Two-Color Polarization Spectroscopy Technique for Probing Collision-Induced Resonances in Nitric Oxide

摘要

We report here the development of a two-color laser-induced polarization spectroscopy (TCPS) technique for probing collisional-induced resonances in nitric oxide (NO). Independent pump and probe lasers with slightly different frequencies were used for pumping and probing two different transitions. Injection-seeded optical parametric generators (OPG) with pulsed dye amplifiers (PDA) were used as sources of single frequency mode pump and probe laser beams which impart the ability to perform high-resolution spectroscopy of NO. It was expected that strong TCPS signals will be generated when the pump and probe transitions share a common upper or lower energy level. However, our initial measurements for NO in a buffer of N_2 showed significant signals from numerous other transitions that did now share a common energy levels with the pump transition. Rotational transfer from close to the lower and upper levels that are directly coupled by the pump laser occurs on time scales short compared to the pump and probe laser pulse lengths. When the rotational transfer preserves the Zeeman state anisotropy induced by the pump laser, TCPS signals from resonances which do not share a common level with the pump transition will result. TCPS spectra are recorded by setting the pump laser frequency in resonance with the R_(11)( 11.5) transitions, and then tuning the pump laser in the frequency range that includes the P_(11)(11.5) and P_(11)(13.5) transitions that share common lower and upper levels, respectively, with the R_(11)(11.5) pump transitions. TCPS spectra are recorded for mixture so 1% NO in buffer gases of He, N_2, and Ar. Significant collision-induced resonances are observed for the He and N_2 buffer gases, but are essentially absent for the Ar buffer gas.