Mechanism of Indirect Photon-Induced Desorption at the Water Ice Surface

Electronic excitations near the surface of water ice lead to the desorption of adsorbed molecules, through a so far debated mechanism. A systematic study of photon-induced indirect desorption, revealed by the spectral dependence of the desorption (7–13 eV), is conducted for Ar, Kr, ${\mathrm{N}}_2$, and CO adsorbed on ${\mathrm{H}}_2\mathrm{O}$ or ${\mathrm{D}}_2\mathrm{O}$ amorphous ices. The mass and isotopic dependence and the increase of intrinsic desorption efficiency with photon energy all point to a mechanism of desorption induced by collisions between adsorbates and energetic H/D atoms, produced by photodissociation of water. This constitutes a direct and unambiguous experimental demonstration of the mechanism of indirect desorption of weakly adsorbed species on water ice, and sheds new light on the possibility of this mechanism in other systems. It also has implications for the description of photon-induced desorption in astrochemical models.

Figure 1

Upper panel: photodesorption spectrum (desorption yield as a function of photon energy) of argon for $1{\mathrm{ML}}_{\mathrm{eq}}$ of argon adsorbed on 100 ML compact amorphous solid water c-ASW (black line). Also shown for comparison are the absorption spectrum of amorphous ${\mathrm{H}}_2\mathrm{O}$ ice from Lu et al. [49] (green line, arbitrary units) and the photodesorption spectrum of Argon from 20 ML pure Argon ice (red line, scaled $1/200$). Lower panel: photodesorption spectrum of argon $1{\mathrm{ML}}_{\mathrm{eq}}$ on c-ASW (black spectrum of the upper panel) divided by ${\mathrm{H}}_2\mathrm{O}$ absorption (green spectrum of the upper panel).

Figure 2

Photodesorption spectra (black lines) of ${{}^{15}\mathrm{N}}_2$ from ${{}^{15}\mathrm{N}}_2$ adsorbed on c-ASW (top panel), Kr from Kr adsorbed on c-ASW (middle panel), and ${}^{13}\mathrm{CO}$ from ${}^{13}\mathrm{CO}$ adsorbed on c-ASW (bottom panel). The absorption spectrum of c-ASW from Lu et al. (green line) is reproduced on the top panel. On the bottom panel, the photodesorption spectrum of 20 ML pure CO is also shown (scaled $1/19$) and fitted under the $\mathrm{CO}/H_2\mathrm{O}$ photodesorption spectra to estimate the contribution of indirect desorption, indicated by the blue arrow.

Figure 3

Indirect desorption yields for each system as a function of the fraction of kinetic energy transferred in the collision of an H/D atom with the adsorbate (see text for the formula). The indirect desorption yield is measured as the photodesorption yield at 9 eV at the maximum of the first electronic state of water ice. Error bars only reflect the signal to noise ratio of the desorption spectra, not errors in the relative calibration of the signals and coverage of the different species (see Supplemental Material [42]). The green line is the desorption probability model from Fredon et al. [53] for a binding energy value of ${\mathrm{E}}_{\mathrm{bind}}=70$, 100, or 130 meV (upper, middle, and lower curves) and an initial H/D kinetic energy of 2.9 eV.