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 Department of Chemistry and Biochemistry Southern Illinois University, Carbondale 1 Sir Peter Mansfield Magnetic Resonance Centre University of Nottingham, UK 2 DAMOP 2009 U. Virginia OP Limitations: [Xe]cell and Laser Output
Highest PXe requires low [Xe]cell (<50 torr) Xe / Rb collisions reduced P , capping P Rb Xe Due to interplay of PXe, [Xe]cell, overall NMR signal reaches steady-state (increases in [Xe]cell compensated by PXe losses). e.g., Meersmann @ co-workers Hard to achieve simultaneously high PXe and [Xe]cellJCP 2003. 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 Volume Holographic Grating (VHG)-Narrowed LDAs Bulk slabs of photosensitive glass with Bragg planes of varying (ni) Retro-reflects narrow emission band into laser elements, forcing lasing at injected .
Narrows LDA output with high efficiency, tolerance High power and narrow (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 VL1 (Spectra-Physics Comet) ~26 W (50 A), FWHM=0.27 nm
VL1 -offset can be tuned by varying driving current. VL2 (Spectra-Physics Integra); ~55 W (96 A), FWHM=0.49 nm VL1 OP Apparatus
Cell: Rosen et al., Rev. Sci. Instrum., Instrum., 70, 70, 1546 (1999). 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. P measured Xe via NMR at 9.4 T Saha, Nikolaou, Whiting, Goodson, Chem. Phys. Lett., 428, 268 (2006). Initial Studies of Temperature and offset on PXe ~3-fold (W for W) improvement in PXe when switching from Standard
(non-narrowed) LDA to VL1. Temperature curve for two lasers similar; mild OP conditions & modest fraction of light absorbed laser power limited. 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
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 High transmittance PXe tracks PRb. PRb increases as laser 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, JMR, 197, 197, 249 (2009). Effect of Laser Flux and [Xe]cell on PXe 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 Effect not due to cell contamination, collection efficiency, or laser energydependent mechanisms Remain laser-power limited, as PXe rises linearly with flux (except at low [Xe]). Whiting et al., JMR, JMR, 197, 197, 249 (2009). Low-Field NMR and Laser Retro-reflection
Measure Xe NMR in situ (Magritek Aurora, w/ home-made pulse/detect coil and cell mounts (PTFE) and Bucking coil (~100fold reduction in noise) 2 mirror retro-reflects laser light ~30% free increase in free increase in PXe (under nominal conditions) Effects of Cell Temperature, [Xe]cell on PXe Optimal cell temperature (Topt) is strongly dependent on [Xe]cell.
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 PXe build-up rate increases with OP cell exhaust temperature (SE increases with [Rb]). Dividing initial build-up slope by [Xe]: estimate of OP efficiency: 90 C
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 SE increases with temperature, PRb and PXe may decrease due to poor cell illumination. SE also depends on SE pathway (binary vs. vdW) dependent on OP gas composition. Recent High-Field PXe Values
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 80 W (@fiber, 60-70 W @cell) ~0.3 nm ~1.5 nm tunability Barlow et al., ENC Conf. (2009). QPC PXe , %T vs. [Xe]cell, Tcell
When PXe maxed (at TOPT), for peak absorbance matches 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 In presence of [Xe], Rb D lines: Broaden Shift Grow an increasingly
large (red-side) shoulder! Xe-dependent Rb lineshape may be contributing to observed effects. Summary
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
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