Multiphoton excitation spectroscopy of the B 1Σ+ and C 1Σ+ Rydberg states of CO

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Abstract
Two‐photon absorption spectra of the CO 3s B 1∑+←←X 1∑+ (v’,v‘)=(0,0), (1,1), (2,2), (1,0), and 3pσ C 1∑+←←X 1∑+ (0,0) transitions have been observed in a room temperature cell and in an atmospheric pressure hydrocarbon flame using fluorescence (B 1∑+→A 1Π) and ionization detection. The ratio of the transition strengths for the B 1∑+←←X 1∑+ (0,0) Q‐branch head is found to exceed 200:1 for excitation with two identical linearly polarized photons compared with two circularly polarized photons. Weak O and S branches are observed for the first time for the B 1∑+←←X 1∑+ (0,0) transition and show the expected 2:3 polarization ratio. These measured polarization ratios are well described by two‐photon absorption theory for ∑←←∑ transitions. In contrast, an earlier study by another group obtained a value of 0.7:1 for the Q‐branch polarization ratio which did not fit the predictions of two‐photon absorption theory.