Photodissociation dynamics in the UV laser photolysis of DNCO: Comparison with HNCO

Using the laser photolysis/laser-induced fluorescence (LP/LIF) pump-probe technique, the gas phase photodissociation dynamics of room temperature DNCO were studied at photolysis wavelengths of 193 and 248 nm. D atoms produced via DNCO ((X) over tilde(1)A') + hv --> D(S-2) + NCO((X) over tilde (2) Pi) were detected by (2p P-2 <--- 1s S-2)-LIF using tunable narrow-band Lyman-alpha laser radiation (lambda(L alpha) approximate to 121.5 nm) generated by resonant third-order sum-difference frequency conversion of pulsed dye laser radiation. By means of a photolytic calibration method absolute cross sections for the direct photochemical D atom formation were measured to be sigma(D)(193 nm) = (5.5 +/- 0.5) x 10(-20) cm(2) molecule(-1) and sigma(D)(248 nm) = (1.2 +/- 0.2) x 10(-21) cm(2) molecule(-1). For the wavelength of 193 nm, the measurement of the optical absorption cross section sigma(DNCO)(193 nm) = (2.6 +/- 0.2) x 10(-19) cm(2) molecule(-1) allowed the direct determination of the D atom product quantum yield of phi(D)(193 nm) = (0.21 +/- 0.03). At the wavelength of 248 nm, where the optical absorption cross section was too small to be measured with reasonable accuracy, a kinetic calibration method was used to measure product quantum yields for D atom, phi(D)(248 nm) = (0.77 +/- 0.10), and for ''spin-forbidden'' ND(X (3) Sigma(-)) formation, phi(ND)(248 nm) = (0.23 +/- 0.10). From the D atom Doppler profiles, measured under collision-free conditions, the fraction of the available energy released as product translational energy was determined to be f(T)(D + NCO) = (0.44 +/- 0.04) for 193 nm and f(T)(D + NCO) = (0.64 +/- 0.17) for 248 nm photolysis wavelengths. In addition, using polarized photodissociation laser light, measurements were carried out in order to determine the anisotropy of the D atom product angular distribution. It was found that the value of the anisotropy parameter beta decreases from -(0.45 +/- 0.09) for a photolysis wavelength of 193 nm to -(0.01 +/- 0.10) at 248 nm, suggesting a moderately anisotropic distribution at the shorter wavelength and an effectively isotropic one at the longer wavelength. Comparisons are made with results from recent photodissociation dynamics studies of the isotopomer HNCO. (C) 1997 American Institute of Physics.