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Two-photon Precision Spectroscopy of H 2 +

Two-photon Precision Spectroscopy of H 2 +. Jean-Philippe Karr Albane Douillet Vu-Quang Tran, PhD Laurent Hilico. Vladimir Korobov. Rachidi Osseni, post doc Jofre Pedregosa, post doc Franck Bielsa, PhD Tristan Valenzuela, post doc. outline. Motivations Experimental status

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Two-photon Precision Spectroscopy of H 2 +

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  1. Two-photon Precision Spectroscopy of H2+ Jean-Philippe Karr Albane Douillet Vu-Quang Tran, PhD Laurent Hilico Vladimir Korobov Rachidi Osseni, post doc Jofre Pedregosa, post doc Franck Bielsa, PhD Tristan Valenzuela, post doc

  2. outline • Motivations • Experimental status • Theoretical progress

  3. mp/me measurement Why ? • Fundamental constant determination mp/me codata 1836.152 672 45 (75) 4.1 10-10 Fine structure constant a = 1/137,03… • e- g-2 measurement, G. Gabrielse 2008 a-1 = 137.035 999 084 (51) (3.7 10-10) a-1 = 137.035 999 037 (91) (6.6 10-10) • h/MRb measurement, F. Biraben 2010 h/mRb : 7. 10-10 → 4.2 10-10 mx/my < 10-10 → 3.5 10-10

  4. mp/me measurement Why ? • Fundamental constant time-variations Astrophysics and spectroscopy H2, HD, NH3, CO, HCO+, HCN … Dt ~ 1010 years red shifts analysis Laboratory physics SF6spectroscopy Proposals on CaH+, MgH+, SrH+, …, GeBr+ Ultra narrow lines Low polarisability Low linear Zeeman effect HD+, H2+ at 10-16 • QED test on simple molecules

  5. mp/me measurement How ? mP and mein atomic unitsare determined separately through RF measurements in Penning traps. Accuracy • Electron mass Larmor to cyclotron frequency ratio C, O, Si • Proton mass :cyclotron frequencies, using 12C4+. R.S. Van Dyck, Jr. et al., in Trapped Charged Particles and Fundamental Physics AIP Conf. Proc. 457, pp. 101-110 (1999). 8.9 10-11 mp = 1.007 276 466 812 (90) me = 0.000 548 579 909 46 (22) 4.0 10-10 Codata 2011 Mp/me = 1836.152 672 45 (75) 4.1 10-10

  6. e- p+ R 248 nm p+ 9.2 µm mp/me Direct optical determination by H2+ spectroscopy Method • Doppler-free Two-photon • spectroscopy • 2+1’ REMPD • Trapped ions • High precision calculations 32.6 THz ( 9.1 µm ) (1091 cm-1) Dn ~1600 Hz Energy (atomic units) expected Internuclear distance (atomic unit)

  7. What do we know on H2+ ? Lundeen group, H2 Rydberg states L Jefferts group, Hyperfine or Zeeman spectroscopy from R.E. Moss, Molecular Physics, 80, 1541 1993. Project challenges v • state selected H2+ ion production • H2+ trapping • REMPD lasers • High precision calculations nexp mp/me Carrington group, Southampton

  8. Two-photon transition probabilities How to choose v→v’ ? v=0 → v=1 transitions 9.1 µm

  9. Two-photon transition probabilities How to choose L→L’ ? Total nuclear spin I=(-1)L L=0, v=0 → L=0, v=1 l=9.128µm L=2, v=0 → L=2, v=1 l=9.166µm L=3, v=0 → L=3, v=1: l=9.205µm Close to a CO2 laser emission line Quantum Cascade Laser available

  10. Experimental setup Hyperbolic Paul trap Quantum cascade laser (QCL) Optical cavity 248 nm KrF excimer Pulsed Laser 2 mm

  11. IR laser source HCOOH : formic or methanoic acid MHz HITRAN v, L

  12. IR source QCL / CO2 beat note Free QCL Quantum cascade Laser 5 MHz O.I. 2 mm QCL / CO2 beat note HCOOH stabilized CO2 laser < 200 Hz Band width ~ 6 MHz RBW : 10 kHz VBW : 1 kHz

  13. IR source • Results • optical power 54 mW • linewidth dn ~ 3kHz • high finesse cavity (~1000) • Faraday optical isolator at 9.2 µm F. Bielsa & al., Optics Letters 32, 1641-1643 (2007) L. Hilico, Rev. Sc. Instr. 82, 096106 (2011) G2ph~0.3 s-1p polarization G2ph~0.07 s-1s+ polarization • HCOOH stabilized CO2 laser Absolute frequency measurement 32 708 391 980.5 (1.0) kHz LPL, Villetaneuse, France F. Bielsa & al. J. Mol. Spectrosc. 247, 41-46 (2008)

  14. The ion trap • = 2p x 14 MHz DC -10 / +10 VAC 150 V r0 = 4.2 mm z0 = 3 mm

  15. H2+ creation: electron impact T=300K G. Werth & Al. Z phys D 28, (1993). v=0 : 12 % v=1 : 19 % Rotational distribution L=2 : 12 % Vibrational distribution Result : 0.07 x 0.12 x 0.6 = 0.5 % Hyperfine structure J=3/2 40% J=5/2 60% Very small !!

  16. Photodissociation at 248 nm 2 3 1 UV 1- ion creation (~ 500) 1,0 s 2- 1 to 30 UV pulses (20 mJ) 0,3 s 3- extraction, time of flight and counting 0.32 mJ signal 1 adjustable parameter ion cloud size experiments 0.85 mm num. simulations 0.83 mm 1.10 mJ 3.25 mJ 11.2 mJ 34.0 mJ 114 mJ Laser pulse number n

  17. Photodissociation at 248 nm Results J.-Ph. Karr & al., Applied Phys. B (2011) • v=0 v=1 population difference L=2, J=5/2 30 pulses at 34 mJ, pv=0 - pv=1 ~ 33% 2.4 % 30 pulses at 114 mJ, pv=0 - pv=1 ~ 86 % 6.2 % drawback : ion losses • Photodissociation yield

  18. Can we perform H2+ REMPD spectroscopy ? Two-photon transitions Photodissociation Trap losses Ion number fluctuations Present experiment signal to noise ratio: 0.27 • H2+ v=0,L=2 population • G2ph Improvements SNR ~ 30

  19. Experimental developments • State selected H2+ ion creation increase v=0 v=1 population difference H2 : v=0, L=0, 1, 2 à 300 K H2 X1Sg+, v=0, L=2 + 3 hn H2 C1Pu-, v=0, L=2 3+1 REMPI + hn mJ 303 nm 10 ns H2+ X Sg+, v=0, L + e- V. Mac Koy Anderson, & Al, Chem. Phys. Lett. 105, 22 (1984) H2+ branching ratios v=0 – v=1 L=2, J=5/2 pop. diff. 0.8 x 1 x 0.6 = 0.48 L v 0 1 1 0.1 Photo-electron yield • 0 0.005 • 1 • 4 0.01 O’Halloran, J. Chem. Phys. 87, 3288 (1987)

  20. Experimental developments • A linear trap for tighter focussing G2ph x 81 waist ÷3 • H2+ sympathetic cooling by laser cooled Be+ ions T = 300 K 7 kHz Second order Doppler effect T = 20 mK negligible

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