Laser intensity effects in the IR multiphoton dissociation of CF2HCl and CF2CFCl

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Abstract
CO2 laser 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 well‐defined CO2 laser intensity, laser excited fluorescence determined the yield of CF2 formed in the v=0 and in the high vibrationally excited v2=5 (Evib=3335 cm−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 given F, increasing I by a factor of 25 (50 vs 2 ns pulse) typically increased CF2(v=0) yield by factors of 8; this I dependence is probably due to power broadening of the discrete levels. The CF2(v=0) yield from CF2CFCl was almost independent of I over this range, which may reflect the coincidence of the 1079 cm−1 R(24) laser frequency with a CF2CFCl Q branch head at 1080 cm−1. The ratio of CF2(v2=5)/CF2(v=0), which is insensitive to discrete levels effects in the excitation process, increases with I for both reactants. This ratio may be expressed as a vibrational temperature, Tv for the CF2 fragments, and varied from about 1400 to 2600 K and from 900 to 1400 K for CF2CFCl and CF2HCl reactants, respectively, as I increased from 55 MW/cm2 to 3.3 GW/cm2 for the 50 ns laser pulses. Arguments are presented relating these results to the establishment of steady‐state excitation conditions and to absorption cross sections in the continuum levels.