spectroscopy of cold molecular ions

Transcription

1 Workshop on an Optical Clock Mission in ESA s Cosmic Vision Program Düsseldorf High-resolution spectroscopy of cold molecular ions B. Roth, J. Koelemeij, I. Ernsting, A. Wicht, S. Schiller Heinrich-Heine Heine-Universität Düsseldorf Acknowledgements: A. Nevsky, M. Okhapkin, V. Korobov*, D. Bakalov * Joint Inst. Nuclear Research, Dubna Institute for Nuclear Research and Nuclear Energy, Sofia

2 Why molecular clocks? S.S. and V. Korobov, PRA 71, (2005) A comparison of an atomic optical clock to a molecular optical clock is (within the Standard Model) sensitive to all nongravitational interactions: ( νat νvib) α φ = bα bφ b ν ν α φ QCD Λ at vib ΛQCD b, b, b O(1) α In gauge unification theories the dependencies of α and Λ QCD on U are correlated (Damour 1999, Langacker et al, Calmet & Fritzsch, 2002) Molecular Clocks are very sensitive probes! φ Λ Λ Λ QCD QCD 40 α α Λ Energy scale of QCD Higgs vacuum field

3 Cold Molecules Schiller and Korobov (2005) The first accurate optical frequency references were molecules: CH 4, OsO 4, I 2 For precision spectroscopy, ultracold, trapped molecules are necessary - reduces various line broadening mechanisms - allows best control over and characterization of systematic effects - vibrational levels in the electronic ground state have lifetimes 10 ms to days Progress in production and manipulation of cold molecules is strong: Ultracold neutral diatomic molecules have been produced by photoassociation from ultracold atoms, e.g. Rb 2 (Pillet et al, 1998) Trapping in an optical lattice demonstrated (Rom et al. 2004) Molecular ions have been cooled and trapped by sympathetic cooling (Aarhus/Düsseldorf) Cold Neutral dipolar molecules have been trapped in electric traps (Rhinhuizen/Berlin/München)..

4 Cold Molecular Ions Molecular ions can be easily produced and sympathetically cooled HD Be T ~ 10 mk Blythe et al., PRL 95, (2005) Large variety of molecular ions possible - By (electron-beam or photo-) ionization of neutral molecules - By chemical reactions - Can choose most suitable ones in terms of systematic shifts, ease of spectroscopy,. - Few-electron molecules can be calculated ab initio, e.g. hydrogen molecular ions (H 2, HD, D 2 ), HeH,. Spectroscopy Electron shelving technique not applicable Options: - Laser-induced reactions (see e.g. Gerlich et al, ) - resonance enhanced multi-photon dissociation (REMPD) - quantum logic spectroscopy (P. Schmidt et al., NIST) - H 3, H 2 D, D 2 H, ArH,N 2 H,BeH, BeD,O 2 H,BaO

5 Ro-vibrational spectroscopy of cold HD - Dipole-allowed transitions - v = 0 to v = 4 overtone transition is accessible to diode laser - Long lifetime ~ 10 ms - No detectable fluorescence use state-selective photodissociation and measure number of remaining HD ions Beginning: End:

6 Results B. Roth et al., Phys. Rev. A 74, (R) (2006) J. Koelemeij et al (subm.) Residual linewidth: - unresolved hyperfine structure - finite temperature - excess micromotion - laser linewidth Theory with ~ 35 MHz broadening (v, J) = (0, 2) (4, 3) Recent results: - measurement of transition frequency at 1.4 µm in agreement with ab-initio theory at < 10 ppb level Theory: Ray & Certain 1976, Ryzlewicz et al. 1982, Carrington et al 1985, Bakalov et al estimate of systematic effects for our trap; but could be improved significantly (magentic field control, minimize micromotion, shutter light beams, etc.)

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8 Energies in HD 22 vibrational levels Dipole allowed transitions, accessible with low-power laser sources - v=0 -> v=1: ~ 5 µm - v=0 -> v=4: ~ 1.4 µm Natural linewidths ~ 10 Hz Rich hyperfine structure - L 4 Sensitivities of energies to m e /m p have been calculated (S.S. and Korobov, 2005) ~ 1.3 THz Fermi contact interaction r r ~ S. e S Sp e Fermi contact interaction r r Spin-rotation interaction ~ S. e S Sd d r r ~ Se. e L p

9 Sympathetic cooling of charged particles Advantages: Efficient, since Coulomb force is long-range General method: independent of nature of particle (only charge and mass relevant) Final state: Coulomb crystal (particles on sites, well-separated) First experiments: Penning trap: Drullinger et al Larson et al Paul trap: Diedrich et al Waki et al 1992 Raizen et al 1992 Baba & Waki 1996 Drewsen et al Cooling laser

10 Sympathetic cooling of charged particles 9 Be 2 P 1/2 313 nm 2 S 1/2 Linear radiofrequency trap Cooling laser

11 Sympathetically cooled HD ions B. Roth et al., PRL 94, (2005) Blythe et al., PRL 95, (2005) Experiment T(Be ) ~ 10 mk Laser propagation Simulations 690 Be and 12 HD Temperature of HD molecules < 20 mk Simulations show very strong coupling between Be and HD If the core was more than 50 mk, there would not be a shell structure Taking into account interactions, laser cooling strength, and observed Be temperatures yields above value

12 Population dynamics in the collision-free regime Energy T rot = 300 K T trans = 20 mk I (1430 nm) = 3.2 mw / mm 2 I (266 nm) = 5.7 mw / mm 2

13 Population dynamics in the collision-free regime Photodissociaton Energy T rot = 300 K T trans = 20 mk I (1430 nm) = 3.2 mw / mm 2 I (266 nm) = 5.7 mw / mm 2

14 Absolute frequency measurement v = 0, J = 2 v = 4, J = 1 Femtosecond frequency comb Feedback Feedback Pump Laser Ti: Sapphire femtosecond laser PCF SHG f REP f 0 laser beatnote MASER unknown optical frequency GPS

15 Summary Molecular optical clocks are useful for fundamental physics, especially for tests of universality of gravitational frequency shift (dedicated satellite mission OPTIS) Ultracold molecules are essential Reached temperatures < 20 mk for molecular ions sympathetically cooled by atomic ions Demonstrated high-resolution spectroscopy of interaction-cooled molecular ions - without fluorescence detection - Vibrational spectroscopy on ultracold molecular ions (HD ) - Could also be applied to atomic ions, possibly highly charged ones - Spectra taken with small ion numbers (~ 1000s) Ongoing work: - Improvement of resolution of HD spectroscopy - Laser with narrower linewidth, steeper trap - Radiofrequency spectroscopy - Quantum logic spectroscopy

16 Other applications of precision spectroscopy of ultracold molecules Precision spectroscopy on simple molecules (e.g. HD, H 2, H 3, ) and comparison with ab-initio theory, measurement of m e /m p - Time-independence of nuclear and electron mass (Schiller and Korobov, 2005 Reinhold et al. 2006) Molecular clock ν molecular ν ~ me mp Atomic clock ν atomic Parity violation: Search so far only with room-temperature molecules (Ziskind et al. 2002) Test of Isotropy of space: Place limits for anisotropy of Coulomb interaction (Kostelecky et al., Müller et al.) ν 0 ν left = ν right ν π 2

17 Ion number measurement

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