The Raman spectrum of the ordered phase of NH4Cl and ND4Cl: Dipole and polarizability derivatives

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Abstract
The polarized Raman spectra of NH4Cl and ND4Cl have been observed at temperatures of the order of 25–35 K such that the crystal is in the ordered Td phase. The transverse, longitudinal frequency separations for modes ν3, ν4, ν5 (the lattice translation) and the Fermi resonant combination ν2+ν4 in NH4Cl have been used to deduce the infrared intensity parameters ∂μ/∂Q. Then, using these quantities together with an estimate of the electronic contribution to the nonlinear susceptibility, and measurements of the relative intensities of longitudinal, transverse Raman line pairs, we calculate ∂α/∂Qj for j=3, 4, 5. Since only the mode j=5 is well determined by this procedure, the final values of the polarizability derivatives are based upon mode 5 as an internal standard along with the intensities of the other modes relative to 5. The theory is developed on the assumption of additive molecular polarizabilities, and the ∂α/∂Qj values reported are those for vanishing local field. Both the ∂μ/∂Q and ∂α/∂Qj parameters appear to satisfy simple isotopic intensity rules. The value found for ∂α/∂Q1 is about 0.4 of the corresponding value in gaseous CH4. Several combination and overtone bands are partially analyzed using polarization, and the components of 2ν6 are shown to be in excellent agreement with theory.