A New Determination of the Binding Energy of Atomic Oxygen on Dust Grain
  Surfaces: Experimental Results and Simulations
  The energy to desorb atomic oxygen from an interstellar dust grain surface,
$E_{\rm des}$, 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 $E_{\rm des}$ for atomic oxygen from dust grain
analogs. The values of $E_{\rm des}$ are $1660\pm 60$~K and $1850\pm 90$~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 $n=10^4$ cm$^{-3}$ and
$G_0$=10$^2$ ($G_0$=1 is the average local interstellar radiation field), the
main result of the adoption of the higher oxygen binding energy is that H$_2$O
can form on grains at lower visual extinction $A_{\rm V}$, closer to the cloud
surface. A higher binding energy of O results in more formation of OH and
H$_2$O on grains, which are subsequently desorbed by FUV radiation, with
consequences for gas-phase chemistry. For higher values of $n$ and $G_0$, the
higher binding energy can lead to a large increase in the column of H$_2$O but
a decrease in the column of O$_2$.
