Narrowing of nuclear magnetic resonance lines in a double frequency RF field

Abstract

The effect of a strong transverse field of frequency ω₁ and amplitude H₁ on the dipolar line width of the nuclear magnetic resonance signal, has been calculated. The resonance signal is supposed to be the response of a weak transverse rf field varying with frequency ω₂. By choosing the value of ω₁ such that γH₁=√2(ω^--ω₁) where \barω is the average Larmor frequency of spins with the same gyromagnetic ratios γ, it is shown that in solids for γH₁≫C_st where C_st is the frequency corresponding to the static dipolar line width in the absence of the strong rf field, the width can be reduced to the order of magnitude C_st²/γH₁. In the case of liquids with 1/τ_c≫C_st, where τ_c is the correlation time of the molecular fluctuations, the line width is already reduced even with only the weak rf field by a factor τ_cC_st compared to that of solids; and to achieve further narrowing the same condition for ω₁ as in solids is required and a still stronger condition γH₁≫1/τ_c for H₁.