Ion–molecule reactions of ammonia clusters with C₆₀aggregates embedded in helium droplets

Abstract

Helium nanodroplets are co-doped with C(60) and ammonia. Mass spectra obtained by electron ionization reveal cations containing ammonia clusters complexed with up to four C(60) units. The high mass resolution of Δm/m≈ 1/6000 makes it possible to separate the contributions of protonated, unprotonated and dehydrogenated ammonia. C(60) aggregates suppress the proton-transfer reaction which usually favors the appearance of protonated ammonia cluster ions. Unprotonated C(x)(NH(3))(n)(+) ions (x = 60, 120, 180) exceed the abundance of the corresponding protonated ions if n < 5; for larger values of n the abundances of C(60)(NH(3))(n)(+) and C(60)(NH)(n-1)NH(4)(+) become about equal. Dehydrogenated C(60)NH(2)(+) ions are relatively abundant; their formation is attributed to a transient doubly charged C(60)-ammonia complex which forms either by an Auger process or by Penning ionization following charge transfer between the primary He(+) ion and C(60). The abundance of C(x)NH(3)(+) and C(x)NH(4)(+) ions (x = 120 or 180) is one to two orders of magnitude weaker than the abundance of ions containing one or two additional ammonia molecules. However, a model involving evaporation of NH(3) or NH(4) from the presumably weakly bound C(x)NH(3)(+) and C(x)NH(4)(+) ions is at odds with the lack of enhancement in the abundance of C(120)(+) and C(180)(+). Mass spectra of C(60) dimers complexed with water complement a previous study of C(60)(H(2)O)(n)(+) recorded at much lower mass resolution.

Cationic Complexes of Hydrogen with Helium

“…Excess energy released during cluster formation and subsequent ionization of the dopant was quickly removed from the nascent ion by the evaporation of helium. Mass spectra demonstrated the solvation of Ne + , Ar + , Kr + and Xe + ,29, 30 and many other atomic, molecular, and cluster ions31–33 in helium. In the study by Toennies and co-workers,26 the nanodroplets were doped with deuterium and subsequently ionized by electron impact.…”

Photoionisaton of pure and doped helium nanodroplets

“…In terms of ion-molecule reactivity, co-doping of C 60 and water or ammonia revealed for the latter a suppression of proton transfer and the appearance of protonated ammonia ions by the C 60 aggregates [26,116].…”

Ion formation upon electron collisions with valine embedded in helium nanodroplets

“…The ions were detected by a micro-channel plate and recorded via a time to digital converter. Additional experimental details have been described elsewhere [31,32]. Special homebuilt software was utilized to deduce ion intensities from the mass spectra taking into account all possible isotopologues [33].…”