Abstract

We have applied XUV-UV two-photon resonant multiphoton ionization (REMPI) to the detection of H₂, an important homonuclear diatomic that is infrared inactive and has its electronic bands in the VUV and XUV spectral regions. The Lyman $\left({\text{B}}^1{\Sigma }_u^{+}\right)$ band and the Werner (C¹Π_u) band are used as the resonant intermediate state in this study. The Werner band is found to be superior because ionization produces predominantly ${\text{H}}_2^{+}$ whereas the Lyman band also produces H⁺ ions with variable intensities. By using the v' = 0 state of the Werner band as the intermediate, we achieved a sensitivity of 1.1 × 10⁶ molecules/cc/quantum state. This is 2 orders of magnitude better than any techniques previously reported for H₂. The ion signal is shown to be proportional to the ground state absorption when ionization from the excited state is saturated. This saturation reduces the MPI process to a linear process and simplifies calibration in the application of this technique to quantum-state-specific detection of H₂. This technique will also be useful as a sensitive detector of other homonuclear diatomics such as F₂, Cl₂, and N₂, which are of great general interest in chemistry, combustion, and semiconductor fabrication.

© 1985 Optical Society of America