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 an-photon transition, this generation occurs mostly on thenth and (n pm 1)th harmonic. For an irradiating field2H_{1} cos omegat of pulsation ω such thatnomega= gamma H_{0}=omega_{0}where H0is the steady field and θ is the angle between H0and H1the amplitudesSmin{n}max{n}andSmin{n}max{n+1}of these components are given bySmin{n}max{n} = f_{n}(theta) (H_{1})^{n},Smin{n}max{n+1} = cos theta f_{n}(theta) (H_{1})^{n+1}. We have verified these equations in a low-field NMR experiment (H_{0} = 0.7Gs/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(n - 1) omega, nomega, or(n + 1) omega, we have directly detected multiphoton transistions. The above equations have been verified forn = 2ton = 5. There are slight discrepancies at high excitation which may be explained taking into account a large saturation.