Far infrared optical properties of polycrystalline alkali halogenates

Abstract

The infrared spectra of polyatomic ionic substances contain two main types of fundamental vibrational modes in the infrared region; internal modes due to the vibrations within the polyatomic ionic species, and external modes due to the lattice vibrations. Some of the internal modes and all of the external modes of NaClO3 , NaBrO3, KCl03 and KBrO3 have been analyzed and discussed in this dissertation. Since these alkali halogenates were found to be highly absorbing solids in the infrared region, a polycrystalline thin film that was approximately 1.0 micron thick was used in obtaining transmission spectra. These films were deposited on high density polyethylene substrates by using a solvent evaporated technique. A critical evaluation of their uniformity, thickness and amount of infrared scattering has been made. Quantitative far infrared transmission spectra, i.e. containing a precision of 0.5% in final transmission values, were calculated by performing a Fourier transformation on an experimental interferogram obtained from a R.I.I.C. FS720 interferometer. This method involving a computer program (described in the appendix), was examined for various types of systematic errors and the results were compared with those obtained from a conventional type of grating spectrometer. After obtaining a reliable transmission spectrum, the task of calculating the optical indices, n and k, associated with a specific polycrystalline sample, was accomplished by using either a classical dispersion, self-bracketing-search (SBSTR) technique or by using a Kramers-Kronig (KK) relationship. Either of these methods employed a single set of transmittance values in order to calculate the values of n and k. The SBSTR method was based on the systematic adjustment of the oscillator strength, Sj, damping constant, γj, and the resonant frequency, υj, according to their relationship in the independent damped harmonic oscillator (DHO) model. The SBSTR computer program minimized the integrated square of the deviation between the calculated transmittance spectrum and the experimental transmittance spectrum. The resultant values of Sj, γj, υj were used in the discussion of optical properties of these polycrystalline alkali halogenates. The υj values were correlated with group theory in order to assign symmetry types to respective lattice modes. Translational or rotational attributions were assigned to the respective modes by comparing their relative intensities. Absolute intensities and dipole moment derivatives were calculated for the υ2 and υ4 internal vibrational modes in the KBrO3 and NaBrO3 polycrystalline substances, the υ4 internal vibrational modes of NaClO3 and KClO3 , and all the lattice modes of each substance that were observed in the far infrared, i.e., five lattice modes in NaClO3 , seven lattice modes in NaBrO3 , four lattice modes in KClO3 and three lattice modes in KBrO3. The deviation between the experimental lattice spectra of the alkali halogenates and the DHO model was shown to be surprisingly small in comparison to the deviation found in similar lattices of LiF and MgO. These diatomic lattices exhibit a considerable amount of phonon-phonon coupling which was found to be absent in the case of the alkali halogenates. In addition to these unusual results, the alkali halogenate spectra contained lifetimes of the excited states that were generally an order of magnitude shorter than those found for LiF and MgO. This phenomena was observed by comparing the damping constants of these substances. This rapid dissipation of energy in the alkali halogenate crystalline lattice is discussed in terms of internal and external combination vibrational modes. These combination modes were directly observed on mid-infrared transmission spectra of thin NaClO3 and NaBrO3 platelets.