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.

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The direct observation of free A, B, and C excitons with n=2 in hexagonal bulk gallium nitride (GaN) by two-photon spectroscopy is reported. From these data, the band gaps, exciton binding energies, and hole masses for the three uppermost valence bands are calculated.

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