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599The temperature dependent lineshape of luminescence lines which are due to inelastic scattering of electrons with free excitons is calculated. The results are compared with experimental findings in ZnO, CdS, CdSe, the copper halides, and, especially, with new experiments on CuBr. For the latter substances approximate values of electron and hole masses are given: me = 0.5m0, mh = 2.0m0 for CuCl, me = 0.28m0, mh = 1.4mo for CuBr, and me = 0.3m0, mh = 2.4m0 for CuI. In CuBr a new line is found which may be due to exciton-hole or exciton-exciton intraband scattering.Die temperaturabhtingige Linienform von Lumineszenzlinien, die ihren Ursprung in der inelastischen Streuung von Elektronen mit freien Exzitonen haben, wird berechnet. Die Resultate werden mit experimentellen Befunden an ZnO, CdS, CdSe, den Kupferhalogeniden und im besonderen mit neuen Experimenten an CuBr verglichen. Fiir die effektiven Elektron-und Lochmassen der Kupferhalogenide werden folgende Werte angegeben: me = = 0,5%, mh = 2,0m0 in CuCl, me = 0,28%, mh = l,4mo in CuBr und me = 0,3%, mh = = 2,4m0 in CuJ. In CuBr wird eine neue Emissionslinie gefunden, die als Exziton-Lochoder Exziton-Exziton-Intrabmdstreuung interpretiert werden kann. IntrodnotionNear their band gap, semiconductors show some new luminescence lines, if the excitation intensity is increased. In order to identify the origin of these lines, one may study their energetic position, lineshape, and integrated intensity as functions of excitation intensity, temperature, and pressure. For T > 70 K luminescence lines are observed in spontaneous and stimulated emission, which shift strongly to lower quantum energies if the temperature is increased. Usually, these emission lines are explained as due to inelastic scattering of electrons with free excitons [l to 61.Let us discuss the following process shown in Fig. 1: An excitonic polariton with momentum K interacts via Coulomb interaction with an electron, having an initial momentum k. The polariton scatters to a final state Q and an energy tzo which is smaller than the resonance energy Am, = E , -EE= (Eg being the gap energy, EEx the exciton binding energy). Thus the polariton may leave the crystal with a certain probability as a luminescence photon. Since energy and momentum are conserved in the scattering process, the electron is scattered to a final state q, obeying q = k + K -Q.

CuBr nanocrystals ͑NC's͒ embedded in a borosilicate glass matrix have been studied by transmission pump-and-probe experiments with both spectral and temporal resolution. Transient as well as persistent spectral hole-burning phenomena are observed in the excitonic absorption of small NC's ͑mean radius less than 4 nm͒ when picosecond and nanosecond dye-laser pulses excite selectively NC's of a specific size at low temperatures. The holes consist of a zero-phonon line ͑at the photon energy of the excitation͒ and marked phonon sidebands. This structure is explained by a strong exciton-optical-phonon interaction. A value of about 1 of the Huang-Rhys factor S is obtained from the Stokes shifts between absorption and photoluminescence spectra. From the width of zero-phonon holes, a lower limit of 2.3 ps of the exciton dephasing time T 2 is deduced. This value is compared to the one obtained by transient four-wave-mixing measurement ͑6.4 ps͒ at low excitation intensities. When the radii of NC's decrease, both types of spectral holes become broader and the persistent spectral-hole burning is more efficient.