Relaxation of the excited states of arsenic in strained germanium

The relaxation times of the lower p states of the arsenic donor in a germanium crystal strained along the [111] crystallographic direction are studied. Measurements are performed by the pump–probe method, using free-electron laser radiation. The states are excited from the $1s (\Gamma_1)$ ground state. The experimentally measured decay times of the $2p_0$, $3p_0$, and $2p_\pm$ states are 1.3, no more than 0.2, and 0.4 ns, respectively. It is shown that the relatively high relaxation rate of the $2p_\pm$ state is defined by the interaction with intravalley TA photons.