Time-resolved experimental and computational study of two-photon laser-induced fluorescence in a hydrogen plasma

The time profile of the fluorescence light emission of atomic hydrogen in a n expanding plasma beam after pulsed excitation with a nanosecond laser is studied, both experimentally and computationally. Ground state PI atoms in an expanding Ar-H cascaded are plasma are excited to the p=3 level using tw o-photon laser excitation at 205 nm. The resulting fluorescence is resolved in time with a fast photomultiplier tube to investigate the occurrence of quenching. A fluorescence decay time of (10+/-0.5) ns is measured under all circumstances, indicating that there is a complete l mixing of the p=3 sub levels. A time-resolved collisional radiative model is developed to model p ulsed laser induced fluorescence for a large range of plasma parameters. Th e model calculations agree well with the experimental results over the enti re range of conditions and indicate that two-photon LIF can strongly influe nce the local electron and ion densities, resulting in a "self-quenching'' of the laser-induced H fluorescence.