Preparation and use of laser polarized xenon

 

The main limitation of NMR spectroscopy resides in its inherent low sensitivity which results from the small involved energies. Indeed the nuclear Zeeman energy (ghB0), even in the largest static magnetic fields is always typically 10 000 smaller than the thermal energy (kT). One of the solutions to circumvent this low sensitivity consists in transferring the polarization from a more polarized system and then in working in a transient way. The solution, we explore for about 10 years now, uses circularly polarized photons as a source. This laser beam allows the selective depopulation of one of the two magnetic ground states of Rubidium atoms. The induced electronic polarization is transferred by cross-relaxation to xenon nuclear spins.

 


Experimental apparatus.
On the left the lasers and the optics. At the centre the pumping cell and the coils for creating the magnetic field, on the right the electronics and the pumping group.

 

Currently we typically reach polarization up to 50% inside the high field static magnetic field. This means that three out of four nuclear xenon spins point in the same direction. At thermal equilibrium in a 11.7T magnet the xenon polarization is equal to 1.1 10-5. Thanks to the optical pumping step an improvement by a factor ~45 000 is achieved.

 


Example of xenon signal enhancement obtained by optical pumping.

 

Combined to the chemical sensitivity of xenon, this large improvement allows two types of applications:

 

        Xenon NMR

o       Biosensing using laser polarized xenon

        Polarization transfer from xenon to other nuclei

o       Exploration of protein hydrophobic cavities by laser-polarized xenon

o       Non linear NMR using highly concentrated and polarized xenon page

 

 

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