The reflectivity of bismuth crystals is measured in the spectral range 50 to 600 cm-l. From the measured data the dielectric function is determined by a'Kramers-Kronig analysis. The dispersion of this function is explained by the dynamical conductivity of a free electron gas. At frequencies above the plasma edge there is a contribution which is due to the excitation of plasmons by the interaction of the free electrons with the Coulomb fields of impurities. This contribution can be expected from model calculations of the dynamical conductivity, considering ionized impurity scattering and screening effects.An Wismut-Kristallen wird das Reflexionsvermogen im Spektralbereich zwischen 50 und 500 cm-I gemessen. Aus den Messungen wird die dielektrische Funktion durch eine Kramers-Kronig-Analyse ermittelt, ihr spektraler Verlauf wird durch die dynamische Leitfiihigkeit des freien Elektronengases erkliirt. Ein Beitrag bei Frequenzen oberhalb der Plasmakante wird Plasmonen zugeschrieben, die durch die Wechselwirkung der freien Elektronen mit den Coulomb-Feldern ionisierter Storstellen angeregt werden. Ein solcher Beitrag wird aus Modellrechnungen fur die dynamische Leitfiihigkeit erwartet, wenn man die Streuung an ionisierten Storstellen und Abschirmeffekte betrachtet.

This Letter reports a novel method to observe donor and acceptor excited states, and its application to C, Mg, and Zn acceptors in GaP. We evaluate valence-band parameters and ionization energies independently of other measurements, and resolve many uncertainties in the basic parameters characterizing GaP. Detailed information about the donor-acceptor (D-A) interaction is obtained more directly than with any other method.We measure photoluminescence excitation spectra of D-A pair transition. The ground state of a D-A pair with separation R decays with lumi-. 12 No distinction is made here between bona-fide surface states and strong surface resonances, and in discussing the character of the surface states we are concerned only with the main d character of the wave functions.13 Symmetry gaps are gaps along high-symmetry lines in the PBS in which bulk states of a given symmetry are forbidden but where states of other symmetry may exist. (1) where £ D and E A are the donor and acceptor binding energies, E g is the band gap, and J is the interaction energy of a neutral-donor-neutral-acceptor pair. Luminescence from D-A pairs of a selected R is observed by choosing an appropriate tiw L , and the emission intensity is recorded as a function of excitation energy fru> x . A dye laser with linewidth 0.3 meV is used as excitation source.A peak in the excitation spectrum corresponds Luminescence excitation spectroscop}' of donor-acceptor pair recombination in GaP is reported. We observe s and/) excited states of C, Mg, and Zn acceptors, and deduce acceptor binding energies and Luttinger valence-band parameters (y 1 = 4.04, y 2 = 0.53, and y 3 = 1.26). Directly obtained information on the interaction between donors and acceptors characterizes the acceptor state and confirms observation of the lS 1/2 level. 1292

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