A New Technique for Gas-phase Kinetic Spectroscopy of Molecules in the Triplet State

A recently developed experimental method is described to populate by Hg photosensitization and observe by absorption spectroscopy the triplet states of polyatomic molecules in the gas phase; detailed discussion and analysis of the experiment are given. The technique can be used to investigate triplet states that cannot be populated efficiently by the intersystem crossing process and to detect triplet absorption bands located within the wavelength range of ground-state absorption of a molecule. A modulation scheme, based on a novel type of Hg resonance light source—an rf-driven toroid capable of modulation frequencies up to 250 kHz—is used for monitoring the optical absorption (by phase-sensitive detection) and measuring the lifetime of the transient species. Sensitivity is comparable with that of flash spectroscopy for transients that decay by a second-order process. Results obtained for several aromatic hydrocarbons are reviewed. New transitions were detected by this method for triplet-state naphthalene, and the absorption spectra and decay kinetics of triplet-state benzene and toluene were measured for the first time. Some applications of the technique to problems other than triplet-state spectroscopy are illustrated or outlined.