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