Ultraviolet and Visible Light Photodissociation of H3+ in an Ion Storage Ring

Ultraviolet and visible photodissociation of a vibrationally excited H3+ ion beam, as produced by standard ion sources, was successfully implemented in an ion storage ring with the aim of investigating the decay of the excited molecular levels. A collinear beams configuration was used to measure the photodissociation of H3+ into H2+ + H fragments by transitions into the first excited singlet state with 266 and 532 nm laser beams. A clear signal could be observed up to 5 ms of storage, indicating that enough highly excited rovibrational states survive on the millisecond time scale of the experiment. The decay into H2+ + H shows an effective time constant between about 1 and 1.5 ms. The initial photodissociating states are estimated to lie roughly 1 eV below the dissociation limit of 4.4 eV. The expected low population of these levels gives rise to an effective cross section of several 10−20 cm2 for ultraviolet and some 10−21 cm2 for visible light. For using multistep resonant dissociation schemes to monitor rotational populations of cold H3+ in low-density environments, these measurements open promising perspectives.