Laboratory evidence for the non-detection of excited nascent H2 in dark clouds

TL;DR

This study provides experimental evidence that nascent hydrogen molecules formed on dust grain surfaces in dark clouds are rapidly de-excited, explaining their non-detection in astronomical observations and refining our understanding of H2 formation energy distribution.

Contribution

The paper demonstrates experimentally that excited nascent H2 is quickly de-excited on water ice surfaces, clarifying the non-detection issue in dark clouds and challenging previous emission predictions.

Findings

Excited nascent H2 is rapidly de-excited on water ice surfaces.

The amount of excited H2 is significantly reduced regardless of ice morphology.

De-excitation occurs efficiently at 10 K on both porous and non-porous ice.

Abstract

There has always been a great deal of interest in the formation of H2 as well as in the binding energy released upon its formation on the surface of dust grains. The present work aims at collecting experimental evidence for how the bond energy budget of H2 is distributed between the reaction site and the internal energy of the molecule. So far, the non-detection of excited nascent H2 in dense quiescent clouds could be a sign that either predictions of emission line intensities are not correct or the de-excitation of the newly formed molecules proceeds rapidly on the grain surface itself. In this letter we present experimental evidence that interstellar molecular hydrogen is formed and then rapidly de-excited on the surface of porous water ice mantles. In addition, although we detect ro-vibrationally excited nascent molecules desorbing from a bare non-porous (compact) water ice film, we demonstrate that the amount of excited nascent hydrogen molecules is significantly reduced no matter the morphology of the water ice substrate at 10 K (both on non-porous and on porous water ice) in a regime of high molecular coverage as is the case in dark molecular clouds.