5875580 nm. Because the 9-10-nm bandpass of the 580-nm interference filter is not negligible in comparison with the width of the 580-nm absorption band, this is an "effective" value of eh,-. By use of a value of 5 X lo4 M-' cm-1 for C~. -,~O a value of 0.18 is obtained for Gk.-. This can be compared with G(free ions) in cyclohexane ~0 . 1 5 . 2~ A value of Gh.greater than G(free ions) is expected because on a time scale of a few hundred nanoseconds the contribution of C02'-(geminate) + Pe -Pee-(geminate) + C02 should be significant. Combmed with the uncertainty in h-, this preclude using the value of GPc.derived here as an accurate measure of G(free ions). COrtC~USiOM

Absolute concentrations of H atoms and OH radicals were measured to determine the product branching ratios for the reactions SiH3 + 0 2 -SiH20 + O H (1 b) and SiH3 + 0 2 -SiH202 + H (IC) by using laser-induced fluorescence (LIF) techniques. The intensity of the VUV-LIF signal of H atoms at Lyman CY wavelength was calibrated against the absolute concentration determined by the simultaneous measurement of the Lyman CY resonant absorption. The calibration for the UV-LIF signal intensity of O H radicals has been performed by the simultaneous measurements of the VUV-LIF of H atoms and UV-LIF of O H produced in the O(1D) + H2 -O H + H reaction system: since the reaction of O(lD) + H2 produces equal amounts of H and OH, the absolute concentration of O H could be known from the H-atom concentration derived from the VUV-LIF intensity of H atoms. It was found that vibrationally excited OH in v"=1 was also produced by reaction l b with a ratio of [OH(v"=l)]/[OH(v"=O)] = 0.49 f 0.08. The yields of H and OH in the SiH3 + 0 2 reaction were derived from the measured absolute concentrations of H and OH combined with the absolute concentrations of SiH3 determined by the time-resolved mass spectrometry. The branching ratios of 0.25 and 0.65 for reactions l b and IC, respectively, were obtained on the basis of kinetic simulations for the yields of H and OH.

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