CO desorption from interstellar icy grains induced by infrared excitation of superhydrogenated polycyclic aromatic hydrocarbons

¹ Center for Interstellar Catalysis, Department of Physics and Astronomy, Aarhus University, 8000 Aarhus C, Denmark
² Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
³ Center for Astrophysics | Harvard & Smithsonian, Cambridge, MA 02138, USA
⁴ HFML-FELIX Laboratory, Radboud University, Nijmegen 6525 ED, The Netherlands

^★ Corresponding authors: thrower@phys.au.dk; liv@phys.au.dk

Received: 14 July 2025
Accepted: 9 October 2025

Abstract

Context. Infrared (IR) radiation dominates dense, interstellar clouds, yet its effect on icy grains remains largely unexplored. Its potential role in driving the photodesorption of volatile species from such grains has recently been demonstrated, providing a crucial link between the solid state reservoir of molecules and the gas phase.

Aims. In this work, we investigate IR-induced photodesorption of CO for astrophysically relevant ice systems containing perhydropyrene (PHP). This fully superhydrogenated version of pyrene is used as an analogue for large carbonaceous molecules such as polycyclic aromatic hydrocarbons (PAHs) and related species, as well as hydrogenated carbonaceous grains. The abundance and range of strong IR absorption bands of these carbonaceous species make them interesting candidates for studying IR-induced effects.

Methods. We present IR spectroscopic and mass spectrometric measurements probing the effects of IR radiation on two ice systems: 1) CO:PHP mixed ice and 2) layered ice with CO on top of PHP. The ices were irradiated with the FELIX IR Free Electron Laser (FEL) FEL-2.

Results. In accordance with previous studies, we confirm that direct excitation of CO is not an efficient pathway to CO desorption, indicating that another energy dissipation mechanism exists. We demonstrate that vibrational excitation of the PHP CH stretching modes leads to efficient CO photodesorption. The derived photodesorption yields are an order of magnitude higher for the layered than the mixed system and comparable to those previously obtained for CO photodesorption from CO on amorphous solid water upon excitation of H₂O vibrational modes. Our results indicate that IR excitation of carbonaceous molecules and grains in dense clouds could potentially play an important role in the desorption of volatile species such as CO from icy grains.