CO2laser pulses of 2, 10, and 50 ns duration, for which the temporal profile was approximately rectangular, were used in the multiphoton dissociation of low pressure CF2HCl and CF2CFCl. Probing a region of wellhyphen;defined CO2laser intensity, laser excited fluorescence determined the yield of CF2formed in thev=0 and in the high vibrationally excitedv2=5 (Evib=3335 cmminus;1) levels as a function of fluence (F) and intensity (I) over a factor of 100 variation. In the dissociation of CF2HCl by pulses of a givenF, increasingIby a factor of 25 (50 vs 2 ns pulse) typically increased CF2(v=0) yield by factors of 8; thisIdependence is probably due to power broadening of the discrete levels. The CF2(v=0) yield from CF2CFCl was almost independent ofIover this range, which may reflect the coincidence of the 1079 cmminus;1R(24) laser frequency with a CF2CFClQbranch head at 1080 cmminus;1. The ratio of CF2(v2=5)/CF2(v=0), which is insensitive to discrete levels effects in the excitation process, increases withIfor both reactants. This ratio may be expressed as a vibrational temperature,Tvfor the CF2fragments, and varied from about 1400 to 2600 K and from 900 to 1400 K for CF2CFCl and CF2HCl reactants, respectively, asIincreased from 55 MW/cm2to 3.3 GW/cm2for the 50 ns laser pulses. Arguments are presented relating these results to the establishment of steadyhyphen;state excitation conditions and to absorption cross sections in the continuum levels.
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