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1	Author	Christovam Mendonça, SaidR. Rabbani	Requires cookie*
	Title	Anharmonic Librations and the Temperature Dependence of NQR Frequencies
	Abstract	The temperature dependence of NQR frequencies of nuclei in molecules undergoing quasi-har-monic rotational oscillations is calculated. Following the model proposed by Bayer, the instanta-neous orientation of the electric field gradient principal axes is assumed to change from its average orientation as a result of rotational oscillations, while the EFG magnitude remains constant. The eigenstates and energy eigenvalues of a rotator in a potential of the form V(0)= V 2 0 2 — K 4 0 4 are calculated by standard perturbation methods, followed by the evaluation of statistical averages of square angular amplitudes. For molecular crystals with more than one molecule in a unit cell the angular displacements are expressed in terms of normal coordinates, and the same procedure is followed. The final expression for the change in the NQR frequency consists of two terms, the first one being related to the harmonic part of the potential and the second one to the quartic term. At the absolute zero it yields a nonzero contribution to <ö 2 >, and for high temperatures it reproduces the nonlinear character of v Q (T) observed experimentally. The model has been tested with measure-ments on solid Cl 2 performed by Nakamura and Chihara, combined with libration mode frequencies measured in Raman spectroscopy by Cahill and Leroi and analysed according to our anharmonic libration picture. The fitting of our equation to experimental data supports Cahill and Leroi's hypothesis about the existence of an undetected libration mode in Cl 2 Raman spectra of frequency near 60 cm"
	Reference	Z. Naturforsch. 51a, 698—704 (1996); received November 20 1995
	Published	1996
	Keywords	NQR, Bayer's Model, Anharmonic Vibrations
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	TEI-XML for	default:Reihe_A/51/ZNA-1996-51a-0698.pdf
	Identifier	ZNA-1996-51a-0698
	Volume	51

2	Author	Christovam Mendonça, SaidR. Rabbani	Requires cookie*
	Title	Anharmonic Lattice Vibrations and the Temperature Shift of Raman Spectral Lines
	Abstract	The temperature dependence of the average lattice mode frequency of a molecule undergoing librational motion in a quasi-harmonic potential has been calculated with the purpose of finding an explicit relationship between the observed shift of Raman spectral lines with temperature and the anharmonic term in the rotational potential. Calculations were carried out both for uncoupled and coupled oscillators. The equations obtained with this model provide good fittings for solid Cl 2 and benzene data. This result can be applied to the analysis of the temperature dependence of NQR frequencies in molecular crystals.
	Reference	Z. Naturforsch. 51a, 716—720 (1996); received November 20 1995
	Published	1996
	Keywords	Vibrational Spectroscopy, Raman Spectra, Anharmonic Vibrations, NQR
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	TEI-XML for	default:Reihe_A/51/ZNA-1996-51a-0716.pdf
	Identifier	ZNA-1996-51a-0716
	Volume	51