Abstract

Recent developments on coherent XUV sources by frequency tripling of tunable dye laser radiation provide new means of preparing atomic and molecular states for studying their spectroscopic proper ties, especially in the region <100 nm. We have studied the photoionization of H₂ in the region of 71 nm by a stepwise two-photon excitation technique using the ${\text{B}}^1{\Sigma }_u^{+}$ and the C¹π_u states as the resonant intermediate. The first photon at ~96 nm is derived from a tunable XUV source recently developed in our laboratory. The second photon is the UV output of a second dye laser. This excitation coupled with the detection of negative H⁻ ions has uncovered several new spectroscopic features that could not be observed in positive ion detection due to an intense continuum in this region. In contrast to previous one-photon studies¹ our spectrum suggests that a H⁻ continuum that couples to the excited H₂ states exists. Intense spectral features in this continuum are assigned as heretofore unobserved Rydberg series converging to the high vibrations of ${\text{H}}_2^{+}\left(v+\ge 9, {\text{N}}^{+}=1,3\right)$ levels. The efficiency of H⁻ production and the effect of the nature of the intermediate state on this efficiency are discussed.

© 1985 Optical Society of America