Application of Laser Dispersion Method in Apparatus Combining H Atom Rydberg Tagging Time-of-Flight Technique with Vacuum Ultraviolet Free Electron Laser

Photodissociation of H_2S in the VUV region plays an important role in the atmospheric chemistry and interstellar chemistry. To date, however, few studies have been focused on this topic. In this article, we have described a laser dispersion method applied in the apparatus combining the high-n H atom Rydberg tagging time-of-flight technique with the vacuum ultraviolet free electron laser (VUV FEL). The Lyman-\alpha laser beam (121.6 nm) used in the H-atom detection was generated by the difference frequency four-wave mixing schemes in a Kr/Ar gas cell. After passing through an off-axis biconvex LiF lens, the 121.6 nm beam was dispersed from the 212.6 nm and 845 nm beams due to the different deflection angles experienced by these laser beams at the surfaces of the biconvex lens. This method can eliminate the background signal from the 212.6 nm photolysis. Combined with the VUV FEL, photodissociation of H_2S at 122.95 nm was studied successfully. The TOF spectrum was measured and the derived total kinetic energy release spectrum was displayed. The results suggest that the experimental setup is a powerful tool for investigating photodissociation dynamics of molecules in the VUV region which involves the H-atom elimination processes.