Transcript Host-Guest Inclusion Complexes Ordered within Thermotropic

Preparation of Laser-Polarized Xenon at High Xe
Densities and High Resonant Laser Powers Provided
by Volume Holographic Grating-Narrowed LDAs
Boyd M. Goodson1, Nicholas Whiting1, Panayiotis
Nikolaou1, Neil Eschmann1, Michael J. Barlow2
1Department
of Chemistry and Biochemistry
Southern Illinois University, Carbondale
2Sir
Peter Mansfield Magnetic Resonance Centre
University of Nottingham, UK
DAMOP 2009
U. Virginia
OP Limitations: [Xe]cell and Laser Output
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Highest PXe requires low [Xe]cell (<50 torr)
 Xe / Rb collisions  reduced PRb, capping PXe
 Due to interplay of PXe, [Xe]cell, overall NMR
signal reaches steady-state (increases in [Xe]cell
compensated by PXe losses).
 Hard to achieve simultaneously high PXe and
[Xe]cell  limiting some applications.
Typical LDAs: high powers, low costs
(but) broad, uneven lineshapes.
 Inefficient use of laser output.
 Collision broadening (high p, T)
Frequency-narrowed LDAs.
 External Cavities
 Volume Holographic Gratings
e.g., Meersmann @ co-workers
JCP 2003.
Volume Holographic Grating (VHG)-Narrowed LDAs
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Bulk slabs of photosensitive glass
with Bragg planes of varying (ni)
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Retro-reflects narrow emission
band into laser elements, forcing
lasing at injected l.
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Narrows LDA output with
high efficiency, tolerance
High power and narrow Dl
(at low cost, w/ high ease of
use)
 more efficient absorption
under milder OP conditions
 higher resonant laser fluxes
‘Tunability’ of a ‘Standard’ VHG-LDA
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‘VL1’
(Spectra-Physics
Comet)
~26 W (50 A),
FWHM=0.27 nm
VL1 l-offset can be
‘tuned’ by varying
driving current.
VL2
(Spectra-Physics
Integra);
~55 W (96 A),
FWHM=0.49 nm
VL1
OP Apparatus
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Var. gas density/composition available for
loading via custom manifold.
Nominal conditions: 300 torr Xe,
backfilled with N2 to ~2,000 torr total.
T: ~70-120 oC, t ~5-15 min.
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Cell: Rosen et al., Rev. Sci. Instrum., 70, 1546 (1999).
PXe measured
via NMR at
9.4 T
Saha, Nikolaou, Whiting, Goodson, Chem. Phys. Lett., 428, 268 (2006).
Initial Studies of Temperature and l-offset on PXe
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~3-fold (W for W) improvement in PXe when switching from ‘Standard’
(non-narrowed) LDA to VL1.
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Temperature curve for two lasers similar; mild OP conditions &
modest fraction of light absorbed  laser power limited.
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Increased current  higher
flux, closer to Rb D1 
lower PXe!
Benefit from slight offset by
allowing for better
illumination of OP cell.
Measuring Rb Electron Spin Polarization, PRb
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Laser absorbed ‘quasi-homogenously’
Monitor small changes in amount of
light transmitted when Bo is cycled.
 Efficient depletion pumping of
ground-state m sublevels.
 Provides in situ estimate of PRb
Tracking PXe, PRb, vs. Cell Illumination
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High transmittance
 PXe tracks PRb.
PRb increases as laser l
approaches Rb D1, while PXe
and transmittance both fall.
Although PRb (along z) is
optimal near Rb D1, poor
transmittance indicates
inferior cell illumination.
 PXe is greatest at an intermediate offset--where both PRb
and transmittance are high.
Whiting et al., JMR, 197, 249 (2009).
Effect of Laser Flux and [Xe]cell on PXe
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Expected: smooth, monotonic decrease in PXe with rising [Xe]cell.
Instead: PXe increases with [Xe]cell, peaks (at ~300 torr), then
decreases but remains uncharacteristically high at elevated [Xe]cell.
Potentially useful for situations where simultaneously high PXe and
[Xe] are desired.
