Study of the reactions H2+ + He→HeH++H and HeH+ + H→H2++He using crossed beam techniques

Abstract

The proton transfer reactions H2+ + He→HeH++H and HeH+ + H→H2++He have been studied in the laboratory energy range 1–25 eV. The first process, which is endothermic for ground state reactants, has been studied over the indicated energy range with different degrees of vibrational excitation present in the H2+ particles. Below 9 eV, the reaction is found to depend strongly on vibrational energy while above this energy the reaction cross sections appear to depend mainly on the kinetic energy of the reactants. The exothermic reverse reaction exhibits a 1 / ν dependence in the low-energy region indicating an ion-dipole mechanism. Methods of measuring the degree of dissociation obtained in forming the hydrogen atom beam are discussed.

Ion chemistry in the early universe

“…This behavior therefore favors the HeH + as a more likely survival candidate vis-á-vis the LiH + case, thereby providing a good molecular coolant acting during the primordial star formation. In the same figure we also report the experimental rate at 300 K given by Karpas et al (1979); the rates included in the GP98 model are taken from the experimental data of Rutherford & Vroom (1973). As seen earlier for the cross sections, the experimental rates are in good agreement with our quantum results (see also Table 2).…”

“…From the figure, the non-Langevin behavior of the cross-sections for this ionic reaction is clear since, instead of independent of E at lower energies, they go through a marked maximum around 10 −2 eV. In the same figure, we report the experimental (thick line, from Rutherford et al 1973) and the extrapolated (dotted line, from Linder et al 1995) cross-sections that reach collision energy values as low as around 10 −2 eV. The agreement between our theoretical results and the experimental findings, both measured and extrapolated, is reassuringly good and confirms the physical reliability of the present NIP method for handling ionic reactions.…”

“…1 are very different from those for LiH + (Bovino et al 2010a) across the same range of energies. These differences are related to the different microscopic mechanisms of the reaction pathways between the two Rutherford & Vroom (1973). The dotted line is obtained from the formula given by Linder et al (1995) by extrapolating the Rutherford's data.…”

“…The elementary reactions indicated in either direction by process (2) have been studied both experimentally (Rutherford & Vroom 1973;Karpas et al 1979;Tang et al 2005) and theoretically Palmieri et al 2000;Xu et al 2008;Ramachandran et al 2009), because of its peculiar resonant features detected experimentally. Several potential energy surfaces (PES's) have been obtained and employed, using quasi-classical trajectories, close coupling (time-independent and time-dependent) methods to study the inverse of the reaction (2).…”

“…Crossed-beam experimental data for energies higher than 0.2 eV were presented by Rutherford & Vroom (1973), fitted by Linder et al (1995) and then included in the early universe model calculations of Schleicher et al (2008), but no direct calculations have been available thus far. In this paper, we therefore report an accurate analysis, from ab initio data, that deals with the above reaction.…”