The ortho/para ratio of H2 newly formed on dust grains

This paper considers experimental and theoretical studies of the H-2 formation process on solid surfaces in an attempt to determine the ortho/para ratio (OPR) of H-2 newly formed on dust grains. Estimates of OPRs and formation temperatures of H-2 on icy mantles, silicate dust, and carbonaceous dust are given based on a new model. The OPR of H-2 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 final vibration-rotation distribution of the product H-2 is determined as the result of a relaxation process of the metastable states of H-2 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.