High-resolution spectroscopy of chemical intermediates

Abstract

Infrared and UV-vis laser spectroscopy has been used to study a number of small free radicals, with an emphasis on those exhibiting large amplitude vibrational motion or a strong breakdown of the Born-Oppenheimer approximation. Recently, we have detected gas phase infrared absorption spectra of the ethyl (C2H5) and carboxyl (HOCO) radicals for the first time. The C2H5 spectrum shows clear evidence for nearly free rotation of the CH2 radical center relative to the CH3 end of the molecule, while the HOCO spectrum appears to be somewhat simpler although analysis is far from complete. Following spectroscopic characterization of these species, infrared absorption will offer a precise and sensitive method for monitoring the radical concentrations on a state-by-state basis. Other species including NCS, C3, and NCO have been studied by laser induced fluorescence in a supersonic free jet expansion. Highly excited vibrational states have been accessed by optical double resonance (stimulated emission pumping) and the experimental data demand extensions of the standard effective Hamiltonian treatments describing the energy levels of these and similar species as increasing amounts of energy are placed in the internal modes.