RATE CONSTANTS FOR THE DEEXCITATION OF THE BENDING VIBRATIONAL LEVELS OF NCO(X)OVERTILDE BY HELIUM, NEON, ARGON, KRYPTON AND XENON

Rate constants at room temperature (ca. 295 K) are reported for the de-excitation of the (0, 1, 0) and (0, 2, 0) bending vibrational levels of the NCO radical in its ground electronic state. Radicals were produced by infrared multiple-photon dissociation of phenyl isocyanate and their relaxation as a function of time after the photolysis pulse was followed by laser-induced fluorescence. Rate constants were found to be independent of the vibronic components of each level. For the (0, 1, 0) level, values, in units of 10-13 cm3 molecule-1 s-1, were found to be 5.40 ± 0.7, 1.2 ± 0.13, 4.7 ± 0.9, 9.8 ± 1.4 and 21.7 ± 6.1 for the collision partners He, Ne, Ar, Kr and Xe, respectively. For the (0, 2, 0) level, de-excitation rate constants were approximately a factor of 2 higher than these, showing the same trend with mass of rare gas. The ordering of the rate constants, Ne < He ≈ Ar < Kr < Xe, is not that expected from either Schwartz-Slawsky-Herzfeld (SSH) or vibrational close coupling infinite order sudden approximation (VCC-IOSA) theories, and a non-adiabatic mechanism is proposed to explain the results