PULSED-LASER PHOTOLYSIS OF CHROMIUM AND MOLYBDENUM HEXACARBONYLS AT 308 NM - SIMULTANEOUS MONITORING OF OPTICAL AND ACOUSTIC EMISSIONS

An acoustic technique developed earlier was applied to measure the mol ar number of fragments produced in the XeCl-laser-induced gas-phase ph otolysis of Cr(CO)(6) and Mo(CO)(6). The bluish-green Cr and Mo emissi ons were also monitored and correlated with the acoustic signal. For l aser fluence ranging up to about 25 mJ/cm(2) (38 mJ/cm(2)), the Cr (Mo ) optical signal varied with laser fluence as y = dx(3), in agreement with the model that highly excited metal atoms were produced by direct three-photon processes. The acoustic signal varied with fluence as y = ax + bx(2), consistent with the model that most fragments were forme d via sequential and direct two-photon processes. Quite expectedly, al l coefficients a, b and d vary linearly with carbonyl partial pressure . Both optical and acoustic signals showed onset of saturation at appr oximate to 25 mJ/cm(2) (38 mJ/cm(2)) for the case of chromium (molybde num) hexacarbonyl, suggesting that all photolysis channels were equall y affected by the depletion of parent molecules. By assuming a common depletion mechanism, upper limits on the branching ratios of the vario us photolysis channels were estimated.