Gas-phase spin relaxation of 129 Xe

Transcription

1 Gas-hase sin relaxation of 19 T 1 = 100 hours! Brian Saam Deartment of Physics Worksho on Physics & Alications of Polarized Noble Gases University of Virginia, 19 May 009

2 Filling the Polarization Bucket 100% γ se P zation Polari 0% = Measured with NMR T 1 Relaxation 1 To maximize P, want γ se >> AND <P Rb > 1. γ se limited by available laser light. Goal: understand and minimize.

3 Longitudinal Sin Relaxation in Noble Gases = wall + gradient + intrinsic = 1 T 1 Container 1/ wall = h 30 G. Density indeendent! gradient = B D B Negligible in our exeriments. 0 B K 19 sin-rotation interaction: C K (R) N K Occurs during binary collision AND during the lifetime of a molecule. Prior to work on dimers: Gas hase T 1 tyically a few tens of minutes for 19 ; assumed dominated by wall interactions!

4 Early Studies of Intrinsic 19 Relaxation [] E.R. Hunt and H.Y. Carr, Phys. Rev. 130, 30 (1963). Assumes binary collisions only (transient dimers). Lowest density studied is [] = 50 amagats. bulk h/amagat (T 1 5 h for 1 atm )

5 Transient vs. Persistent Dimers K intrinsic = + t C K (R) N K τ Transient Dimers Persistent Dimers K binary collisions of duration τ t 1s. t []. -1 t 5 h amagat: Hunt and Carr (1963); Moudrakovski, et al. (001). Form/break u in 3-body collisions. Last for lifetime τ 1 ns (until next collision). t is indeendent of [] (for fixed gas comosition).

6 Theory of Persistent-Dimer Relaxation* = 3 c K N τ ( [ ] ) h 1+ Ω τ κ Mean-squared sin-rotation interaction energy. Power sectrum J(ω) for field fluctuations τ ~ 10-9 seconds for [] = 1 amagat. ω/π =118MHzforB 11.8 B 0 =1T T. Can often assume Ω τ << 1 (fast-fluctuation limit). Chemical Equilibrium Coefficient κ [ ] [ ][ ] [] 1 τ Fraction of atoms bound in molecules, assumes [ ] << []. Key oint for SEOP regime, where Ω τ << 1: τ is indeendent of []; looks like wall relaxation! *See: B. Chann, et al., Phys. Rev. Lett. 88, (00).

7 Low-field ( mt) Results* 4 = κ c K 3h N [] τ 4 = κ c K 3h N 1 k 1+ r B 1 ([B]/[]) = ure- rate Correction for nd gas vdw With: 1 τ [ ] + k [ B]+ ] L = k B r k B B / k B Need constant wall (asymtote). *B. Chann, et al., Phys. Rev. Lett. 88, (00). No observed [] deendence for fixed gas comosition (inset). vdw, wall, r B extracted from fits. vdw 0.5 h -1, ten times faster than binary collisions at 1 atm!

8 High-field Exeriments: Theory = 3 c K N h τ 1+ Ω τ κ ( [ ] ) Magnetic-field decouling term imortant for large Ω, small τ. chemical- + Θ ck N μbb Additional term due to 0 c 6 K 3 h 15 h M sr + M csa shift anisotroy (CSA) interaction has deendence on. B 0 = τ 1+ Ω τ κ ( ) ( sr csa ) M + M [ ]

9 High-Field Exeriment: NMR Probe & Cell Field strengths B 0 : 1.5 T (17.7 MHz) 4.7 T (55.3 MHz) 8.0 T (94. MHz) T (166 MHz) 6.7 cm diam sherical measurement cell. Silicone-coated. Contains no Rb (HP gas transferred in). Long and robust wall relaxation times.

10 19 Persistent-Dimer Relaxation: Results* = ( M sr csa + M ) ( κ [ ] ) ( ) sr csa αkα [G] 1+ Ω τ = + κ M M rearam. kα [G] + Ω τ Total gas density: G] = [] + [N ] [ concentration (FIXED): α []/[G] 1 Breaku coefficient: τ = k [G] α High density: indeendent of density. T 1 5 h Low density: magnetic-field suression of. (wall time subtracted out). decreases for decreasing [] at fixed total gas density [G]. *B.N. Berry-Pusey, et al., Phys. Rev. A 74, (006); B.C. Anger, et al., Phys. Rev. A 78, (008).

11 100-Hour Gas-Phase T 1 for 19 Inferred wall relaxation time: T 1 (wall) = 175 h!! Obeys the Driehuys Axiom concerning HP xenon.

12 Quadratic Deendence on Alied Field sr csa αk ( ) α[g] κ M + M kα [G] + Ω = Combined interaction strength of SR and CSA Data consistent with CSA interaction strength M csa roortional to B 0. Intercet is roortional to SR interaction strength M sr. Characteristic crossover field B 0 16 T.

13 So what about [] = 1 amagat; B 0 = 30 G? Table II taken from: B.C. Anger, et al., Phys. Rev. A 78, (008). We ve measured T 1 = 5.75 h in a large 1 cm diam sherical borosilicateglass cell (DMDCS-coated) at 30 G, 100 C. (Still limited by wall relaxation!)

14 Imrovement to Flow-through 19 Polarizer*? Long narrow cell ( 1 m long 4 cm diam). [] 1 amagat; use sectrally narrowed diode-laser array. Counterroagation of gas and laser light. Cryogenic (LN ) freeze out, searation, and storage of xenon (T K). *See talk B4.0000, tomorrow 10:4 am. Goal: gas-hase storage cell (no cryogenics) with 3 storage time of frozen having T 1 10 h. (Preliminary atent alication filed.)

15 Summary We have thoroughly (exhaustively) characterized intrinsic gas-hase T 1 -relaxation of 19 due to ersistent dimers an imortant limit to roduction, accumulation, and storage of HP 19. Transient-dimer contribution (binary collisions). Persistent-dimer contribution (van der Waals molecules). Cometes (and gets confused) with wall relaxation in many cases, because of density-indeendence of. Possibility of cryogen-free accumulation and storage with -3 longer storage times; significant imrovement for state-of-the-art method in olarizing 19.

16 Hyerolarized Gas Research Grou Faculty Brian Saam David Ailion Gernot Laicher Graduate Students Ben Anger (ostdoc at Leiden) Geoff Schrank (graduating Summer 009) Eric Sorte Zayd Ma Undergraduates Brittany Berry-Pusey (grad. at UCLA) Kimberly Butler (UU med. school) Laurel Hales Allison Schoeck Oliver Jeong (HS student) Thanks to M.S. Conradi for numerous helful discussions.

17 Room-Temerature Wall-Relaxation Rate vs. B 0 Fast-fluctuation limit. Intrinsic relaxation rate i subtracted out. L t i fit i ld Lorentzian fit yields correlation time for wall interaction of 4 ns.

18 Simle Model for Surface Relaxation of Gases Container of 19 with uniformly relaxing stable surface S V, [] wall S = η v V η 1/ wall tyically ranges from 10 to 100 min for 19 in carefully reared glass vessels. η surface relaxivity For ballistic collisions with a uniformly relaxing surface, wall is indeendent of gas density [].