2015ApJ...801..120H

The energy to desorb atomic oxygen from an interstellar dust grain surface, E_des, is an important controlling parameter in gas-grain models; its value impacts the temperature range over which oxygen resides on a dust grain. However, no prior measurement has been done of the desorption energy. We report the first direct measurement of E_des for atomic oxygen from dust grain analogs. The values of E_desare 1660±60 and 1850±90 K for porous amorphous water ice and for a bare amorphous silicate film, respectively, or about twice the value previously adopted in simulations of the chemical evolution of a cloud. We use the new values to study oxygen chemistry as a function of depth in a molecular cloud. For n = 10⁴/cm3 and G₀= 10² (G₀= 1 is the average local interstellar radiation field), the main result of the adoption of the higher oxygen binding energy is that H₂O can form on grains at lower visual extinction A_V, closer to the cloud surface. A higher binding energy of O results in more formation of OH and H₂ O on grains, which are subsequently desorbed by far-ultraviolet radiation, with consequences for gas-phase chemistry. For higher values of n and G₀, the higher binding energy can lead to a large increase in the column of H₂O but a decrease in the column of O₂.