Jet-cooled radical spectroscopy has been developed for its potential application to high resolution infrared spectroscopic studies of large radicals. In general, radicals containing more than three atoms heavier than hydrogen can not be studied in a room temperature cell using high resolution techniques. For such large species, the infrared spectrum becomes congested and unresolvably complex because of the presence of overlapping rotational lines and vibrational hot bands. By cooling radicals in a supersonic expansion, excited rotational and vibrational levels are depopulated, giving simplified spectrum.
In this technique, radicals are produced inside a slit supersonic nozzle by 193 or 248 nm excimer laser photolysis of a gas mixture consisting of 1% suitable precursor seeded into 1-11 atm carrier gas (typically helium). To reduce the vibrational temperature of the hot radicals produced upon photolysis, the radicals are thermalized by collisions with the room temperature helium inside the slit thermalization region before expansion. The radicals are then cooled rotationally in the subsequent expansion, and their transient absorption is probed downstream of the slit orifice by a tunable, computer-controlled color center laser.
The jet-cooled infrared spectroscopy technique was first tested on small radicals with known high resolution spectra. Small radicals such as NH$\sb2$, OH, and CH$\sb3$ have been observed in the jet with excellent sensitivity and low rotational temperatures. Rotational temperatures ranging from 13-25K and signal-to-noise ratios of 30-150 have been obtained for these radicals. Additionally, rotational temperatures of 10K have been observed for trans-nitrous acid, a stable species produced upon photolysis of nitric acid in the jet.
Jet-cooled infrared spectroscopic studies of larger radicals were initiated since the technique proved successful in the production, cooling, and detection of small radicals. Spectroscopic searches were made for CH$\sb2$OH, t-HOCO, CH$\sb3$NH, C$\sb2$H$\sb3$, C$\sb3$H$\sb5$, and OH-Ar (radical van der Waals complex). Although no new radical species were conclusively observed in the jet photolysis experiments, the initial results of searches for these radicals, including the observation of several stable molecules produced upon photolysis of various precursors, have been described.