Strong resonance enhancement of the CN two-photon absorption B 2Σ+←X 2Σ+(3,0) by the A 2Πi, v’=4 level.

We have observed a strong two-photon absorption in the B 2Σ+←X 2Σ+(3,0) band of CN by means of a resonant enhancement through the A 2Πi, v’=4 level. Many lines are seen in the two-photon spectrum due to multiple single-photon near resonances in the A 2Πi←X 2Σ+(4,0) band. The detuning of the laser from these resonances varies from less than one to hundreds of wave numbers, producing unusually large intensity variations in the two-photon spectrum. This effect is not observed in two-photon transitions far from resonance. Resonant enhancement is observed over a range from N=5 to 20. We know of no other molecular two-photon transition in which a near resonance produces such dramatically varying intensities over a short range of rotational levels. A calculation of the line strengths for these transitions reproduces the major features of the spectrum. [ABSTRACT FROM AUTHOR]