DFG Universität Konstanz Humbolt-Universität Berlin O. Vogelsang, S. Orlov J. Mlynek, A. Peters D. Weise Optik-Zentrum Konstanz Ultra-sensitive Detection of Molecular Oxygen using Cavity-enhanced Absorption Spectroscopy (NICE-OHMS) European Cold Molecule Network Meeting December 13th, 2002 NICE-OHMS (Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy) First results y Cavity-enhanced absorption spectroscopy y Detection schemes y The electronic structure of O2 y Why molecular oxygen? Outline 9 (?) 2 µB 9 9 2 µB none yes 9 (?) Buffer-gas cooling [2] Evaporative cooling [3] Magnetic moment [1] Electric dipole moment [1] Hyperfine structure A. Boca et al., J. Chem. Phys. 112(8), 2000 J.L. Bohn, PR A 62(032701), 2000 A.V. Avdeenkov et al. PR A 64(052703), 2001 [1] [2] [3] no ≈ 10-4 D 9 (?) 9 9 Low field seeking ground state [1] 16O18O 0 0.205 % 18O 5/2 0.038 % 17O 17O 2 0 99.747 % 16O 16O 2 ISOTOPOMERS Nuclear spin Natural abundance ISOTOPES Properties of the oxygen isotopomers / Need for precision spectroscopy Electric dipole-dipole interactions with 16O18O Internal rotational and vibrational degrees of freedom y Molecular BEC y Ultracold Chemistry Test of the Symmetrization Postulate with buffer-gas cooled 16O2 Measurement of precise absorption cross sections (e.g. for astronomy) Study of molecular structure Verification of theoretical intermolecular PESs One of the simplest paramagnetic diatomic species y Test of fundamental questions in Physics Prospects of ultracold oxygen samples y Doppler-free spectroscopy (?) y CW Spectroscopy for continuous monitoring Windows limit possible wavelength range Limited optical access Low laser powers (mW) y Compatible with cryogenic setup Expected initial density in the trap: 1012 molecules/cm3 100x increased sensitivity @ 200 mK (reduced Doppler-width, increased occupation of initial state) y Detection limit of < 1010 molecules / cm3 @ 300 K y Scheme should be very general Requirements for oxygen detection - + u 3 - X Σg 3 AΣ 3 C ∆u 3 B Σu 1 c Σu a1∆g IR Atmospheric System (924 - 1580 nm) -1 A ≈ 0.00021 s Atmospheric System (538 - 997 nm) -1 A ≈ 0.089 s (A-Band) 1 + - b Σg Herzberg System III (257 - 263 nm) -1 A ≈ 0.00001 s ? Herzberg System II (254 - 272 nm) -1 A ≈ 0.0001 s ? Herzberg System I (243 - 279 nm) -1 A ≈ 0.001 - 1 s ? Schumann-Runge Ba nds (175 - 468 nm) 7 -1 A ≈ 2 ⋅10 s The electronic structure of molecular oxygen O2 O-O + O2 Σ+ ↔ Σ + , Σ- ↔ Σ - g↔u ∆Λ = 0, ±1 ∆S = 0 Selection Rules A ≈ 3.8 ⋅107 s-1 Rubidium D2 Line 3 - X Σg + u - AΣ C3∆u 3 3 B Σu / Use of Vacuum UV lasers and optics is too challenging y Laser induced fluorescence (LIF) between 175 and 177 nm Strongly decreasing absorption coefficient y Absorption measurements above 175 nm Highest absorption coefficient Continuum / no laser needed y High sensitivity absorption measurements below 175 nm Detection in the Schumann-Runge System O2 O-O + O2 / Detection via B3Σu- state is too involving Moving O2+ is influenced by the magnetic field Maximum electric field for O2+ detection is limited by the buffer-gas Ionization potential: 97265 cm-1 (103 nm) y Photoionization from the ground state One-photon dissociation @ 204 nm from the excited b1Σg+ state y Two-step Photodissociation (Eppink et al., 1998) Detection of the fragments e.g. through Two Photon Allowed Laser Induced Fluorescence @ 226 nm or Photoionization @ 204 nm Dissociation energy: 41260 cm-1 (242 nm) y Photodissociation of O2 around 175 nm Photodissociation and Photoionization 3 - X Σg + u AΣ C3∆u 3 B3Σu- O2 O-O O2+ 759 760 R branch 761 762 Rubidium D2 line: 6⋅105 cm-1 αmax ≈ 7⋅10-6 cm-1 @ 296 K, 1 mbar 764 Wavelength [nm] 763 Q branch forbidden (Σ→Σ) y Extremely low absorption