Rate constants for the chemical and physical quenching of Cd(53PJ) by H2O and D2O Original Research Article

The deactivation processes of thermally equilibrated Cd(53P1) and Cd(53P0) by H2O and D2O were examined by employing a laser excitation-depletion technique. The concentration of the ground-state Cd(51S0) atoms was measured in both the presence and absence of an intense saturable pump laser pulse. In accordance with chemical reactions of excited triplet cadmium, the concentration of Cd(51S0) decreases. From this decrease, the ratio of the chemical quenching rate to the physical one can be determined. The branching ratio for the chemical quenching channel was determined to be 0.91 ± 0.07 for H2O, while that for D2O was 0.71 ± 0.06. The most probable product for the chemical channel is CdOH(CdOD) + H(D). Combined with the decay rate measurements, the rate constant for the chemical quenching by H2O was determined to be 7 times larger than that by D2O at 469 K. This large isotope effect should mainly be attributed to the difference in the zero-point energies.