Electronic Spectra of Crystalline NaCl and KCl

The real (${\epsilon }_1$) and imaginary (${\epsilon }_2$) parts of the dielectric response function, together with the energy-loss function Im ($\frac{1}{\hat{\epsilon }}$), of freshly cleaved single crystals of NaCl and KCl were obtained in the region 5-28 eV by Kramers-Kronig analysis of near-normal-incidence reflectance spectra. Data were obtained over the entire region at room temperature, and from 5-11 eV at 77°K. The low-temperature spectra of both materials reveal a definite distinction between exciton and interband spectra. Fine structure corresponding to the $n=1\mathrm{}$ and $n=2\mathrm{}$ members of the ${\Gamma }_{15}\left(\frac{3}{2}\right)$ and ${\Gamma }_{15}\left(\frac{1}{2}\right)$ exciton series were clearly resolved at 77°K, using a spectral resolution of 2Å. The oscillator strength available for interband transitions has been largely exhausted below 15 eV in both materials, and the reflectance and dielectric response have low magnitudes at higher energies. Plasma-resonance effects occur in both materials, and were identified from the $\mathrm{Im}\left(\frac{1}{\hat{\epsilon }}\right)$ function. The optical data are in good agreement with directly measured electron energy losses. The ${\epsilon }_2$ spectrum of NaCl bears a strong resemblance to that of MgO, and an attempt was made to analyze the observed structure in terms of a band structure similar to that calculated for MgO. On this basis, nearly all of the main spectral features below 14 eV could be identified in terms of direct interband transitions. Strong transitions producing $M_0$ edges were assigned to the symmetry points $\Gamma$, $L$, and $X$. A peak at 11.2 eV results from near degeneracy of the ${\Delta }_5\to {\Delta }_1$ and ${\Sigma }_4\to {\Sigma }_1$ transitions. The band gap for the lowest-energy direct transition (${\Gamma }_{15}\to {\Gamma }_1$) was estimated to be 8.97±0.07 eV at 77°K. The spectrum of KCl is more complex than that of NaCl, because of the low-lying $d$-like states in the conduction-band structure. The $X_3$ level lies below $X_1$, and the exciton associated with it (excited from ${X_5}^{\prime }$) produces the temperature-dependent peak at 9.38±0.03 eV (77°K). The band gap (${\Gamma }_{15}\to {\Gamma }_1$) lies at 8.69±0.07 eV at 77°K, and the remainder of the analysis is analogous to that for NaCl and MgO.