electrons. This restdts in a homogeneous. and isotropic excitation of the gas in the reaction chamber, and allows to initiate a discharge at very low pressure, where the primary electron geometrical mean free path before collection by the wall (50-60 cm [2]) is larger than the electron-molecule inelastic collision mean free path:
241. The Ho line spectrum centered at 486.1 run was recorded with a low wavelength scanning velocity (0.0045 A s-l)_ The wavelength scale on the recorder was calibrated with known Ar lines in the 480490 _.nrn region, emitted from a low-pressure Ar discharge. The instrumental resolution, estimated from the Ar+ line profde shown in fig. la, is better than 0.085 A (fwhm). For each gas, H, line spectra were recorded at several electron energies: 27,32,37,42,47,58,69 and 1lOeV. 70
23 November 1984 HP_ 4861 A
Fig. 1. (a) Profile of an AI+ line, emitted from a low-pressure Ar multipole discharge, giving the instrumental wavelength resolution (b) HP line spectra obtained from CH4, SiH4 and CcH4 multipole discharges generated with 110 eV primary electrons.
3. Results 3. I. Spectral liizeshape Experimental He line spectra obtained in discharges of SiH4 and GeH4 with 110 eV primary electrons are presented in fig. lb. For the sake of comparison, the H, line spectrum obtained in a discharge of CH4, with the same electron energy, is also given. The excellent agreement between the present measurements on methane and those obtained by Ogawa et al. [24] in an electron beam apparatus, not only at ~100 eV but also at lower energies, confirms that the multipole discharge operated at low pressure is well suited to the study of electron-molecule collision processes_ The resolution used appears good enough to induce negligible instrumental distortion of the lineshapes, and no parasitic contribution of other lines are expected nor observed. In order to perform differentiation of the lineshape (see below), each recorded spectrum was converted into a discrete set of points
13 November 1984
CHEMICAL PHYSICS LETTERS
Volume 112, number 1
F(hk) where A&= kbkis the distance in waveleR~h
Wavelength
(A)
from the line center and the wavelength step &A= 0.0235
A_ Intensity measurements
alon& the spectrum
have been made after noise averaging and subtraction
of the baseline. The We_center was adjtisted by checking the symmetry so that F(k&X) = F(-k6A): The resulting smoothed lineshapes, normalized to the Mensity maximum; are presented in fig. 2a for SiH4 and Energy leV) 2 4 6
Wavelength [A) 0 9.5 -0.5
Electron
Impact Energy
(b)
Fig 3. (a) Normalized Ho lineshapes obtained from lowpressure germane discharges at various primary electron enerpies. The accuracy, linited by the noise averaging and the subtraction of the baseline on the esperimf+a.l spectra, is indicated by the enor bars at the maximum and on the wings. (b) Tmnslational energy distniution of H(rz= 4) atoms produced by electron impact dissociation of GeH4. 4
2
faf
Ekctron
Impact Energy
6
W
Fig. 2, (a) Normaiiz&i HP lines&apesobtained from Iowpressure silane discharges at variotis primary election ener. gies. The accuracy, liqited by $be _~oiseaveragingand _tie sub-action of the basetie on the experimental spectra, is indlca
fig. 3a for Gel&. The accuracy of these measurement% Limited by the signal-to-noise ratio on the spectrum is indicated by the error.bars at the maximum and on the wings.
The observed bro~de~g
of the IAsline is ascribed 71
Volume 112, number 1
CHEMICAL PHYSICS LETTERS
to the Doppler effect and all the other causes of broadening (Stark or pressure broadenings) are neghgible in the present experimental conditions. Assuming an isotropic velocity distribution function for the excited H atoms, their TED R(E) is obtained by differentiating the lineshape F(h).according to the relation Pl,l4] ll(,Y) = - dt;‘(h)/dh
,
with
where c is the light velocity, IIZ the atom mass, and X, the wavelength at the center of the line. The assumption of an isotropic velocity distribution of the fragments is not justified a priori in an electron beam experiment, Kouchi et al. 1271 pointed out the importance of the angular ~st~bution and the polarization effect arising from magnetic subIevc1 populations of H(U) on simulated Doppler profiies. Indeed, in the case of CH4, Ogawa et al. 1241 did detect a small polarization effect of the Hl(rz = 4) fragment emission but con&de to a negligible effect of this anisotropy on the derivation of the TED within experimental errors. Anyhow, in our multipole apparatus, the primary electron trajectories have no preferential orientation with respect to the opficai dctection axis so that the light emission from the excitation region is perfectly isotropic and unpola~zed due to statistical averaging.
