4C1 - Nonlinear effects in an NMR experiment

We have studied multiple-photon transition in a low-field nuclear magnetic resonance (NMR) experiment. It is well known that a saturation of the NMR occurs when using a sufficiently high alternating magnetic field which induces multiple-photons transitions. We show theoretically that these multiphoton transitions are accompanied by an harmonic generation. For a -photon transition, this generation occurs mostly on the th and ( )th harmonic. For an irradiating field t of pulsation ω such that where H0is the steady field and θ is the angle between H0and H1the amplitudes and of these components are given by , . We have verified these equations in a low-field NMR experiment ( Gs/s), polarizing first a flowing liquid in a high magnetic field. The liquid then flows in a modified Bloch spectrometer. The receiving coil is still perpendicular to the steady field but it is possible to adjust the angle θ between the axis of the emission coils (H1field) and the steady-field H0. Using a synchronous detection at the output of the receiving coil on the pulsation , or , we have directly detected multiphoton transistions. The above equations have been verified for to . There are slight discrepancies at high excitation which may be explained taking into account a large saturation.