VL2
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Effect not due to cell
contamination, collection
efficiency, or laser energydependent mechanisms
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Remain laser-power limited,
as PXe rises linearly with flux
(except at low [Xe]).
Whiting et al., JMR, 197, 249 (2009).
Low-Field NMR and Laser Retro-reflection
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Measure Xe NMR in situ
(Magritek Aurora, w/ home-made
pulse/detect coil and cell mounts
(PTFE) and ‘Bucking coil’ (~100fold reduction in noise)
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2” mirror retro-reflects laser light
 ~30% “free” increase in PXe
(under nominal conditions)
Effects of Cell Temperature, [Xe]cell on PXe
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Optimal cell temperature (Topt) is strongly dependent on [Xe]cell.
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Lower [Xe]cell  higher Topt (and vice-versa)
Increase in [Xe]cell gives higher NMR signal  even at ~1400 torr.
Independent of [Xe]cell, Topt is poorly sensitive to both [N2] and total
cell pressure.
Effects of [Xe] and Topt on PXe Build-up
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PXe build-up rate increases with OP cell exhaust temperature
(gSE increases with [Rb]).
Dividing initial build-up slope by [Xe]: estimate of OP efficiency:
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90 °C
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Xe
Complex dependence on both
Tcell and [Xe]cell
At fixed (lowish) Tcell, slope
follows our PXe trend; OP at
Topt gives expected trend
Although gSE increases with
temperature, PRb and PXe
may decrease due to poor
cell illumination.
gSE also depends on SE
pathway (binary vs. vdW)
 dependent on OP gas
composition.
Recent High-Field PXe Values
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Low [Xe] favors high T, vice-versa
Among highest PXe values achieved at such high [Xe]cell
Enhancements: ~60,000 at 50 torr Xe, and >12,000 at 2000 torr Xe
Origin of interplay of temperature, Xe concentration?
Tunable VHG-LDA with ‘On-Chip’ Grating
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80 W (@fiber, 60-70 W @cell)
~0.3 nm Dl
~1.5 nm tunability
Barlow et al., ENC Conf. (2009).
QPC
PXe , %T vs. [Xe]cell, Tcell
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When PXe maxed (at TOPT), l for peak absorbance matches l for peak PXe
As Tcell goes up, peak absorbance deepens, red-shifts, and broadens
Clear benefit to OP at each [Xe]'s TOPT (esp. lower Tcell for increased [Xe])
Rb absorbance dependent on [Xe]; shift much greater than expected
(~0.2 vs. ~0.04 nm at 2000 torr Xe).
2000 torr Xe
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In presence of
[Xe], Rb D lines:
 Broaden
 Shift
 Grow an
increasingly
large (red-side)
shoulder!
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Xe-dependent Rb
lineshape may be
contributing to
observed effects.
Summary
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VHG-narrowed LDAs: high laser flux / narrow linewidths
 high Rb absorption efficiency  up to 3-fold improvement in PXe
Slight offset from Rb D1 demonstrated to enhance PXe
PRb monitored via changes in laser transmission while cycling Bo.
Anomalous dependence of PXe on [Xe]cell from interplay of Topt, [Xe]cell
Exploiting this effect + Further optimization  high PXe (including
~55%, ~32%, ~23%, and ~11% at 50, 300, 500, 2000 torr Xe).
‘On-chip’ grating gives narrowed (<0.3 nm), tunable, 80 W LDA
Origin of Topt / [Xe]cell not (yet) understood; [Xe]cell-dep Rb D spectrum
could be contributing factor.
To achieve best PXe at given [Xe]cell, all OP parameters should be
optimized
Results could have impact on other SE OP designs, Alkali metals
(Cs, K), and noble gases (He, Kr), as well as applications
Acknowledgements
OP Team
Nick Whiting
Panayiotis Nikolaou
Neil A. Eschmann
Dr. Michael J. Barlow
(University of Nottingham, UK)
Other Group Members
Kassie Chaffee, Ping He
Indra Saha, Jennifer Shapiro
Laura Walkup, Laura Buck
Kyle Power
Shavonne Montgomery
Support
NSF (CAREER & REU)
Research Corporation
School of Medical and Surgical
Sciences-University of
Nottingham, UK
GE Healthcare-Amersham