THE IMPORTANCE OF THE PLUME EJECTION TIME FOR THE FRAGMENTATION YIELDS OBSERVED IN THE UV ABLATION OF MOLECULAR VAN-DER-WAALS FILMS - ABLATION OF CHLOROBENZENE FILMS AT 248 NM

The efficiency of photolysis in the ablation of thick C6H5Cl films at 248 nm is probed over 40-300 mJ/cm(2) by means of time-of-flight quadr upole mass spectrometry. Up to approximate to 150 mJ/cm(2), the photol ysis yield is considerably smaller than that in the gas-phase. We argu e that because of plume ejection well after the laser pulse, photolysi s occurs exclusively in the film, and as a result, permanent dissociat ion is limited by the operation of efficient recombination processes c haracteristic of condensed phases. Fragmentation starts being importan t only at higher fluences, at which desorption during the laser pulse is indicated to be significant.