Collision-independent detection of molecular two-photon excitation by time-resolved parametric four-wave mixing
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Copyright 2015 John Wiley & Sons, Ltd. We report collision-independent detection of gas-phase nitric oxide (NO) via a two-pulse, femtosecond (fs) time-resolved parametric four-wave-mixing (PFWM) optical-probe scheme. This approach exploits the broadband two-photon excitation of rovibrational manifolds associated with resonant electronic molecular transitions, allowing species-specific detection. The observed time-domain fs-PFWM spectral signature associated with this broadband electronic two-photon excitation process is shown to exhibit negligible dependence on colliding-partner concentrations at short (<10 ps) time delays, even in the presence of species that strongly quench excited-state fluorescence. Furthermore, by employing a hybrid fs/ps PFWM scheme, the broadband two-photon absorption spectrum of the molecular species of interest is demonstrated to be resolved in the wavenumber domain without need to scan the probe-pulse delay. Copyright 2015 John Wiley & Sons, Ltd.