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 molar number of fragments produced in the XeCl-laser-induced gas-phase photolysis of Cr(CO)₆ and Mo(CO)₆. The bluish-green Cr and Mo emissions were also monitored and correlated with the acoustic signal. For laser fluence ranging up to about 25 mJ/cm² (38 mJ/cm²), the Cr (Mo) optical signal varied with laser fluence as y =dx³, 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 + bx², consistent with the model that most fragments were formed via sequential and direct two-photon processes. Quite expectedly, all coefficients a, b and d vary linearly with carbonyl partial pressure. Both optical and acoustic signals showed onset of saturation at ≈ 25 mJ/cm² (38 mJ/cm²) for the case of chromium (molybdenum) hexacarbonyl, suggesting that all photolysis channels were equally affected by the depletion of parent molecules. By assuming a common depletion mechanism, upper limits on the branching ratios of the various photolysis channels were estimated.