مرکز منطقه ای اطلاع رساني علوم و فناوري - 4C1

Abstract :

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 $n$ -photon transition, this generation occurs mostly on the $n$ th and ( $n \\pm 1$ )th harmonic. For an irradiating field $2H_{1} \\cos \\omega$ t of pulsation ω such that $n\\omega = \\gamma H_{0}=\\omega _{0}$ where H0is the steady field and θ is the angle between H0and H1the amplitudes $S\\min{n}\\max {n}$ and $S\\min{n}\\max {n+1}$ of these components are given by $S\\min{n}\\max {n} = f_{n}(\\theta) (H_{1})^{n}$ , $S\\min{n}\\max {n+1} = \\cos \\theta f_{n}(\\theta) (H_{1})^{n+1}$ . We have verified these equations in a low-field NMR experiment ( $H_{0} = 0.7$ 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 $(n - 1) \\omega , n\\omega$ , or $(n + 1) \\omega$ , we have directly detected multiphoton transistions. The above equations have been verified for $n = 2$ to $n = 5$ . There are slight discrepancies at high excitation which may be explained taking into account a large saturation.