We have investigated the effects of isotopic composition on the band gap of CuCl on a series of samples made out of the stable isotopes 63 Cu, 65 Cu, 35 Cl, and 37 Cl. Besides specimens containing elements with the natural abundances, we have measured samples with monoisotopic sublattices as well as artificial mixtures of isotopes. With nonlinear ͑two-photon absorption, second-harmonic generation͒ and linear ͑luminescence͒ optical spectroscopy we find that the fundamental gap of CuCl increases by 364͑18͒ eV/amu when increasing the Cl mass. However, it decreases by 76͑5͒ eV/amu when increasing the Cu mass. Using a two-oscillator model for the lattice dynamics of CuCl we show that these rates are consistent with the anomalous increase of the band gap with increasing temperature. These effects can be traced back to the strong p-d mixing in the copper halides. From the temperature dependence of the band gap of CuBr we also estimate the changes of its gap with isotopic composition.

We report the first high resolution photoluminescence studies of isotopically pure Si (99.896% (28)Si). New information is obtained on isotopic effects on the indirect band gap energy, phonon energies, and phonon broadenings, which is in good agreement with calculations and previous results obtained in Ge and diamond. Remarkably, the linewidths of the no-phonon boron and phosphorus bound exciton transitions in the (28)Si sample are much narrower than in natural Si and are not well resolved at our maximum instrumental resolution of approximately 0.014 cm(-1). The removal of the dominant broadening resulting from isotopic randomness in natural Si reveals new fine structure in the boron bound exciton luminescence.

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The dependence of the E 0 direct gap of Ge, GaAs, and ZnSe on isotopic masses at low temperatures has been investigated. Contributions of the variation of the lattice parameter to the gap shift of the binary compounds have been evaluated by using a volume-dependent lattice dynamics, while local empirical pseudopotential techniques have been employed to calculate gap shifts due to electron-phonon interaction. The dependence of these terms on the lattice-dynamical model and on the q→0 extrapolation of the pseudopotential form factors has been investigated. The contributions of the optical and acoustical modes to the isotopic shift are analyzed. The results are compared to previous experimental data, in the case of germanium, and to lowtemperature reflectance measurements performed as part of this work on GaAs samples with different isotopic gallium composition. Particular attention has been paid to the differences in the effects of changing either the cation or the anion masses. The temperature dependence of the E 0 gap of ZnSe has also been calculated, and reasonable agreement with experiment has been found. ͓S0163-1829͑96͒02032-2͔