XUV resonant multiphoton ionization of H2

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 ( B 1 Σ u + ) 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 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.