Noncascade relaxation of photoexcited electrons on ionized donor centers has been observed in silicon doped by arsenic (Si:As) at low temperatures. Emission spectra of the Si:As terahertz intracenter laser give evidence of specific channels for the electron relaxation through low-lying donor states. The dominating relaxation channels strongly depend on the initial energy distribution of the nonequilibrium carriers. A relaxation step may exceed not only the energy gap to an adjacent lower-lying donor level but also the characteristic energy step as set by the energy and momentum conservation requirements for intravalley acoustic phonons.
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