The differentiation is approxhnsted by
of the experimental
lineshapes
E& = fmL;L [(I&,, 1 + x, - ~~~)~~~~I 2 Due to the limited precision of a given set ofpoints F&j, the fi(&) computed curves have a “noisy”appcarance. By allowing slight changes of the 4X,) values within their respective error bars. one can obtain an estimate of the envelopes of this noise. It has been chosen to represent these envelopes instead of the result of one computation with an arbitrary set of values for P’&). The results are given in fig. 2b for Sill, and fig. 3b for GeH4. For both molecules, three energy components of the H(rr = 4) fragments can be identi~ed as the elec-
23 November 1984
tron impact energy increases from the HP emission threshold up to 110 eV_ At electron energies 27 and 32 eV, the TED seems to consist of a single component and its peak lies at about 0.4 eV for SiH,+ and 0.3 eV for GeHd. At higher electron energies a second component appears with a maximum at about 2.5 eV for SiH4 and 2 eV for GeH4. Finally a third broad component in the 3-6 eV region is unambiguously present in the TED obtained with 69 and 110 eV electrons. These three components are denoted as components 1,2 and 3 respectively.
4. Discussion The successive appearance of components I,2 and 3 in the TED of I-Z(~Z = 4) as a function of e?ectron impact energy on the parent molecule, can be related to the onsets observed in the HP excitation functions [3]_ For SiH, (respectively GeH,) three onsets have been found at 19 * 0.5 eV (respectively 18.5 k 0.5 ev), 22.5 F 1.0 eV (respectively 22.0 * 0.5 ev) and 40 i: 3 eV (respectively 38 f 1 ev). Each onset corresponds to the AI? of a new dissociation process which could be characterized by a specific traI~slation~~ energy of the excited hydrogen fragments, depending on the potential energy surfaces of the dissociative superexcited or ionized states of the parent molecule 17, 251. However it does not seem possible to establish a one to one reIations~p between the three APs in the HP excitation function and the three components in the H(u = 4) TED. In fact, component 2 appears only at 37 i 2 eV electron energy and is probably related to the third AP of HP for both molecules. Then it follows that component 1 involves the undistinguished cont~butio~ of dissociation processes associated with both the first and second APs of HP: Finally component 3, although not correIated with any onset in the HP excitation function must originate from high energy dissociation processes in the 50-70 eV region. In ref. [3] it had been shown that, at the first AP of HP at least part of the formation of H(n = 4) may proceed via dissociative superexcited Rydberg states converging on the (3al)-l (respectively (4al)-I) ionized states of Silil, (respectively GeH4) [28]. In the case of silane, one can assign to these states the dissociation into SiH,(X) f H(lr = 4) for which the cat cufated dissociation limit (for fragments at rest) is
CHEMICAL
Volume 112, number 1
PHYSICS
16.8 +-0.1 eV [3]. The difference between the first AP of HP and this first dissociation limit represents an excess energy of 2.2 + 0.6 eV imparted to the fragments, which is higher than the peak energy of component 1 but compatible with its energy distribution. However dissociation into SiH(X) + Hz + H(rz = 4) with a dissociation limit of 18.3, i 0.1 eV [3] can also be correlated with the first AP of HP since the excess ener,T is 0.8 t- 0.6 eV in this case. The formation of H(rr = 4) at the second AP of HP proceeds also via dissociative superexcited states to which several dissociation processes can be assigned: SiHz(X)+H(~z=1)+H(n=4)
19_4eV,
SiH-JA) + H(JZ= 1) + H(IZ= 4)
21.1 eV,
SiH(A) + H,(X) + H(rz = 4)
21.2 eV _
The origin of component 2 at the third AI’ of HP may be the dissociative excitation of a series of doul bly excited states converging on a third ionized state of the parent molecule (possibly the doubly ionized states (2t2)-2 (respectively (3t-Jm2) of SiH, (respectively GeH,)). The existence of this series of states has been suggested by the analysis of the isotope effect in silane [3] _Many dissociation processes, where H(rr = 4) can have an ionized fragmentation partner, could be assigned to these states. It is striking to notice that while the HP lineshapes and consequently the H(n = 4) TEDS obtained from the dissociation of SiH4 and GeH4 Iook quite similar, they are very different from those obtained from the dissociation of CH, (see fig_ lb and ref. [24]). This is another aspect of the particular behaviour of CH4 in the series of isovalent Xl-l, molecules, as &eady illustrated in comparative studies of photoelectron spectra [28] and ionization mass spectra [3,29], or comparative ab initio computations [30], where it appears that the structure and stability of C@ are very different from those of the otherMb ions. In fact the parallel between ionization and TED of excited H fragments is not fortuitous according to the ion core model [31] if the superexcited states leading to the dissociation into H(/z> 3) are essentially high Rydberg states converging to the various ionized states of the parent molecules. The Rydberg character of these superexcited states is supported by several experimental evidences including the close analogy between @, H metastable, and H(tz = 4) TEDS in the case of CH,
LETl-ERS
23 November 1984
[24) and also the power law dependence of H Balmer emission cross sections as a function of the principal quantum number [3] for the series of XII4 molecules.
