Ion-molecule reactions in He-4 droplets: Flying nano-cryo-reactors

Ion-molecule reactions are studied inside large (approximate to10(4) atoms) very cold (0.37 K) superfluid He-4 droplets by mass spectrometric detection of the product ions. He+ ions initially formed inside the droplets by electron impact ionization undergo charge transfer with either embedded D-2, N-2, or CH4. For D-2 this charge transfer process was studied in detail by varying the pickup pressure. For either N-2 or CH4 the reagent ions were formed by this charge transfer and the reaction pathways of the secondary reactions N-2(+)+D-2, CH4++D-2, and CH3++D-2 each with an additionally embedded D-2 molecule were also determined from the pickup pressure dependencies. In several cases, notably He.N-2(+) and CH3D2+ reaction intermediates are observed. The analysis is facilitated by the tendency for molecular ion products to appear without (or with only very few) attached He atoms whereas the atomic ion products usually appear in the mass spectra with several attached He atoms, e.g., He-m.D+ ions with up to m=19. (C) 2005 American Institute of Physics.