  Using a quantum-mechanical close-coupling method, we calculate cross sections
for fine structure excitation and relaxation of Si and S atoms in collisions
with atomic hydrogen. Rate coefficients are calculated over a range of
temperatures for astrophysical applications. We determine the
temperature-dependent critical densities for the relaxation of Si and S in
collisions with H and compare these to the critical densities for collisions
with electrons. The present calculation should be useful in modeling
environments exhibiting the [S i] 25 {\mu}m and [S i] 57 {\mu}m far-infrared
emission lines or where cooling of S and Si by collisions with H are of
interest.

Atomic Physics;Cosmology and Nongalactic Astrophysics