Measurement of the adsorption energy difference between ortho- and para-D2 on an amorphous ice surface

Molecular hydrogen interaction on water ice surfaces is a major process taking place in interstellar dense clouds. By coupling laser detection and classical thermal desorption spectroscopy, it is possible to study the effect of rotation of D-2 on adsorption on amorphous solid water ice surfaces. The desorption profiles of ortho- and para-D-2 are different. This difference is due to a shift in the adsorption energy distribution of the two lowest rotational states. Molecules in J('')=1 rotational state are on average more strongly bound to the ice surface than those in J('')=0 rotational state. This energy difference is estimated to be 1.4 +/- 0.3 meV. This value is in agreement with previous calculation and interpretation. The nonspherical wave function J('')=1 has an interaction with the asymmetric part of the adsorption potential and contributes positively in the binding energy.