Rate constants for the de-excitation of the bending vibrational levels of NCO(X) 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.