A snowflake's chance in heaven
  We consider the survival of solid H2 in the diffuse interstellar medium, with
application to grains which are small enough to qualify as dust. Consideration
of only the thermal aspects of this problem leads to the familiar conclusion
that such grains sublimate rapidly. Here we show that charging plays a critical
role in determining the sublimation rate, because an electric field helps to
bind molecules to the grain surface. A key aspect of the charging process is
that the conduction band of solid hydrogen lies above the vacuum free-electron
energy level, so low-energy electrons cannot penetrate the solid. But they are
attracted by the dielectric and by positive ions in the matrix, so they become
trapped in vacuum states just above the surface. This charge-separated
configuration suppresses recombination and permits overall neutrality, while
supporting large electric fields at the surface. Charging ceases when the
potential energy just outside the electron layer coincides with the conduction
band minimum. By that stage the heat of sublimation has increased tenfold,
effecting a huge reduction in the sublimation rate. Consequently hydrogen
grains may be able to survive indefinitely in the diffuse ISM. There are good
prospects for identifying H2 grains, if they exist, as fully localised surface
electrons should exhibit discrete energy levels, with a corresponding spectral
line signature.
