Signatures of Quantum-Tunneling Diffusion of Hydrogen Atoms on Water Ice at 10 K

Reported here is the first observation of the tunneling surface diffusion of a hydrogen (H) atom on water ice. Photostimulated desorption and resonance-enhanced multiphoton ionization methods were used to determine the diffusion rates at 10 K on amorphous solid water and polycrystalline ice. H-atom diffusion on polycrystalline ice was 2 orders of magnitude faster than that of deuterium atoms, indicating the occurrence of tunneling diffusion. Whether diffusion is by tunneling or thermal hopping also depends on the diffusion length of the atoms and the morphology of the surface. Our findings contribute to a better understanding of elementary physicochemical processes of hydrogen on cosmic ice dust.

Figure 1

Delay-time spectra of photodesorbed H (gray line) and D (black line) atoms from ices at 10 K: (a) ASW, (b) PCI for the atomic flux of $1.7\times {10}^{13}\text{atoms}{\mathrm{s}}^{-1}$, (c) ASW, (d) PCI for the atomic flux of $7.9\times {10}^{14}\text{atoms}{\mathrm{s}}^{-1}$. The red lines are fits to the data derived assuming Maxwell-Boltzmann distributions with translational temperatures in range 25–50 K for the spectra of the H and D atoms.

Figure 2

Upper: $\mathrm{D}/\mathrm{H}$ signal intensity ratio of the delay-time spectra as a function of the flux of H (D) atoms, obtained from the PCI (open triangles) and ASW (filled circles) surfaces. The right $Y$ axis is the ratio of surface diffusion rates calculated from Eq. (2). The shaded area represents the thermal-diffusion dominated region derived from Eq. (4). Lower: enlarged view in the low flux region.

Figure 3

Schematic illustration of the diffusion of atoms on a surface for (a) low atomic coverage and (b) high atomic coverage. For lower fluxes, the average distance between adatoms is significant [case (a)] and, therefore, each atom is required to migrate a long distance across the steps and boundaries to encounter its reaction partner. For higher fluxes, adatoms locate nearby and thus can encounter through the tunneling diffusion.