Fluorescence-detected infrared– and Raman–ultraviolet double resonance in acetylene gas: studies of spectroscopy and rotational energy transfer

Abstract

Fluorescence-detected Raman–ultraviolet and infrared–ultraviolet double resonance (DR) spectroscopy enables state-selective studies of rotational and vibrational energy transfer in gas-phase acetylene (C2H2) and its deuterated isotopomers (C2HD, C2D2). The Raman–UV DR approach entails pulsed coherent Raman excitation in the ν2 rovibrational band of C2H2(g), followed by fluorescence-detected rovibronic probing of the resulting rovibrational population distributions. Corresponding IR–UV DR experiments employ a line-tunable, pulsed CO2 laser to excite rovibrational transitions in the 2ν4 band of C2HD(g) and in the (ν4 + ν5) band of C2D2(g), with similar fluorescence-detected rovibronic probing. These time-resolved DR spectroscopic techniques provide rotationally specific information on collision-induced molecular energy transfer in acetylene. This paper extends previous Raman–UV DR spectroscopic studies of C2H2 and presents fresh IR–UV DR spectra of gas-phase C2HD and C2D2, including evidence of a novel two-step excitation sequence in which a single CO2-laser pulse promotes C2D2 by successive transitions in the (ν4 + v5) and (2ν4 + 2ν5−ν4−v5) absorption bands. Kinetic measurements and mechanistic observations are also reported for collision-induced rotational energy transfer in acetylene gas, complementing other investigations of rotationally resolved vibrational energy transfer.