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.
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