coefficients 0E+ 00 1E-18 2E-18 3E-18 4E-18 Detection in the Atmospheric A-Band Integrated Cross Section [MHz/molecule cm -2 ] 765 766 P branch 4 K (calculated) 70 K (calculated) 767 296 K (HITRAN Database) 768 - b1Σg+ 1 a ∆g / Not sensitive enough Expected detection limit using pulsed lasers: 1010 molecules / cm3 Anti-Stokes signal is a directed laser beam Result of the third order nonlinear susceptibility χ(3) y Coherent Anti-Stokes Raman Spectroscopy Reduced efficiency through limited solid angle and possible quenching of the b1Σg+ state Favorable branching to singlet-state (≈ 1/60) y Fluorescence spectroscopy y Direct absorption spectroscopy not feasible 3 X Σg Atmospheric A-Band Detection Schemes transmitted signal PZT vacuum chamber λ = 760.8854 nm Length 15 cm → ∆ν = 625 kHz Cavity Finesse 1600 power monitor & wavemeter y Preliminary experiment @ room temperature Iso lator Frequency Lock y Increased absorption length through the use of a resonator Cavity-Enhanced Absorption Spectroscopy nm 76 0 er e La s Dio d 4 Freq u en cy 3 9 4,0 0 0 3 + x GH z 5 6 7 8 Pin ≈ 3 mW y Detection limit: 1 mbar @ 296 K (2⋅1016 oxygen molecules / cm3) 2 0 1 2 3 4 5 6 -6 1 1e-6 mba r 5.1 mba r 10 mba r 20 mba r 31 mba r 43 mba r 57 mba r 67 mba r 89 mba r 130 mba r Results Transmission [10 ] Improve by 6 orders of magnitude Shot-noise limited Optimized Precision Scanning • Detection limit • Signal to noise ratio • Cavity transmission • Frequency measurement → Frequency Reference → Good mirrors → FM technique → Ultra-high-finesse cavity Only pulsed operation / Alternative: Cavity ringdown spectroscopy Demonstrated for the Oxygen A-Band by L. Gianfrani et al. (1999) Integrated absorption sensitivity 10-14 cm-1 Developed for molecular overtone spectroscopy / molecular frequency standards Combination of Cavity-Enhanced and FM Spectroscopy J. Ye, J.L. Hall, 1997 (JILA) / Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy 9 • Test of principle Necessary Improvements νFM νFM = FSR FSR y Sidebands serve as intensity reference NICE-OHMS Principle NICE-OHMS Normal FM Cavity Modes Frequency Scan Ti:Saph AOM Frequency Stabilization EOM1 λ/4 FSR Tracking EOM2 NICE-OHMS Experimental Setup Frequency Lock νFSR = 1.734 GHz PZT NICE-OHMS Signal y INVAR Spacer with maximum side access y Coatings optimized for transmission y R = 1 m → w0 ≈ 220 µm y Length 8.65 cm → FSR = 1.734 GHz, ∆ν < 17 kHz y Specified Finesse >100,000 for 760 – 790 nm NICE-OHMS Resonator y Broadband AR coating (400 – 800 nm) y Indium-sealed windows y Heated capillary for O2 injection NICE-OHMS Experimental Cell Optimal thermal anchoring Use of a Piezo in a buffer-gas environment Thermalization of hot O2 with a cold Helium buffer-gas ? Capillary Injection y Test setup for the implementation in dilution fridge Doppler-free spectroscopy ? Increased sensitivity First experimental data for “cold” O2 y Spectroscopy of molecular oxygen between 300 and 4 K NICE-OHMS on buffer-gas cooled molecular oxygen Compatible with cryogenic environment Only 1 laser needed Low light intensities possible Widely applicable Extremely sensitive y NICE-OHMS in the Atmospheric A-Band Not suitable in a cryogenic environment Feasible, but VUV wavelengths y Absorption spectroscopy in the Schumann-Runge bands Problem: Detection y Oxygen is an interesting candidate for magnetic trapping Summary J. Mlynek, A. Peters O. Vogelsang, S. Orlov y The Cryogenic Magnetic Trap Team For providing the diode laser for the preliminary experiment y Schiller Group (Düsseldorf) y Doyle Group (Harvard) Kurt-Alten-Stiftung, Hannover Optik-Zentrum Konstanz DFG y Financial Support Acknowledgements
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