Degenerate and two-color resonant four-wave mixing applied to the rotational characterization of high-lying vibrational states of formaldehyde (Ã, 1A2)

Degenerate and two‐color resonant four‐wave mixing techniques (DFWM and TC‐RFWM) are applied to determine rotational constants of high‐lying vibrational levels in the first excited singlet state Ã(1A2) of formaldehyde. It has been demonstrated that the sensitivity of the spectroscopic technique is applicable to the low‐density environment of a supersonic molecular beam and to predissociating transitions displaying low fluorescence quantum yield. In addition, we take advantage of the superior selectivity of the double‐resonance method, TC‐RFWM, to isolate and assign transitions in the congested region of the (one‐color) DFWM spectra. The line positions of 25 well‐isolated transitions are determined in the $2_{0}^{2}5_{0}^{1}$ band and yield the rotational constants A, B, C and the origin νe. The accuracy of the constants is determined by performing the same procedure for the $2_{0}^{2}4_{0}^{1}$ band where literature data is available for comparison. Copyright © 2006 John Wiley & Sons, Ltd.