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

Ion-molecule reactions are studied inside large (≅104 atoms) very cold (0.37 K) superfluid 4He 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 D2, N2, or CH4. For D2 this charge transfer process was studied in detail by varying the pickup pressure. For either N2 or CH4 the reagent ions were formed by this charge transfer and the reaction pathways of the secondary reactions N2++D2, CH4++D2, and CH3++D2 each with an additionally embedded D2 molecule were also determined from the pickup pressure dependencies. In several cases, notably He·N2+ 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., Hem·D+ ions with up to m=19