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.