Nonlinear laser spectroscopy in nitric oxide studied through VUV harmonic generation

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Abstract
Two and three‐photon spectra have been obtained by single and double resonance enhancements of the elements of the third‐order nonlinear susceptibility (χ(3)), which describe four‐wave sum mixing into the vacuum ultraviolet (VUV). Results are reported for nitric oxide, in which the A 2Σ+−X 2Π transition is utilized for two‐photon resonant enhancement in χ3ω(3). Relative intensities of the recorded rotational structure agreed well with the predictions of theory based on multiphoton transition probabilities, with the exception of cases where there was accidental double resonant enhancement of χ(3), occurring through both two and three‐photon transition probabilities. Finally, in studies designed to explore double resonance effects in χ(3)(2ω1+ω2), two lasers were used to probe simultaneous contributions from the A 2Σ+ and M 2Σ+ states of NO. Signals were further enhanced by a factor of 103 to values as large as 10−4 of the input dye laser intensity.