State selective doubly enhanced sum-frequency mixing in CO

Doubly enhanced sum-frequency mixing (DESMI) of two collinear laser beams is studied using a pulsed jet of carbon monoxide as the nonlinear medium. The pump laser frequency (ω₁) is in two-photon resonance with a selected rotational component of the A¹Π-X¹Σ⁺ (5,0) transition. Tuning the probe laser frequency (ω₂) to a sequential rotational component of the B¹Σ⁺-A¹Π (1,5) transition leads to strong enhancement of VUV generation at frequency 2ω₁+ω₂. The double enhancement is shown to be subject to firm overall selection rules. It is demonstrated that DESMI can be accurate and efficient for the study of molecular states in the VUV and XUV provided that low laser powers are used. The high selectivity of the four-wave mixing processes investigated allows a better understanding of the competition between four-wave mixing, reabsorption of the sum frequency, and multiphoton absorption. In particular, it is shown that resonance-enhanced multiphoton absorption is an important saturation mechanism at high laser powers.