Resonant multiphoton ionisation-dissociation of H2

Abstract

The authors have studied the five-photon ionisation of H₂ via the four-photon resonance on the E, F¹ Sigma _g⁺, v_E=2 from X¹ Sigma _g⁺, v=0 transition. A dye laser delivering up to 5*10¹¹ W cm^-2 around 3860 AA is focused into an H₂ cell at room temperature and at a pressure of 5*10^-4 Torr. The time-of-flight mass spectrum exhibits comparable signals of H⁺ and H₂⁺ thus demonstrating the existence of an important dissociative mechanism superimposed on the ionisation channel. Different processes are considered to tentatively explain this unexpectedly high probability of the multiphoton dissociation of H₂.