ELSEVIER Journal of Photochemistry and Photobiology A: Chemistry 101 (1996) 1-5 Jom'nalof PI-R311XlqEM~gII~ A:~TBY ArF laser dissociation of trisilane Rosa Becerra a, Milagros Ponz b, Marta Castillejo a, Mohamed Oujja a, Javier Ruiz a,t, Margarita Martin a'* i Instituto de Qu[mica F(sica "Rocasolano' ', C.S.I.C, Serrano 119, 28006 Madrid, Spain b Facultad de Ciencias Experimentales y Ticnicas, Universidad San Pablo CEU, 28660 Boadilla del Monte, Spain Received 17 April 1996; accepted 3 June 1996 Abstract The laser photolysis of trisilane at 193 nm was studied. The final photolysis products were monosilane, disilane and tetrasilanes. The results were indicative of the participation in the photolysis process of at least two different dissociation pathways which account for about 80% of the total parent molecule dissociation. ArF laser multiphoton dissociation of SiaH s in a free jet expansion was also studied. Photofragment fluorescence from SiH(A2A ) and several singlet and triplet excited states of the silicon atom was observed. The rovibrational population distributions in SiH(A2A ) were obtained by spectral simulation. The rotational distributions can be characterized by near-Boltzmann distributions. The average rotational energies in v = 0 and v = 1 are 1550 and 1140 cm- t respectively and the average vibrational energy in the observed vibrational levels (v =0-2) is 800 cm- 1. Keywords: Laser photolysis; Photofragment fluorescence; Trisilane 1. Introduction The smallest stable silicon hydrides, silane and disilane, have been the subject of photochemical and spectroscopic studies [ 1-3]. In recent work, the role of stable silicon hydrides as photochemical sources of silicon hydride radicals and other relevant information on their kinetic properties and thermochemistry have been reviewed [4]. A knowledge of the photofragmentation channels of silanes is of importance in order to understand and control the chemical vapour dep- osition of silicon thin films. Moreover, new transient silicon hydride species, some still uncharacterized, are expected to be formed as primary dissociation products [2]. In addition, experimental evidence has indicated that certain selective processes may take place in the photodissociation of silanes. In the vacuum UV photodissociation of monosilane, the only silicon-containing products observed are Si atoms and elec- tronically excited silylidyne [ 1]. In the photolysis of disilane, SiH(A2A) is formed promptly; however, ground state SiH(X21-I) cannot be observed within approximately 100 ns after the photolysis pulse [ 5 ]. For the next stable silicon hydride, Si3Hs, the onset of absorption shifts towards longer wavelengths with respect to * Corresponding author. Tel.: + 34 1 561 94 00; fax: + 34 1 564 24 31. Present address: Departamento de Ffsica Aplicada, Facultad de Cien- cias, Universidad de Mfilaga, Spain. 1010-6030/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved P11S1010-6030(96)04427-9 monosilane and disilane [3]. UV photolysis of Si3H8 has been reported to be a source of Sill2 [4,6], but little infor- mation about other photodissociation processes is available. In this work, we have undertaken a study of the ArF laser photolysis of trisilane. We report our initial results on the single-photon photolysis processes, and the observation of several excited photofragments following laser multiphoton dissociation of trisilane. 2. Experimental details The experimental set-up was similar to that used in previ- ous work [7]. Samples of trisilane kept in a slush bath at - 15 °C were expanded into a vacuum chamber through a pulsed solenoid valve (General Valve Corporation; diameter, 0.5 mm; typically operated at a pulsed voltage of approxi- mately 60 V and approximately 300 p~s). About 10 mm below the nozzle the molecular beam was perpendicularly crossed by the output of an ArF laser. The laser beam was focused approximately 40 cm behind the interaction region. Photo- fragment fluorescence produced in the interaction region was imaged by a lens onto the entrance slit of a 0.5 m mono- chromator and the dispersed fluorescence was viewed by a photomultiplier (Hamamatsu R928). The time-integrated photomultiplier signals were fed to SRS245 gated integrators