Laser-induced decomposition of dimethyl cadmium on a quartz substrate

The photochemistry of adsorbed dimethyl cadmium at submonolayer levels on a fused quartz surface has been investigated at 193 and 248 nm using a rare-gas fluoride excimer laser. The desorbed gaseous products, which include CH2, CH3, CH4, C2H4, C2H5, C2H6, Cd, CH3Cd and (CH3)2Cd, have been detected by time-of-flight mass spectrometry with either electron impact or resonance-enhanced multiphoton ionization. The translational energies of these desorption products could be characterized in terms of Maxwell-Boltzmann temperatures, TMB‘s, which depend strongly on sample dosage (surface coverage), laser fluence and photon energy. The TMB‘s of the C1 and C2 hydrocarbon species were found to be much lower than those of Cd, CH3Cd and (CH3)2Cd, suggesting that the electronic excitation/relaxation mechanism may be involved in the desorption of Cd and Cd-containing species. The rotational temperature of the CH3 radical, determined by analysis of REMPI spectra, was found to be much colder than its translational temperature. A realistic mechanism is proposed for the UV-photochemistry of adsorbed (CH3)2Cd.