Influence of the substrate on the density and infrared spectra of the adsorbed methanol ice of different thicknesses using molecular dynamics simulation

In the interstellar medium, several complex organic molecules are found, and of these molecules, methanol is the simplest and the most ubiquitous one. By comparing the observed infrared (IR) spectra from astrochemical data, with laboratory experiments, one can deduce the composition and structure of these astrochemical ices. Computational studies are scarce, yet they could be greatly helpful in understanding the nature of these molecules. On that premise, the present study reports a molecular dynamics study of adsorbed methanol on the KBr substrate at 90 K and 130 K. After validating the potential parameters, two adsorbed phases differing in their thickness along the z-axis: 4 x 10 x 4 (4 layers) and 4 x 10 x 50 (50 layers) were simulated. Depending on this thickness, the IR spectra and density distribution functions were computed for the bottom and top 10 angstrom of the 4 layers of adsorbed methanol and the top, middle, and bottom 10 angstrom of the 50 layers of adsorbed methanol on the KBr substrate. The bottom 10 angstrom of the adsorbed phase exhibit considerable disorder. Additionally, the bands in the IR spectra of these bottom 10 angstrom show widening, referring to a heterogeneous environment. It is further reported that the slower heating and cooling of the adsorbed phase between 90 K and 130 K leads to a complete reversal of the changes seen in heating. Our findings here further clarify the recent observation of the amorphous phase of different astrochemical molecules seen at low temperatures and their crystalline phase seen at relatively higher temperatures.