Volume 123, number 5 CHEMICAL PHYSICS LETTERS 24 January 1986 zyxwvutsrqpon TIME-RESOLVED LIF DETECTION OF SILYLENE IN THE IR MPD OF ETHYLSILANE * D.M. RAYNER, R.P. STEER’, P.A. HACKETI- zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIH Laser Chemrstry Group, Dwwon of Chemutry, Nailonal Research Councrl Canada. 100 Sussex Drive. Ottawa, Ontano, Canada KIA OR6 C.L. WILSON and P. JOHN Department of Chemistry Herrot - W att Unrversrty, Rtccarton, Currre, Edmburgh EH14 MS, UK Received 26 August 1985 Stlylene has been observed m the colhstonless IR MPD of ethylsdane via LIF. A mechanism for the ummolecular dissociation of ethylsdane which is consistent with known shock-induced dissociation and IR MPD data IS advanced. 1. Introduction The widespread use of chemical vapour deposition of amorphous silicon has led to a renewed interest in the chemistry of silanes. Despite this increased activity the primary dissociation pathways of these molecules remains a controversial topic [l-3]. As usual, this is in large part due to the complication introduced by secondary and/or heterogeneous reactions. Infrared multiphoton dissociation (IR MPD) combined with time-resolved product detection provides a powerful method of initiating and following homogeneous gas phase reactions and we have applied these techniques to the dissociation of ethylsllane. Francisco, Joyce and Steinfeld have determined the final products following IR MPD of alkylsilanes and infer that silylene, SiH,, is an important intermediate [4]. The visible absorption of this transient is well known from the work of Dubois [5] and it has recently been detected via LIF following the 193 nm photolysis of phenylsilane [6]. In this Letter we report upon the detection of silylene following IR MPD of ethylsilane and discuss the relevance of these results to the mechanism of alkylsllane dissociation. Issued as NRCC No. 24985. Thus paper was presented at the 17th International Symposium on Free Radicals, Colorado 1985. On sabbatical leave 1984-1985 from Department of Chem- istry, Umversity of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N OWO. 2. Experimental Our pump-and-probe apparatus for time-resolved IR MPD studies is described in detail elsewhere [7]. Briefly, the pump laser was a Lumonics 820 CO, laser operated in TEM,, and single longitudinal modes and employing an optional plasma shutter to remove the tall of the pulse. The probe was a modified Molectron DL16 dye laser pumped by a Lumonics 860 XeCl excimer laser. The beams from the two lasers counterpropagated in a collinear arrangement and were mildly focused at the centre of the reaction cell, the infrared with a 1 m Ge lens to a beam radius of 0.55 mm and the probe with a 600 mm quartz lens. This arrangement ensured that the probe sampled only a region of well defined IR flux, with no spatial averaging. LIF was observed orthogonally through suitable filters (Schott 610 for X(probe) < 573 nm, Schott 630 for)l(probe) > 573 nm) using a cooled Hamamatsu R446 photomultiplier. Scanning of the probe laser wavelength and the delay between the two lasers was controlled by a Tektronix 4052 system which also re- corded the LIF signal, the pulse energy of each laser and the interpulse delay on each firing of the system. Ethylsilane (Petrarch Systems Inc.) was degassed several times and used without further purification. It was flowed through the reaction cell neat or buffered in either helium or nitrogen. 449