Millimeter-wave detection of free radicals using pulsed laser photolysis

Abstract

The application of millimeter-wave microwave spectroscopy to the detection of free radicals and other species generated by pulsed excimer laser photolysis is described. To detect the radical species, a newly developed 140-GHz microwave spectrometer employing a high-Q Fabry–Perot cavity was used. The technique is illustrated by the observation of SO and CS radicals produced by photodissociation at 193 nm of SO2 and CS2, respectively. SO absorption signals from the ground vibrational state transition at 138.17 GHz and the first and second excited vibrational state transitions at 137.28 and 136.45 GHz were measured. CS was detected in several excited vibrational states up to ν=4. Finally, as an example of the application of the technique to chemical kinetic studies, the reaction rate of SO with NO2 was determined. The measured rate coefficient at 298 K of (1.46±0.12)×10−11 cm3 molecule−1 s−1 is in excellent agreement with values reported in other studies. Millimeter-wave spectroscopy is shown to be a sensitive and highly specific probe for the detection of short lived species in gas phase chemical kinetic studies.