J . Phys. Chem. 1992, 96,6688-6693
6688
Doppler Spectroscopy of Hydrogen and Chlorine Atoms from Photodissociation of Silane, Germane, Chiorosiianes, and Chioromethanes in the Vacuum Ultraviolet Region Keniehi Tonokura, Yuxiang MoJ Yutaka Matsumi, and Masahiro Kawasaki* Institute for Electronics Science, Hokkaido University, Sapporo 060, Japan (Received: April 13, 1992)
Silane, germane, chlorosilanes,and chloromethanes are photodissociated at 157.6 nm. SiH4and GeH, are also p h o t d i t e d by a two-photon process at 243.1 nm. Hydrogen and chlorine atoms are detected using multiphoton ionization technique for measurement of Doppler profiles. The kinetic energy distributions of the nascent hydrogen atoms from silane and germane are derived from the Doppler profiles, which are Boltzmann. The average translational energies are 0.6 eV at 157.6 nm (one-photon dissociation) and 0.8 eV at 243.1 nm (two-photon dissociation). Hydrogen atoms from chlorinated compounds have both Boltzmann and non-Boltzmann energy distributions, suggesting both predissociation and direct dissociation processes. Relative quantum yields of hydrogen atoms for chloromethanes are smaller than the stoichiometric ratios.
I. Introduction The vacuum ultraviolet absorption spectrum of SiH4 for the wavelength region 110-160 nm was assigned primarily to Rydberg transitions of the gSiHbonding electrons to the 4s, 4p, and 4d orbitals.'-3 Though the valence excitations into a* molecular orbitals are not seen in the outer-shell excitation spectra of saturated species such as SiH4 and CH4, the mixing of ala* with 4s(a,) in SiH4can be appreciable and is responsible for the dissociation of the (It2, 4s(al)) Rydberg state., Suto and Lee2 reported formation of SiH* (A2 A). However, the fluorescence quantum yield of this emission system is less than 0.015. The excited 6iH4 molecules are likely to dissociateinto fragments that are in the ground electronic states. Table I summarizes the energetics of the dissociation processes. Perkins et aL5 reported two primary processes for SiH4at 147 nm (8.4 eV) based on their observation of H2, Si2H6,Si3Hs,and a solid hydridic silicon film SiH4
-E
SiH3
+
H
SiHP + 2H
(1)
-E
+H
GeH2
+ 2H
+
The first absorption bands of alkyl chlorides are assigned to the ~ * ~ ~ , ?r- 3transitions p with a weak peak near 175 nm.' The second strong band has peaks in the 160-140-nm region for chloromethanes, CH3C1, CH2C12,and CHC13. This band is assigned to the 4s 3p Rydberg transition. The manifold of U*CC, levels is in part overlaid by Rydberg excitations. The 157.6-nm F2laser light excites both the Rydberg and the valence transitions. Because of strong mixing of u * ~ ~u , *, ~ and ~ ,the Rydberg states, photodissociation dynamics of the chloromethanes are expected to be different from those of CCI,. The vacuum UV photoabsorption of chlorosilanes were assigned to Rydberg transitions from C1 lone pair orbitals. The Q* 3p transitions are also excited for chlorosilanes at 157.6 nm. The behavior of chlorosilanes is expected largely to parallel those of the chloromethanes.
-
+-
11. Experimental Section
The experimental setup is the same as that used for a previous study of multiphoton ionization detection of H and C1 atoms from On leave from the Laboratory for Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Science, Dalian, 116011, People's Republic of China.
ref a
b C
+
+ +
+
(2)
The absorption features of germane are essentially the same as those of monosilane and assigned mostly to Rydberg transitions. Direct detection of photofragments has not been reported. Piserchio and L a m p 6 reported the product analysis by a mass spectrometerfor the 147-nm dissociation of GeH4at 2 Torr. They observed H2, G%H6,Ge3Hs,Ge4Hlo,and solid hydridic germane film or powder. These products were explained by assuming the following primary processes GeH3
TABLE I: Eaergetics of Diacocirtion Processes molecule products MIeV SiH4 SiH3 H 3.91 SiH2 + 2H 6.65 GeH4 GeH3 + H 3.7 GeH2 + 2H 6.2 SiH2CI2 SiHCI2 + H 3.8 2.4 SiH2Cl + C1 SiHCl C1 + H 7.2 SiHC13 SiC13 + H 4.0 SiHC12 + CI 2.6 SiC12 C1 + H 6.9 CH3C1 CH2C1 H 3.9 3.5 CH3 + C1 CHI + C1+ H 8.4 CH2C12 CHCl2 + H 3.5 CHCl + C1+ H (7.0) CHClp CCI3 H 3.9 CC12 + C1 + H (7.4) 26.
C
d d e
d e
References 22 and 23. Reference 24. Reference 25. Reference Estimated value.
photodissociation at 157.6 nm (one-photon) and 243.1 nm (twephoton)! Sample gases in an effusive beam were introduced through an effusive beam nozzle into a vacuum chamber, which was maintained at a pressure of 5 X Torr. An unpolarized dissociation excimer laser beam (Lambda Physik, EMG 101 MSC-F2, 10-ns pulse width) was collinearly aligned with a linearly polarized probe laser. The 157.6-nm laser light was attenuated to 0.2 mJ/pulse by meshed screens and focused by a CaF2 lens (f = 350 mm). Resulting H and C1 atoms were detected by the resonance enhanced multiphoton ionization technique. An XeCl excimer laser pumped dye laser (Lambda Physik, FL 3002) was used as the probe laser, and the output was fed into a BBO crystal for frequency doubling. The UV light (
