New measurements on water ice photodesorption and product formation under ultraviolet irradiation

TL;DR

This study measures water ice photodesorption yields and product formation under UV irradiation, providing data crucial for understanding gas-phase water in cold astrophysical regions.

Contribution

It presents new quantitative measurements of water ice photodesorption yields and identifies photoproducts, advancing knowledge of ice photochemistry in space environments.

Findings

Photodesorption yield of water ice at 8 K is 1.3×10^{-3} molecules per photon.

Photodesorption yield of deuterated water at 8 K is 0.7×10^{-3} molecules per photon.

Water ice photodesorption can explain gas-phase water in cold regions without thermal desorption.

Abstract

The photodesorption of icy grain mantles has been claimed to be responsible for the abundance of gas-phase molecules toward cold regions. Being water a ubiquitous molecule, it is crucial to understand its role in photochemistry and its behavior under an ultraviolet field. We report new measurements on the UV-photodesorption of water ice and its H$_2$, OH, and O$_2$ photoproducts using a calibrated quadrupole mass spectrometer. Solid water was deposited under ultra-high-vacuum conditions and then UV-irradiated at various temperatures starting from 8 K with a microwave discharged hydrogen lamp. Deuterated water was used for confirmation of the results. We found a photodesorption yield of 1.3 $\times$ 10$^{-3}$ molecules per incident photon for water, and 0.7 $\times$ 10$^{-3}$ molecules per incident photon for deuterated water at the lowest irradiation temperature, 8 K. The photodesorption yield per absorbed photon is given and comparison with astrophysical scenarios, where water ice photodesorption could account for the presence of gas-phase water toward cold regions in the absence of a thermal desorption process is addressed.