Date

Thursday 24 Jul 2025

Time

15:15 - 15:45

Location

BW018

Supervisor

Thanja Lamberts

2nd reviewer

Jörg Meyer

Jury

Irene Groot

Context. In the interstellar medium (ISM), molecules can be formed in both the gas phase and adsorbed on dust grain surfaces. These surfaces act as meeting sites for atoms and molecules and can facilitate reactions not occurring normally in the gas phase. Upon formation, molecules can either stay adsorbed on the dust grain to freeze out and form a mantle, or return back to the gas phase via various methods, including UV-induced photodesorption, thermal desorption, and chemical desorption. However, the process of chemical desorption, such as how the exothermicity is redistributed, is poorly understood.

Aims. This thesis adapts the Sudden Vector Projection (SVP) model to predict the likelihood of chemical desorption. Its accuracy is tested by analysing four astrochemical systems with experimentally known desorption behaviour, involving the following reactions: H + CO −−→ HCO, SO2 + H −−→ HOSO, SO2 + H −−→ HSO2, OH + H2 −−→ H2O + H, HS + H −−→ H2S and H2S + H −−→ HS + H2.

Methods. Transition state (TS) and intrinsic reaction coordinate (IRC) calculations were performed to model the reaction paths on various CO and water surface clusters. An inhouse adaptation of the SVP model was used to derive the directionality of desorption. Results. It was found that desorption angles for HCO range between 80◦ and 86◦ compared to the surface normal, indicating no feasible desorption path. HS on a water trimer also shows no desorption (86◦). In contrast, HOSO (44◦ - 55◦) and HS on a water octamer (49◦) do exhibit desorption behaviour. H2O shows more vertical angles (36◦ - 58◦), but its exothermicity (-14 kcal/mol) is insufficient to overcome the binding energy (12 - 17 kcal/mol).

Conclusions. The calculated desorption angles coincided with desorption behaviour observed in experiments, showing that the SVP model is a promising tool for gaining insight in energy redistribution and desorption directionality. While the model cannot predict desorption probabilities without molecular dynamics, this work has shown that SVP provides a valuable starting point for understanding chemical desorption.