A three-step mechanism explaining ultraviolet-induced CO desorption from ice CO

Ultraviolet (UV) radiation-induced desorption of CO ice is a phenomenon that occurs in some cold parts of the universe, and has often also been reproduced in the laboratory. Although this phenomenon is now well documented, the molecular mechanisms underlying it remain to be discovered.

Researchers from the University of Lille and Sorbonne University, as part of the French ANR PIXyES project led by Mathieu Bertin, recently carried out a study investigating this mechanism through a combination of experiments and molecular simulations. Their article, published in Physical Examination Lettersdescribes a three-step mechanism that could give rise to UV photon-induced CO ice desorption.

“In the interstellar medium (ISM), molecular matter is found mainly in the coldest and densest regions,” Maurice Monnerville and Jean-Hugues Fillion, the paper’s lead authors, told Phys.org.

“These areas are stellar nurseries, where stars and planets come to life, like the inner parts of protoplanetary disks and prestellar clouds. About 200 different molecular species, ranging from the simplest, like hydrogen and water (H₂H₂O, CO,…) to more complex ones like methanol (CH₃OH) coexist with tiny grains made of silicates and carbons. »

In some of the coldest regions of the universe, with extremely low temperatures of around 10 K, all molecules (except H₂) stick to the surface of tiny grains, forming icy layers. These layers consist mainly of condensed water and various other substances, such as carbon monoxide (CO) and carbon dioxide (CO).₂).

“These interstellar ices act as a crucial reservoir of molecular matter in cold regions of the universe,” the authors explain.

“In these coldest regions, anomalous abundances in the gas phase were detected, even though the species should be frozen on dust grains due to the extremely low temperature. So how can we explain the desorption of these molecules in the cold regions of space? “To understand these unexpected abundances, a non-thermal desorption phenomenon explaining the detection of these molecules in the gas phase is necessary.”

One process that may explain the high abundance of gas molecules in parts of the universe where temperatures are particularly low is desorption induced by UV photons from surrounding stellar emission, filtered through atomic hydrogen (7 to 13.6 eV). Many physicists have recently explored this phenomenon in depth, in particular the UV photodesorption of CO.

“CO ices serve as a potential starting point for complex chemistry leading to the formation of methanol and the resulting highly diverse organic chemistry,” the authors said. “For these reasons, VUV photodesorption of solid CO has for decades been the subject of a wide range of experimental studies aimed at providing absolute desorption efficiencies to the astrochemical community.”

Previous research by Jean-Hugues Fillion’s research group at the LERMA laboratory at Sorbonne University found evidence suggesting that the UV-induced CO desorption mechanism is largely indirect. This essentially means that the desorbing molecule is not necessarily the one absorbing the photon, but rather that this desorption process is primarily driven by energy transfer between the excited molecule and the surface molecule.

However, until now, this desorption mechanism has remained poorly understood, as neither theoretical nor experimental work has been able to account for all of its associated molecular properties. The main objective of the recent study by Monnerville and colleagues was therefore to characterize the mechanism, with emphasis on the nature of the previously reported energy transfer and the properties of the desorbed molecules.

“We have developed a concerted strategy between theory and experimentation,” Monnerville said. “The PCMT team from the University of Lille used Ab Initio Molecular Dynamics (AIMD), a sophisticated mixed quantum/classical simulation technique based on density functional theory (DFT) to further elucidate the mechanism of transfer of energy.”

“At the same time, the Parisian team carried out a new pulsed laser-induced photodesorption at a selected excitation energy in the VUV using the SPICES ultrahigh vacuum system, providing data on the vibrational and translational energy distribution of the molecules of photodesorbed COs that can be directly compared to AIMD results.”

Simulations carried out by part of this research group based at the University of Lille revealed that the desorption of CO ice induced by UV radiation was based on a mechanism involving three key steps. During the first of these stages, an excited molecule vibrates within the ice, retaining the vibrational energy initially deposited within it.

“Subsequently, the excited molecule and one or two nearby CO molecules begin to attract each other and consequently gain translational energy, leading to their collision via a ‘kick event’,” a explained Monnerville.

“The colliding molecules then initiate motion and interact with other molecules in the ice, resulting in a cascading energy transfer effect. Essentially, the translational and rotational energy acquired in the second stage is transferred to the surface CO molecules, allowing them to overcome the binding energy of the aggregate and desorb.”

Notably, the three key steps described by Monnerville and colleagues are partly aligned with the well-known DIET (Desorption Induced by Electronic Transition) mechanism. This mechanism was previously hypothesized to be a possible cause of VUV irradiation-induced desorption of interstellar ice analogs, but this study is the first to describe it in detail via simulations that are also consistent with the experimental observations.

“The key to successfully describing this complex process and achieving perfect agreement with experimental observations lay in the use of computationally intensive simulation techniques, which enabled a more accurate representation of this complex dynamic system,” he said. declared Monnerville.

“AIMD techniques were crucial to accurately characterize the interaction between a vibrationally excited CO molecule and its neighbors, thereby initiating the desorption process, a facet where previous theoretical studies had failed.”

The recent work of this team of researchers constitutes a significant contribution to the study of molecular processes in ultracold environments. Remarkably, this is the first to provide detailed simulations of the mechanism behind UV-induced CO ice desorption, fully aligned with experimental observations.

“Our discovery of a three-step mechanism (vibrational excitation, kick, desorption) allows us to explain a complex process in relatively simple terms,” the authors said. “It is in fact the simplicity of this mechanism that makes it significant. It is entirely plausible that this primary mechanism could be used by the astrophysical community to theoretically explain the desorption already observed in more complex interstellar ices.”

In the future, the experimental methods and simulation tools implemented by the Lille and Sorbonne University teams could be used to study the photo-desorption of a wider range of complex ice mixtures. The researchers are now also working on a machine learning-based potential energy surface (PES) model, trained using data collected during their ab initio molecular dynamics (DFT) calculations.

“This high-dimensional neural network PES will allow us to perform more and longer molecular dynamics simulations of the CO desorption process on a more representative model of the CO ice surface, while significantly reducing costs of calculation,” added Monnerville.

“In addition, we are conducting new experimental and theoretical studies of more complex interstellar ice analogs, such as CO₂SCAM₂, and CO:NO, using similar methodologies. Finally, a new experimental approach will be tested to reveal the angular distribution of photodesorbed molecules. This will be achieved by implementing a velocity map imaging detector, a powerful detection technique well known for gas phase applications, although its development is challenging for the study of molecules desorbed from cold substrates . »

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