RAMAN-ULTRAVIOLET DOUBLE-RESONANCE IN ACETYLENE - ROVIBRATIONAL STATE PREPARATION AND SPECTROSCOPY

We report time-resolved optical double resonance spectroscopic experiments in which gas-phase acetylene molecules are selectively prepared and monitored in discrete rotational states of the v2 = 1 (C = C stretch, 1974 cm-1) vibrational level. This is achieved by pulsed coherent Raman excitation and laser-induced fluorescence detection. State-selective spectra of single rovibrational states are presented under effectively collision-free conditions. Several new rovibronic bands in the A <-- X absorption system of acetylene are identified in this way, owing to the enhanced sensitivity and spectral simplification of our Raman-optical double resonance technique. Investigations of C2H2(g) concentrate on rotationally resolved vibronic bands of the form 2(1)(0)3(0)x(where x = 1,2,3,...), exploring spectroscopic subtleties such as axis switching. The method has also been extended to the 2(1)(0)3(0)x4(1)0 vibronic bands of C2H2(g), by Raman excitation in the (nu-2 + nu-4 - nu-4) hot band, and to studies of the deuterated isotopomers, C2HD(g) and C2D2(g). Two distinct experimental strategies are demonstrated, in terms of their utility for spectroscopic assignment and energy transfer applications. One such approach comprises a rovibronic fluorescence excitation spectrum, recorded with fixed Raman excitation frequency. The alternative approach yields state-selected Raman spectra, with the Raman excitation frequency varied and the rovibronic excitation wavelength fixed.