Overhauser studies in two-spin systems

Abstract

Expressions have been derived for fractional changes in intensity of double resonance transitions for weak irradiation of two spin systems for random field and dipole-dipole interactions. The relative fractional changes are shown to be independent of the strength and small offsets of the irradiation and the strength of the observing field. The fractional changes are found to be quite sensitive to the relaxation mechanisms, thereby allowing a simple interpretation of the observed fractional changes in terms of the relaxation mechanism operative in the spin system. Overhauser experiments have been performed on the AX spin system of 2,2-dichloroacetaldehyde, studied in different sample conditions and the AB spin system of 1-bromo-1-chloroethylene. All the degassed samples have been shown to have contributions from both random field and dipole-dipole interactions, the magnitudes of which have been obtained in each case. An analysis of sources of errors in these experiments indicates that even for strengths of irradiation much smaller than the observed linewidths in the spectra, splittings of the connected transitions are important for quantitative estimates of the intensities. The diagonal elements of η, the change in density matrix due to the observing field, are shown to be negligible in the present experiments.