Abstract
We present time-resolved measurements of the relaxation between the orbital states of the shallow acceptor boron in silicon. The silicon host was enriched Si-28, which exhibits life-time broadened absorption lines. We observed a wide range of T1 lifetimes from 6ps to 130ps depending on the excited state and the pump intensity. The fastest transients have not been observed previously in the time domain, and they are caused by the phonon relaxation responsible for the small-signal frequency domain line-width. We identify the slower components with an ionisation/recombination/cascade pathway.