  We develop a computationally inexpensive model to examine the dynamics of
boson-assisted electron relaxation in solids, studying nonequilibrium dynamics
in a metal, in a nodal superconductor with a stationary density of states, and
in a nodal superconductor where the gap dynamically opens. In the metallic
system, the electron population resembles a thermal population at all times,
but the presence of even a fixed nodal gap both invalidates a purely thermal
treatment and sharply curtails relaxation rates. For a gap that is allowed to
open as electron relaxation proceeds, effects are even more pronounced, and gap
dynamics become coupled to the dynamics of the electron population. Comparisons
to experiments reveal that phase-space restrictions in the presence of a gap
are likely to play a significant role in the widespread observation of
coexisting femtosecond and picosecond dynamics in the cuprate high-temperature
superconductors.
