摘要
本文报道了三光子共振条件下CO的A^1Π←X^1Σ^+跃迁的多光子电离光谱,并研究了谱线强度的反常压力效应以及谱线的位移和不对称加宽。通过园偏振激光实验,证实了此光谱来源于CO分子吸收其自身介质中的激光诱导的三次谐波;并利用相位匹配条件在理论上解释了实验中所观察到的谱线强度的反常压力效应。与此同时,研究了AC Stark效应引起的谱线位移和不对称加宽,并首次对此效应进行了定量计算,其线果和实验观测值吻合得很好。
Three-photon resonance-enhanced multiphoton ionization (REMPI) spectra of CO A^1Π(v')←X^1Σ^+(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.
基金
国家自然科学基金
