The formation mechanism of molecular hydrogen on icy mantles of interstellar dust

Abstract

The fundamental processes of H2 formation via H + H → H2 on the surfaces of icy mantles of interstellar dust have been investigated consistently within a single model based on a classical molecular dynamics (MD) computational simulation. As a model surface for icy mantles of dust grains, an amorphous water ice slab was generated at 10 and 70 K under periodic boundary conditions. The first and second incident H atoms were then ‘thrown’ on to the model surface. Two MD procedures were employed: (i) the H2)O molecules were treated as rigid (hard ice model); (ii) the intramolecular vibrational modes of H2O were included (soft ice model). The amorphous water ice slabs produced by our MD simulations are found to be good models for the surfaces of icy mantles of dust grains. For the various fundamental processes of H2 formation on the dust surface, the following results emerge. (1) For the sticking of an H atom on to the surface, a sticking probability that depends on the temperature of the incident H atom is obtained. (2) For the diffusion of an H atom on the surface, it is found that the first incident H atom diffuses via the thermal hopping mechanism at the first stage, and then it is always trapped in one of the stable sites on the amorphous ice. The migration length and time have been calculated for the mobility of the incident H atom before it is trapped. The time‐scales of thermal diffusion and desorption of H atoms after trapping have also been estimated. (3) For the reaction of two H atoms on the surface, the following three reaction patterns are observed: (i) H2 is produced via the Langmuir–Hinshelwood mechanism; (ii) H2 is produced via the Eley–Rideal mechanism; (iii) the almost elastic collision of two H atoms occurs without H2 being formed. The effective reactive cross‐section is estimated at about 40 Å2. The reaction probabilities are found to be near unity. (4) For the ejection of H2 from the ice surface, the product H2 is subsequently ejected after the reaction process, using part of the excess energy derived from the H2 formation. The average lifetime of ejection is about 400–600 fs. Most of the ejected H2 molecules are found to be in vibrationally and rotationally excited states.

MD simulation for H2 formation on amorphous ice

“…our MD simulation (Takahashi et al, 1999a). It was found that the ejection process of the product H 2 via the direct ejection mechanism occurred subsequently after the two H atoms' reaction process.…”

“…The present definition of the reaction probability is not a global one, but a microscopic one. The results are shown in Table 5 (after Takahashi et al, 1999a) and they are found to be almost unity.…”

“…The reaction process of two hydrogen atoms on the icy surface of dust grains was studied by the third stage of our MD simulation (Takahashi et al, 1999a). Two reaction cases were observed in which H 2 molecules are produced on the dust surface (see, e.g., Williams, 1993;Herbst, 1995).…”

“…3. The 3rd stage of MD simulation-three cases in the reaction process of two H atoms on the amorphous water ice (after Takahashi et al, 1999a).…”

The Formation of H 2 and HD with the Master Equation Approach

“…In the Langmuir-Hinshelwood mechanism, the adsorbed H atoms quickly equilibrate and diffuse on the surface of the grain either by thermal activation or by tunneling. When two adsorbed H atoms encounter each other, an H 2 molecule may form (Williams 1968;Hollenbach & Salpeter 1970;Smoluchowski 1981;Aronowitz & Chang 1985;Duley & Williams 1986;Pirronello & Averna 1988;Sandford & Allamandolla 1993;Takahashi et al 1999;Farebrother et al 2000). The steady-state production rate of molecular hydrogen, R H2 (cm −3 s −1 ) can be expressed by…”