The Ortho/Para Ratio of H 2 Newly Formed on Dust Grains

Abstract

This paper considers experimental and theoretical studies of the H2 formation process on solid surfaces in an attempt to determine the ortho/para ratio (OPR) of H2 newly formed on dust grains. Estimates of OPRs and formation temperatures ofH2 on icy mantles, silicate dust, and carbonaceous dust are given based on a new model. The OPR of H2 newly formed on dust grains differs from the value based on the nuclear spin statistics or the LTE value at the temperature of gas or dust. The finalvibration-rotation distribution of the product H2 isdetermined as the result of a relaxation process of the metastable states of H2 adsorbed on dust grains via the dynamical adatom-surface interaction. We calculated the OPRs and formation temperatures by the new definitions with the formation pumping models built by Takahashi & Uehara. Test calculations with unrealistic, too small rotational energy distributions (only 1.4%-1.5% and 0.14%-0.15% of the formation energy, 4.48 eV), give anomalous OPRs because of imbalance between the odd-J and even-J populations in the rotational distribution. On the other hand, calculations with the original formation pumping models with realistic rotational energy distributions (14%-15%) give OPRs of nearly 3.0 on any dust surface and formation temperatures of 3600-4200 K.