OVERTONE SPECTROSCOPY BY VIBRATIONALLY ASSISTED DISSOCIATION AND PHOTOFRAGMENT IONIZATION

A new method for obtaining overtone spectra in the gas phase is presented: IR and UV laser light is focused into a flow cell. The IR excites nu = 3 or nu = 4 CH-stretch overtones in the molecules studied, CHCl3, CH3Cl, CF2HCl and (CH3)3CCl. Subsequent excitation by UV photon absorption dissociates the molecule. The photofragments are probed by multiphoton ionization using the same frequency as the UV dissociation laser. By scanning the IR laser while monitoring the ionization yield, overtone spectra have been obtained. The new technique requires sample pressures below 1 mbar, and the absorption path is the focus region, only. The method is thus suitable for overtone spectroscopy in a supersonic jet expansion, or to study molecules with low vapour pressures.