Impact of surface heterogeneity on IR line profiles of adsorbed carbon monoxide on models of interstellar grain surfaces

Abstract

Surface heterogeneity of model amorphous silica films used as a model for interstellar grain surfaces is revealed through the application of the pre-exponential optimized inversion method to previously reported sub-monolayer thermal desorption studies of carbon monoxide (CO) desorption. The impact of that surface heterogeneity, as represented by the coverage dependence of the CO activation energy for desorption from the amorphous silica surface, on the IR spectroscopy of the CO stretching vibration is explored through vibrational line profile synthesis. Comparison is then made to previous investigations of CO line profiles on this surface and on amorphous solid water as reported in Taj et al. (2017, 2019a). A tentative conclusion is drawn that CO vibrationally promoted desorption from, and diffusion on, the amorphous silica surface may be responsible for the correspondingly short vibrational excited state lifetime of CO on that surface. The contrast with CO on amorphous solid water, where direct and rapid vibrational relaxation into the solid water phonon bath occurs, is highlighted. The consequences of this from the standpoint of CO deposition on grain surfaces are discussed.

Thermal Desorption of Interstellar Ices: A Review on the Controlling Parameters and Their Implications from Snowlines to Chemical Complexity

“…A lower binding energy of 1418 K was measured for a 0.1 ML adsorbed on SiO x surface . Recently Taj et al reported values between 750 and 1100 K for a monolayer of CO adsorbed on amorphous SiO x . O . We used the value estimated by Minissale et al on oxidized HOPG.…”

Physics and chemistry on the surface of cosmic dust grains: a laboratory view

“…coming to the fore with studies likewise on a variety of carbonaceous [168], siliceous [167,198,199] and ice surfaces [168,198,200]. As an illustrative example, we consider the study of CO desorption from amorphous silica by Taj and co-workers [198,199].…”

“…coming to the fore with studies likewise on a variety of carbonaceous [168], siliceous [167,198,199] and ice surfaces [168,198,200]. As an illustrative example, we consider the study of CO desorption from amorphous silica by Taj and co-workers [198,199]. Figure 12 shows the R 2 (the sum of the squared error of the fit) data versus pre-exponential factor for these measurements.…”

“…It also revealed that the line width of the CO vibrational band, 2.3 cm -1 , suggests a vibrational lifetime of 1.15 ps on the amorphous silica surface perhaps consistent with vibrational predissociation of the CO from the surface. Reproduced with permission from [199] (Figure 1a).…”