General Aspects of NQR and Temperature

In the discussion of NQR results in the frame of the Townes-Dailey theory, the influence of temperature on NQR frequencies is normally ignored. The temperature dependence of NQR spectra is considered here as a crystal field effect and in the following we shall discuss some of its aspects. 

Most Of the NQR spectra have been investigated at 77 °K. There are many reasons for choosing this particular temperature as the standard temperature in this branch of spectroscopy. (The standard pressure is 1 atm). 

NQR is intrinsically a solid state experiment. At 77 °K all except a very few elements and compounds are in the solid state and therefore accessible to NQR experiments as long as the substance incorporates nuclei with / 1/2. 

The spin-lattice relaxation of nuclei with / 1 /2 is mainly due to nuclear quadrupole relaxation and the relaxation time at a temperature of 77 °K is still fairly short (of the order of milliseconds for many nuclei of interest to chemists). Therefore the experimental methods usually applied do not involve relaxation problems. The full RF fields available — for instance, with a superregenerative NQR spectrometer — may be applied successfully, resulting in an improved signal-to-noise ratio for the resonance at 77 °K as compared with room temperature. There are exceptions to this rule (e.g. 14N NQR spectroscopy where saturation problems come into play at 77 °K). 

At 77 °K many thermal motions of the molecules in the solid, still excited at room temperature, are frozen in. The amplitudes of the vibrations are small and the temperature coefficient a is quite often found to be smaller than at room temperature (see Fig. IV. 1). 

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