Laboratory measurements and modeling of the desorption energy of molecules on interstellar dust

<p indent="0mm">Desorption energy stands as a pivotal physical parameter in the realm of interstellar chemistry modeling, directly influencing the temperature at which molecules transition from the solid state to the gas phase. Interstellar dust primarily consists of amorphous silicate or carbon grains, typically coated with an icy mantle composed of mixed ices such as H₂O, CO₂, CO, NH₃, CH₄, and CH₃OH. The intricate nature of these dust surfaces renders the desorption process of interstellar molecules highly complex, necessitating comprehensive laboratory investigations. Laboratory measurements of desorption energy predominantly rely on temperature-programmed desorption (TPD) experiments. In these experiments, volatile molecules are deposited onto a substrate of interest, which is subsequently heated to facilitate desorption. A mass spectrometer monitors the desorption process, yielding TPD curves. This paper offers a concise overview of the TPD experimental procedure and its technical intricacies. Particular emphasis is placed on the significance of conducting TPD experiments at very low surface coverage and under ultra-clean conditions to ensure accurate and reliable results. The paper also critically examines the drawbacks and technical challenges associated with existing TPD experiments, proposing potential improvements to address these issues. As a representative case study, TPD data for CO molecules on a non-porous amorphous solid water surface are presented. In this experiment, CO molecules of varying coverage were deposited from a collimated molecular beam onto an amorphous water ice surface and subsequently heated for TPD measurement. At low coverage, the desorption curves exhibited a shared trailing edge. When coverage exceeded one monolayer (full surface coverage), the TPD curves displayed a typical zero-order desorption pattern, characterized by a common leading edge. The distribution of CO desorption energy was derived from the experimental data. TPD data analysis typically employs the Polanyi-Wigner equation. By comparing numerical solutions of this equation with experimental TPD data, the desorption energy value can be determined. This work explores how desorption curves depend on initial surface coverage, desorption energy (or its distribution), and heating ramp rate. Cosmic dust grains, composed of amorphous silicate or carbonaceous material, are naturally coated with amorphous water ice. Consequently, the corresponding desorption energy on these surfaces forms a continuous distribution rather than a single value. A direct inversion of the Polanyi-Wigner equation can be utilized to obtain the desorption energy distribution, and the impact of this broad distribution on TPD profiles is demonstrated. Laboratory TPD experiments typically span minutes to hours, whereas the warm-up timescale for interstellar clouds can extend to millions of years. Based on the Polanyi-Wigner equation, it is shown that desorption peak temperature correlates positively with the warm-up ramp rate. This implies that desorption temperatures under interstellar conditions are significantly lower than those observed in laboratory experiments. Establishing a relationship between desorption peak temperature and interstellar desorption temperature is not straightforward. Through simulations using the Polanyi-Wigner equation, this paper presented a figure from which one can easily look up the desorption temperatures at astronomical timescales, providing the desorption temperature at laboratory timescales. Finally, the discussion addresses some unresolved challenges in experimental measurement and modeling related to desorption energy.