Infrared optical properties and vibrational behavior of anisotropic crystals: Orthorhombic Ba

Abstract

The infrared (ir) optical properties and vibrational behavior of anisotropic absorbing crystals in spectral regions including strong resonances are considered. In particular, the optical parameters, namely the complex dielectric tensor and refractive indexes, of barium nitroprusside trihydrate (BNP), Ba[Fe(CN${)}_{5}$NO]\ensuremath{\cdot}${3\mathrm{H}}_{2}$O (orthorhombic, space group ${C}_{2v}^{5}$), in the range 1500--2300 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, which includes the strongly polar NO stretching mode, are determined. To this purpose, transversal electric (TE) reflectance data on (001) planes of BNP in the spectral range 250--4000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ were analyzed by the Kramers-Kronig method. The values obtained for the optical parameters were employed to get transversal (TO) and longitudinal (LO) frequencies of the NO optic stretching mode of symmetry species ${B}_{1}$ (polarized along the crystal a axis) and to show the impossibility of obtaining sharp peaks for accurate frequency measurement of the TO mode by transmission spectroscopy because too-thin crystals films (less than 1 \ensuremath{\mu}m thick) would be required. The dielectric tensor was employed to predict correctly the position and shape of the NO reststrahlen band due to the LO mode ${B}_{1}$ in the reflectance spectrum from the (100) face.This band occurs only for oblique incidence and transversal-magnetic (TM) polarization of the ir beam. The computed TM-polarized, off-axis transmittance spectra through bc crystal plates of BNP in the NO stretching region are in good agreement with spectroscopic data and exhibit a sharp, strong absorption band at 1982 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ which is due to the coupling of the LO mode ${B}_{1}$ with the ir radiation. Our results show that strongly absorbing LO modes polarized perpendicularly to the plate can be easily identified and their frequencies measured accurately in relatively thick samples of an anisotropic substance. Finally, a least-squares fit to reflection data of the calculated TE reflectance on a (001) plane of BNP, derived from a damped harmonic-oscillator model for the dielectric constant along a, leads to a value for the transition strength of the NO stretching which is in accordance with values obtained by independent spectroscopic measurements.