Abstract:

The three-photon resonant ionization spectroscopy of H2 B1∑u+-X1∑g+ (12,0) and C1Ⅱu-X1∑g+ (2,0) exhibits abnormal pressure effects. Some of them are observed for the first time. Pressure-induced abnormal line shift, line broadening, line splitting and spectral intensity variations can be well explained by a mechanism of reabsorption of the third harmonic generation in the medium. Approximate expressions considering the phase matching requirement yield lineshifts, -splitting and -intensities at different pressures in good agreement with the experimental results. Two similar spectra are detected by two tandem electrode pairs in a gas celll which give experimental evidence for THG reabsorption mechanism. The failure to obtain ion signal from circularly polarized light also supports the above mechanism.

Key words: Molecular hydrogen, Abnormal pressure effects, 3-Photon REMPI, Third harmonic generation