Excitation spectroscopies of impurities in CdSe

Two kinds of excitation spectroscopies, namely, resonant Raman scattering and photoluminescence excitation spectroscopy, have been used to study impurities in CdSe. At excitons bound to neutral donors we observe enhancement of a Raman mode at 109 ${\mathrm{cm}}^{-1\mathrm{}}$ which we identify as due to electronic Raman scattering in which the donor is excited from the $1S$ to the $2S$ state. At excitons bound to neutral acceptors in CdSe we observe enhancement of two new Raman peaks at 189 and 257 ${\mathrm{cm}}^{-1\mathrm{}}$, respectively. The former is identified as an electronic Raman mode of the acceptor in CdSe in which the hole is excited from the ground state (derived from the ${\Gamma }_9$ valence band of CdSe) to an excited state (derived from the ${\Gamma }_7$ valence band). The peak at 257 ${\mathrm{cm}}^{-1\mathrm{}}$ is tentatively identified as a local vibrational mode associated with substitutional Na. We also observe enhancement of four donor-acceptor pair transitions in CdSe when they are resonantly excited (other authors have labeled such pair transitions as selectively excited). Two such resonantly excited pair transitions involve excited states of the donor and acceptor in CdSe. The remaining two are assigned to phonons. A qualitative theory is also proposed to explain the dependence of the enhancement on excitation frequencies.