  We measure magnetization changes in a single crystal of the single-molecule
magnet Fe8 when exposed to intense, short (<20 $\mu$s) pulses of microwave
radiation resonant with the m = 10 to 9 transition. We find that radiation
induces a phonon bottleneck in the system with a time scale of ~5 $\mu$s. The
phonon bottleneck, in turn, drives the spin dynamics, allowing observation of
thermally assisted resonant tunneling between spin states at the 100-ns time
scale. Detailed numerical simulations quantitatively reproduce the data and
yield a spin-phonon relaxation time of T1 ~ 40 ns.
