Using a combination of photostimulated desorption and resonance-enhanced multiphoton ionization techniques, OH radicals photodesorbed from amorphous solid water at low temperatures are detected. It was found that the OH photodesorption can be caused by a one-photon process on the surface even at 532 nm, where both isolated OH and ${\mathrm{H}}_2\mathrm{O}$ are transparent. Quantum chemical calculations reveal that the electronic excitation of OH dramatically changes depending on the adsorption sites and the number of surrounding ${\mathrm{H}}_2\mathrm{O}$ molecules, and that OH having three hydrogen bonds with neighboring ${\mathrm{H}}_2\mathrm{O}$ molecules can absorb photons of around 532 nm, which supports the experimental findings.

• Received 21 October 2019
• Revised 21 July 2020
• Accepted 29 October 2020

DOI:https://doi.org/10.1103/PhysRevA.102.052822