Four-wave mixing spectroscopy of NO E 2Σ+ state

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Abstract
The E 2Σ+ state of NO (nitric oxide) has been probed by four‐wave mixing spectroscopy via the A 2Σ+ state. In the present scheme, the pump laser frequency ω1 was fixed on the A 2Σ+ (v’=0 and 1)←X 2Π3/2(v‘=0) two‐photon transition, while the probe laser frequency ω2 was remained variable to monitor the E 2Σ+←A 2Σ+ electronic system. The intensity of the resultant coherent VUV radiation (ωVUV=2ω1+ω2) was strongly wavelength dependent. The analysis of the spectrum revealed the following two respects: (1) The intensity of VUV radiation was enhanced by the resonance of ω2 to rotational levels of the E 2Σ+ state as well as of 2ω1 to the A 2Σ+ state. (2) The spectral structure corresponding to the E 2Σ+←A 2Σ+ system was governed by different rotational selection rules from ordinary single‐photon transitions. These aspects were discussed in terms of the third order nonlinear process in isotropic media and of the two‐photon line intensities for the A 2Σ+←X 2Π system.