Abstract

A novel Raman-optical double-resonance (ODR) technique is described in detail and applied to studies of molecular glyoxal (C2H2O2). The technique employs a pulsed excitation sequence, consisting of coherent Raman pumping of a molecular rovibrational transition followed by rovibronic probing through visible laser-induced fluorescence (LIF). The experiments demonstrate a 103-fold enhancement of sensitivity, relative to established coherent Raman spectroscopic methods, and enable individual sets of O-, P-, Q-, R-, and S-branch Raman transitions to be distinguished with high specificity under effectively collision-free conditions. For trans-glyoxal, a typical excitation sequence is X˜, v = 0, (J″, K″) → X˜, v2 = 1, (J, K) → Ã, v′ = 0, (J′, K′), where X˜ and à denote the ground (1Ag) and first excited (1Au) electronic singlet states, respectively, and ν2 is the symmetric CO stretching mode of vibration. There is also evidence of contributions from hot bands involving sequences in the low-frequency modes, ν7 and ν12. The Raman-ODR spectra are analyzed to yield new spectroscopic constants for the X˜, v2 = 1 vibronic level of trans-glyoxal. Variation of the time delay between Raman-excitation and LIF-probe pulses has permitted direct observation of collision-induced rotational relaxation in the ground electronic manifold of glyoxal at a rate that is ~6.5 times gas-kinetic.

© 1985 Optical Society of America

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