Measurement of spin-lattice relaxation times on a high resolution CW NMR spectrometer using audio modulation pulses

Abstract

Nuclear magnetic resonance lines with relatively short spin-spin relaxation times ( T 2) may be inverted for the purpose of measuring their spin-lattice relaxation times ( T 1) by applying a large audio frequency field modulation pulse to a high resolution sideband detection NMR spectrometer. The frequency, amplitude, and length of such a pulse is evaluated and the trajectory of the magnetization as it is inverted is approximately calculated. It is found, in agreement with Anderson's cw result, that the spins invert in a time π γ H 1 J 1 (β), as if a field H 1 J 1 (β) had been applied, where H 1 is the actual rotating rf field amplitude, J 1 is the first Bessel function and β equals γH m ω m , where H m is the modulation amplitude and ω m is the modulation angular frequency. The implementation of this technique on a Varian HA-100 spectrometer equipped with a Varian Spectro System accessory is described. Examples of the use of the technique are given. One example is on a sample with a T 1 of 342 msec.