Non-linear NMR using highly polarized and concentrated xenon


When concentrated, highly polarized xenon induces large average dipolar field experienced by all spins. This magnetic field results from the superposition of distant dipolar interactions which are not averaged out by Brownian motions. It allows polarization transfer as we have recently shown. The shape of the xenon NMR is also modified with the appearance of narrow lines superimposing on the main resonance, broadening of the lines,…


Example of a clustering effect.
Due to the distant dipolar field resulting from high polarization and high xenon concentration, the xenon resonance line is no longer a well behaved Lorentzian but the shape (linewidth structure) becomes much more complicated.


Non linear effects as MASER can also be observed when xenon is polarized with a negative spin temperature.


Example of a spontaneous burst of rf irradiation (MASER).
Highly polarized and concentrated xenon with negative spin temperature is dissolved in cyclohexane and inserted inside the NMR coil. No rf excitation is applied.


In fact we have recently observed and reported the unexpected multiple maser emissions, although polarized xenon is prepared in batch modes. We have been able to show that they are chaotically initiated by noise and lead to a spatial inhomogenenous organization of the xenon magnetization.


Example of multiple chaotic maser emissions.
a Time-dependence of the 129Xe signal (FID) monitored during more than 8.5 min. b This insert corresponds to the first maser; it illustrates the existence of frequency beats even for the largest magnetization. c Fourier transform of the first maser in absolute value mode. The existence of individual narrow peaks is clearly visible. d Same representation as in c but for the last emission. Due to the long duration of this emission the linewidth is very narrow, FWMH = 0.103 Hz, a value incompatible with the static magnetic field homogeneity. e Fourier transform of the modulus of the signal acquired during the last emission. The much narrower linewidth, FWMH = 0.034 Hz, illustrates the fact that the FID exhibits a frequency sweep during the emission, resulting from the decrease of the average dipolar field. For this experiment, the sample contained 80 mmol.L-1 of xenon polarized at about 27% with a negative spin temperature.


All these effects are at the centre of the DIPOL project, financed by ANR, run in collaboration with P.J. Nacher’s team at LKB. A recent collaborative study has led to the development of a characterization of these distant dipolar fields thanks to a selective two-pulse sequence combined with gradients. Numerous echoes were observed and the results confronted to extensive numerical simulations. The latter reveals the key importance of finite sample size effects.


Series of echoes observed on a dissolved laser-polarized 129Xe sample.
These echoes result from the presence of large distant dipolar fields due to large magnetization which allow the refocussing of magnetization. The use of gradient pulses allows one to distinguish them and to probe different characteristic length-scale.


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