Laser intensity dependence of mulitphoton excitation vs collisional relaxation in CF2HCl and CF2CFCl

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Abstract
CO2 laser pulses for which the intensity vs time profile is rectangular (10 or 50 ns duration) were used in the multiphoton excitation of CF2HCl and CF2CFCl dilute in high pressure (400 Torr) argon. Energy deposition was measured by optoacoustic detection and CF2 product yield by laser‐excited fluorescence. Even at low yield (e.g., 10−3) more than 100 photons were absorbed per CF2CFCl molecule, while for an identical yield, CF2HCl absorbed 200 times less energy. For the same laser fluence, the higher intensity 10 ns pulses gave more yield from CF2CFCl (factors up to 500 were observed) than the less intense 50 ns pulses; for CF2HCl, the two intensities give the same yield. For both molecules, the two intensities gave the same optoacoustic signal for a given fluence. These results are related to the dependence on reactant energy of the competing ratio of collisional deactivation to laser excitation.