Eur. Phys. J. D (2017) 71: 45
https://doi.org/10.1140/epjd/e2017-70610-4

Regular Article

Adsorption energies of H and H₂: a quantum-chemical study^*

¹ Indian Centre for Space Physics, 43, Chalantika, Garia Station Road, 700084 Kolkata, India
² Atomic Molecular and Optical Spectroscopy Division, Physical Research Laboratory, 380009 Ahmedabad, India
³ S. N. Bose National Centre for Basic Sciences, Salt Lake, 700098 Kolkata, India

^a e-mail: ankan.das@gmail.com

Received: 30 September 2016
Received in final form: 4 January 2017
Published online: 28 February 2017

Abstract

The chemical composition of interstellar grain mantle is mostly dependent on adsorption energies of the surface species. Since hydrogen is widespread either in atomic or in molecular form, our aim in this work is to review (by quantum chemical calculations) the variation of the adsorption energies of H and H₂ depending on the nature of the adsorbents. Choice of absorbents was based on relative abundances of interstellar materials. Since carbonaceous and silicate grains are very abundant, we used them as our absorbents. To save computational time, benzene (smallest structure sample of PAHs) is employed as carbonaceous material and for silicate grain, simple cluster of silicon dioxide (silica) (SiO₂)₃ is used. Around dense cloud regions, water is the major constituent of a grain mantle, therefore, usage of binding energies with bare grains is immaterial. To mimic the water as the adsorbents, we use a water-cluster ((H₂O)₆). We found that, for all types of adsorbents considered here, binding energies of H are always lower than those of H₂, whereas, some of the experimental values are just the other way around. Assuming a steady state solution to the rate equation method, we also present the H₂ formation efficiency window in various cases.