Acknowledgement We are grateful to Dr. E. Fabre for lending us his high-resolution monochromator for this experiment.
References [ 1) J. Pen-in and J.P.M. Schmitt, Chem. Phys. 67 (1982) 167. [2] J. Perrin, J-PM. Schmitt, G. de Rosny, 8. DreviIIon, J. Iiuc and A. Llorct. Chem Phys. 73 (1982) 383. [3] J. Perrin and J.F.M. Aarts. Chem. Phys 80 (1983) 35 1. [4] J-P-M. Schmitt, P. Gressier, M. Krishnan, G. de Rosny and J. Perrin. Chem. Phys. 84 (1984) 281. [5 1 R.L. Platzmann, The Vanes 23 (1962) 372. [6] F_J_ de Heer, Intern J. Radiat Phyr Chem 7 (1975) 137. [7] FJ_ de Heer, H-A. van Sprans and G.R. Mohhnann, J. Chim. Phys. 77 (1980) 773. [ 8 1 IL Ito, N. Oda, Y. Hatano and T_ Tsuboi, Cbem. Phyr 17 (1976) 35: 21(1977) 203. [9] R-S. Freund, J.A. Schiavone and D.F. Brader. J. Chem. Phys. 64 (1976) 1122. Polyakova, V-F_ Erko, A.I. Ranyuk and O.S. Pavlichenko, Soviet Phyr JEEP 44 (1976) 921. ill T. Ogawa and hi. Higo. Chem. Phys Letters 65 (1979) 610; Chem. Phys. 52 (1980) 55;56 (1981) 15. [I2 1 hf. Higo, S. Kamata and T. Ogawa, Chem. Phys. 66 (1982) 243; 73 (1982) 99. 113 J. Kurawaki and T_ Ogawa, Chem. Phyr Letters 98 (1963) 274. [ 141 hi. Higo and T. Og3w3_ Chem. Phys. 44 (1979) 279. [I5 ] T. Ogawa, F_ Masumoto and N_ Ishiiashi, Chem Letters (1976) 207. [ 161 hi. Ohno, N. Mouchi, K. Ito, N. Oda and Y. Hatano, Chem. Phys. 58 (1981) 45. [ 17 ] N. Kouchi, K. Ito, Y. Hatano, N. Oda and T. Tsuboi, Chem. Phys. 36 (1979) 239. [ 181 hi_ Higo, T. Ogawa and N. Ishibashi, Chem. Letters ( 1977) 7 39. [ 191 T. Ogawa and J. Kurawaki, Chcm. Phys. Letters 95 (1983) 274. [20] N. Kouchi, M. Ohno. K. Ito, N. Oda and Y. Hatano. Proceedings of the XIth ICPEAC, Kyoto, 1979, p_ 366. [2 1] I. Nishiyarna, T. Kondow and K. Kuchitsu, Chem. Phys. Lerters 68 (1979) 333. [22 ] N. Kouchi, M. Ohno, K. Ito, N. Oda and Y. Hatano, Chem. Phys. 67 (1982) 287.
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f23] J. Kurawaki and T. Ogawa, Chem. Phys. 86 (1984) 295. 1241 T_ Ogawa, J. Kurawaki and M. H&o, C&em. Phys. 61 (1981) 181. [25] Y. Hatano, Comments At. Mel Phys. 13 (1983) 259. (261 G,R. ~fohima~, F.J. de Heer and J. LOS, Chem. Phys. 25 (1977) 103. f271 N. Kouchi. I;. Ito, Y. Hatano and N. Oda, Chem. Phys. 70 (1982) 105.
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[28 J A-W. Potts and W.C. Price, Proc. Roy. Sot. A326 (1972) I6.S. [ZS] J.D. Nor&on and ;.C, iraeger, intern. J. lliass Spectrqm, Ion Phys. ll(l973) 289. 1301 M-S_ Cordon, C&m_ Phys. Letters59 (1978) 410. [?I J J-A. Schkvone, R.C. Smyth and R.S. Freund, J. Chem. Phys 63 (1975) 1043.