Abstract: Three-photon resonance-enhanced multiphoton ionization （REMPI） spectra of CO A¹Π（v’）←X¹Σ⁺（v"=0）, where v’= 1,2 or 3, have been obtained by using tightly-focused laser beams under a pressure range of 0.1-13 torr. The spectral intensity manifests adverse pressure effect, i.e., the signal decreases with the increase in pressure above 1.0 torr, and almost disappears at 13 tort. In addition, the appearance of the spectra is closely related to the confocal parameter of laser beam.
In general, under linearly-polarized beam excitation, three-photon resonant transition composes of two path ways, i.e., direct three-photo excitation driven by the fundamental （laser） field and one-photon excitation driven by the third harmonic field. In order to investigate the detailed mechanism of three-photon resonant transition, a circularly-polarized laser beam is employed to suppress the THG and study the allowed direct three-photon excitation independently. Our experimental findings show that the direct three-photon excitation for CO is undetectable and the ionization signal given by linearly-polarized beam excitation is actually produced by THG reabsorption. Hence, unlike the assertions in Xe given by previous investigation that the anomalous phenomena in three-photon excitation are caused by destructive interference, we show that phase-matching, which greatly increases the THG, plays a major role in the three-photon resonance-enhanced ionization spectra, and accounts for both the appearance of the spectra and the adverse pressure effect of the spectral intensity. An analytical treatment is provided to show that the adverse pressure effect is caused by the shift of phase-matching frequency away from resonance as the increase of pressure.
Another interesting observation in three photon REMPI spectra of CO is the noticeable line shifts accompanied asymmetrically broadened line profiles. This has been proved to be caused by ac Stark effect. From the experiment, it is found that the line shift varies for different vibrational band. This is elucidated theoretically with dressed-state theory. A detailed calculation of ac Stark shifts based on the evaluation of dipole matrix element, gives -0.66cm^-1, -0.31cm^-1and 0.87cm^-1for P₁₁of the （1-0）, （2-0） and （3-0） band, respectively, where the negative value stands for red shift while the positive one for blue shift. These results are very close to the experimental observations, viz., -0.4Bern^-1, -0.13cm^-1and 0.77cm^-1for P₁₁of （1-0）, （2-0） and （3-0） band, respectively, Furthermore, the asymmetrically broadened line profile is observed. 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 ac Stark effect coupled with the spatial inhomogeneity of the laser field. According to this model, a numerical calcuation for the line shape is provided which gives fairly good agreement with the experimental observation.