CARS spectroscopy of transient species

The author uses pulsed-laser coherent anti-Stokes Raman scattering (CARS) spectroscopy with nanosecond temporal resolution, to measure the nascent rotational and vibrational state distributions of molecular fragments produced by laser photodissociation under collisionless conditions, and to determine the quantum state distributions of chemical reaction products under single-collision reaction conditions. CARS spectroscopy is also used to detect electronically excited molecules in microwave discharges. Some results from the recent CARS studies of the UV and visible photodissociation of 03 to produce 02(1delta /sub g/ ) and 02(3sigma- /sub g/ ), the reaction of H atoms (produced by HI photolysis) with D2 to form HD, and the observation of electronically excited 02(1delta /sub g/ ) in microwave-excited electric discharges in 02 are discussed. By coupling pulsed-laser CARS with pulsed-laser photolysis, using commercially available pulsed Nd:YAG and Nd:YAG-pumped dye lasers, it is possible to effect photodissociation of a molecule and spectroscopically interrogate its photofragments on a time scale short compared to the mean time between collisions, even at pressures of 1 to 10 torr