EFFECTS OF THE ISOTOPIC COMPOSITION ON THE FUNDAMENTAL GAP OF CUCL

We have investigated the effects of isotopic composition on the band g ap of CuCl on a series of samples made out of the stable isotopes (CU) -C-63, (CU)-C-65, Cl-35, and Cl-37. Besides specimens containing eleme nts with the natural abundances, we have measured samples with monoiso topic sublattices as well as artificial mixtures of isotopes. With non linear (two-photon absorption, second-harmonic generation) and linear (luminescence) optical spectroscopy we find that the fundamental gap o f CuCl increases by 364(18) mu eV/amu when increasing the Cl mass. How ever, it decreases by 76(5) mu eV/amu when increasing the Cu mass. Usi ng a two-oscillator model for the lattice dynamics of CuCl we show tha t these rates are consistent with the anomalous increase of the band g ap with increasing temperature. These effects can be traced back to th e strong p-d mixing in the copper halides. From the temperature depend ence of the band gap of CuBr we also estimate the changes of its gap w ith isotopic composition.