Mechanism of Ultraviolet-Induced CO Desorption from CO Ice: Role of Vibrational Relaxation Highlighted

Although UV photon-induced CO ice desorption is clearly observed in many cold regions of the Universe as well as in the laboratory, the fundamental question of the mechanisms involved at the molecular scale remains debated. In particular, the exact nature of the involved energy transfers in the indirect desorption pathway highlighted in previous experiments is not explained. Using ab initio molecular dynamics simulations, we explore a new indirect desorption mechanism in which a highly vibrationally excited CO (v = 40) within an aggregate of 50 CO molecules triggers the desorption of molecules at the surface. The desorption originates first from a mutual attraction between the excited molecule and the surrounding molecule(s), followed by a cascade of energy transfers, ultimately resulting in the desorption of vibrationally cold CO (similar to 95% in v = 0). The theoretical vibrational distribution, along with the kinetic energy one, which peaks around 25 meV for CO with low rotational levels (v = 0, J < 7), is in excellent agreement with the results obtained from VUV laser induced desorption (157 nm) of CO (v = 0, 1) probed using REMPI.