CO desorption from interstellar icy grains induced by IR excitation of superhydrogenated PAHs

TL;DR

This study demonstrates that infrared irradiation of carbonaceous materials, modeled by perhydropyrene (PHP), can efficiently desorb CO from interstellar ice analogues via indirect energy transfer, even when CO itself is IR-transparent. Layered CO-on-PHP ices exhibit much higher desorption yields than mixed CO:PHP ices, indicating that energy transfer through the CO layer and collective PHP excitations enhance desorption. Direct excitation of CO is ineffective, highlighting the importance of non-desorptive dissipation channels and possible phonon-mediated or IR-emission processes. These findings suggest IR photons, abundant in dense clouds, could play a significant role in maintaining gas-phase CO and influencing the broader gas–ice chemistry, warranting inclusion of IR-induced desorption mechanisms in astrochemical models.

Abstract

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. 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 IR-induced effects. We present IR spectroscopic and mass spectrometric measurements probing the effects of IR radiation on two ice systems: a layered ice with CO on top of PHP, and a CO:PHP mixed ice. These ices were irradiated with IR radiation from the FELIX IR Free Electron Laser (FEL) FEL-2. 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 H$_2$O 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.

Figures (5)

• Figure 1: (a) The structure of perhydropyrene (PHP, C16H26), the fully superhydrogenated version of pyrene. (b) RAIR spectrum of the CO:PHP mixed ice. The main spectral features are marked, and coloured arrows indicate the irradiation wavelengths: the strongest CH stretch of PHP at 3.42 µm (blue) and the CO stretching mode at 4.67 µm (red).
• Figure 2: CO desorption signals detected by the QMS for $m/z=28$ during irradiations of the mixed CO:PHP and layered CO/PHP ices, baselined and offset for clarity. Irradiation energies, $E_\mathrm{irr}$, are denoted above each spectrum. For better visual comparison, the desorption trace for the PHP mode irradiation of the layered ice with higher irradiation energy has been scaled by a factor of 2/3. Each individual desorption peak corresponds to a 5 Hz macropulse of the FEL. The zero-time for irradiation is defined as the onset of the first CO desorption peak.
• Figure 3: Bi-exponential fits to the integrated CO desorption signals derived from the data presented in Fig. \ref{['fig:QMSmain']}, for the 3.42 µm irradiation of the mixed (top panel) and layered (bottom panel) CO-PHP systems. The data for the layered ice has not been scaled.
• Figure 4: RAIR spectra (top) and difference spectra (bottom) for irradiation of the (a) CO:PHP mixed ice and (b) CO/PHP layered ice, focusing on the region around the CO stretching mode. The CO peak shape is different for the mixed and layered ices but represents the same total area and CO column density. The difference spectra represent the irradiations for which QMS data are shown in Fig. \ref{['fig:QMSmain']} and Appendix Fig. \ref{['AppFig:QMSzoom']}, with the band irradiated denoted above each spectrum. The difference spectrum for a wait time of 1 min without irradiation for the layered ice (grey) indicates continuous CO deposition. Each difference spectrum has been baseline corrected by subtracting the mean value in the displayed range, excluding the shaded region around the CO band, and has been offset for clarity, represented by horizontal lines. The off-resonance irradiation energy was $E_\mathrm{irr}=56\,$mJ.
• Figure 5: The CO ($m/z=28$) desorption detected in the QMS for the first few FEL macropulses during all irradiations in this work. The connecting lines are shown as a guide to the eye. All traces have been baselined to the average CO signal before irradiation starts and offset for clarity. The zero-time is defined as the onset of the first peak for the irradiations showing desorption signal.