Vibrational spectra of transition metal hexafluoride crystals. II. Two-particle and mixed crystal spectra as techniques for determination of densities

In part II of this series two techniques for obtaining densities of exciton states are discussed: heavily doped crystals and two-particle overtone and combination bands. It is demonstrated through Raman spectra and calculations that (ν_i + ν₁) combination bands yield very nearly true density of states functions for ν_i in the case for which ν₁ is essentially dispersionless. The mixed crystal method for density of states determinations is compared to the combination band technique and approximate mixed crystal concentrations appropriate for such studies can be calibrated for individual bands. It is pointed out that the overtone method, whenever applicable, is both simpler and more accurate for exciton state studies. Detailed analyses of ν₁ and 2ν₁ show that the major contribution to overtone intensity comes from the second order polarizability derivative and not anharmonic contributions.