Toward a better understanding of interstellar molecular clouds

In deep space, dense molecular clouds consist of both gaseous species and grains of dust. Adsorption of these gas species onto dust grain cores is a key driver of star and planetary formation. On Earth, this process can be studied through temperature programmed desorption (TPD), which determines the desorption rate based on the adsorbate binding energy and a pre-exponential factor.

While the binding energies of most gaseous species are well known, the pre-exponential factor is less precisely determined. Pantaleone et al. evaluated several common models for the pre-exponential factor using quantum mechanical calculations to identify the most efficient approach.

In their tests, the authors calculated the adsorption properties of water, ammonia, and methanol — some common species found in molecular clouds — on both amorphous and crystalline ices using four different formulations of the pre-exponential factor, comparing the results.

“The four pre-exponential factor models differ on the treatment of the adsorbate’s behaviour upon surface attachment,” said author Stefano Pantaleone. “The simplest Hasegawa-Herbst formulation consists of an adsorbate free to scan all the available sites of a surface, Tait’s and vibrational models introduce the translation, rotation and vibration of the adsorbed molecule, while Campbell’s model adopts an empirical formula based on the rotational entropy of the adsorbed molecule.”

While the vibrational model was the most rigorous from a theoretical perspective, they found Campbell’s empirical approach converged to the same result without costly computational modelling.

The authors hope this work will guide consensus on the best models to use and drive a synthesis of theoretical and experimental research.

“Accurate desorption temperature predictions can aid in assessing the likelihood of detecting these species in the JWST infrared spectra of icy mantles in protoplanetary disks,” said Pantaleone.

Source: “The role of the pre-exponential factor on temperature programmed desorption spectra: A computational study of frozen species on interstellar icy grain mantles,” by S. Pantaleone, L. Tinacci, V. Bariosco, A. Rimola, C. Ceccarelli, and P. Ugliengo, Journal of Chemical Physics (2025). The article can be accessed at https://doi.org/10.1063/5.0266978 .