Tunnel Spectroscopy by Dipolar-Driven NMR

Abstract

At low static fields nmr transitions are driven at least in part by the dipole-dipole interactions being modulated by the applied rf field. The result is that the Δm = 2 transition is comparable in intensity with the main Δm = 1 transition. More importantly, transitions which are symmetry forbidden at high field may become allowed at low field. This applies to the tunnelling sidebands of CH₃ and NH₄ groups observed at low temperatures, where the transitions involve a change in proton permutation symmetry. By carrying out nmr in fields of the order of 0-0.02 T tunnel sidebands are easily observed. This has opened a new window of tunnel spectroscopy for tunnel frequencies from 40 kHz - 1 MHz. Several examples will be shown. To maintain reasonable sensitivity, a field cycling procedure is used in which the initial polarisation and final measurement are carried out at high fields. Apart from measuring tunnel frequencies, a number of interesting and unexpected features have been observed. The small tunnel frequencies accessible to this technique are characteristic of CH₃ groups terminating aliphatic chains. It therefore provides a sensitive probe of the environment of CH₃ groups in a very wide range of materials.