Terahertz intracenter photoluminescence of silicon with lithium at interband excitation

Terahertz radiation has been revealed at interband photoexcitation of lithium-doped silicon crystals at liquid helium temperatures. It has been shown that the lines caused by optical transitions of electrons from the $2P$ excited states of lithium centers to the $1S(A_1)$ state of the impurity prevail in the radiation spectrum. The strong suppression of terahertz radiation associated with transitions to the lowest state of the donor $1S(E+T_2)$ as compared to radiation associated with transitions to the $1S(A_1)$ state is explained by the reabsorption of radiation. The radiation spectrum also includes weaker terahertz lines, which can be attributed to the intracenter transitions in donors that are caused by Li-O complexes. The radiation spectrum also exhibits lines at ${\sim}\,12.7$ and ${\sim}\,15.3\,$meV, which are possibly due to intraexcitonic radiative transitions and transitions from continuum to the ground state of excitons.