Raman ultraviolet optical double resonance spectroscopy in gas phase acetylene

The Raman UV double-resonance (Raman UVDR) approach has been implemented with a high SNR in acetylene gas [C₂H₂(g)]. Coherent Raman excitation (by pulsed two-color laser irradiation at 532 and approximately 594 nm) allows selective preparation of C₂H₂ in discrete rotational states of the ν = 1 vibrational level. This excitation is probed by tunable pulsed UV radiation (approximately 240 nm) and detected by broadband UV LIF (300-400 nm). The molecular excitation scheme and the spectra are shown and discussed. The spectra provide kinetic data that elucidate state-to-state rovibrational energy transfer processes involving C₂H₂. In another Raman UVDR spectroscopic approach, state-specific Raman spectra are recorded in which the UV laser probes a single rovibrational state selectively and the Raman excitation frequency is scanned. The observed Raman linewidth of 0.06 cm^-1 is limited by the optical bandwidth of the lasers used.