Three-photon resonance-enhanced ionization spectroscopy of CO(A 1Π←X 1Σ+). II. Line shift/profile and the ac Stark effect

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Abstract
The influence of the ac Stark effect on the line shift and line shape is studied in the three‐photon resonance enhanced ionization spectroscopy of COA 1Π(v’)←X 1Σ(v‘=0), where v’=1, 2, or 3. Experimental results show that the line shift varies from one vibrational band to another and differs for various lines in the same band, although it is to a lesser degree. This is given an adequate theoretical elucidation. A detailed calculation, based on the ac stark effect with evaluation of the dipole matrix element, gives −0.66, −0.31, and 0.87 cm−1 for P11 of the (1−0), (2−0), and (3−0) band, respectively, where the negative values stand for red shift while the positive one for blue shift. These results are very close to the experimental observations, viz., −0.48, −0.13, and 0.77 for P11 of the (1−0), (2−0), and (3−0) bands, respectively. Furthermore, the line shape is found to be asymmetrically broadened. For the blue‐shift line, the red side is much steeper than the blue side, whereas for the red‐shift line, the reverse situation appears. This is again explained as due to the ac Stark effect coupled with the spatial inhomogeneity of the laser field. According to this model, a numerical calculation for the line shape is provided which gives fairly good agreement with the experimental observation.