Pulsed laser photodissociation of chromium hexacarbonyl at 308 nm: simultaneous monitoring of optical and acoustic emissions

A novel, yet simple, acoustic technique was applied to measure the molar number of fragments formed in the XeCl laser-induced gas-phase photodissociation of Cr(CO)₆. The bluish-green Cr emission was also monitored and correlated with the acoustic signal. At 0.16 Torr of hexacarbonyl and for fluences below ≈ 25 mJ/cm², the optical signal varied as the cube of the laser fluence, in agreement with the model that highly excited Cr atoms were produced by three-photon processes. The acoustic signal y (mV) varied with fluence F (mJ/cm²) as y = 1.77F + 0.15F², consistent with the model that most fragments were formed via two sequential single-photon processes. Both optical and acoustic signals showed onset of saturation at ≈ 25 mJ/cm², suggesting that single-photon photolysis was the primary Cr(CO)₆ depletion process. Quite expectedly, with a constant fluence of ≈ 20 mJ/cm² and varying hexacarbonyl pressure, the optical and acoustic signals were linearly correlated.