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Cavity-Enhanced Velocity Modulation Spectroscopy

Cavity-Enhanced Velocity Modulation Spectroscopy. Brian Siller , Andrew Mills, Michael Porambo & Benjamin McCall University of Illinois at Urbana-Champaign. Ions & Astrochemistry. Molecular ions are important to interstellar chemistry Ions important as reaction intermediates

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Cavity-Enhanced Velocity Modulation Spectroscopy

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  1. Cavity-Enhanced Velocity Modulation Spectroscopy Brian Siller, Andrew Mills, Michael Porambo & Benjamin McCall University of Illinois at Urbana-Champaign

  2. Ions & Astrochemistry • Molecular ions are important to interstellar chemistry • Ions important as reaction intermediates • >150 Molecules observed in ISM • Only ~20 are ions • Need laboratory data to provide astronomers with spectral targets C6H6 C6H7+ e H2 C6H5+ C2H2 C4H3+ H C4H2+ C3H2 C3H C C3H3+ e e H2 C3H+ C+ C2H2 C2H e C2H4 C2H3+ e C2H5+ e C+ CH4 CH3+ e CH3OCH3 CH5+ C2H5CN CH3OH, e CH3CN, e H2O, e CH3OH H2 HCN, e CH3CN CH3+ CO, e NH3, e CH2CO CH3NH2 e H2 N, e CH2+ CH HCN H2O H2 H3O+ e CH+ OH H2O+ H2 C OH+ HCO+ H2 H3+ O CO H2 H2+

  3. Ion Spectroscopy Techniques Supersonic Expansion Velocity Modulation Hollow Cathode    High ion column density    Ion-neutral discrimination     Low rotational temperature     Narrow linewidth   Compatible with cavity-enhanced spectroscopy 

  4. Velocity Modulation Spectroscopy • Positive column discharge cell • High ion density, rich chemistry • Cations move toward the cathode +1kV -1kV Plasma Discharge Cell

  5. Velocity Modulation Spectroscopy • Positive column discharge cell • High ion density, rich chemistry • Cations move toward the cathode • Ions absorption profile is Doppler-shifted +1kV -1kV Laser Detector Plasma Discharge Cell

  6. Velocity Modulation Spectroscopy • Positive column discharge cell • High ion density, rich chemistry • Cations move toward the cathode • Ions absorption profile is Doppler-shifted -1kV +1kV Laser Detector Plasma Discharge Cell

  7. Velocity Modulation Spectroscopy • Positive column discharge cell • High ion density, rich chemistry • Cations move toward the cathode • Ions absorption profile is Doppler-shifted • Drive with AC voltage • Ion Doppler profile alternates red/blue shift • Laser at fixed wavelength • Demodulate detector signal at modulation frequency Laser Plasma Discharge Cell Detector

  8. Velocity Modulation Spectroscopy

  9. Velocity Modulation Spectroscopy • Want strongest absorption possible • Signal enhanced by modified White cell • Laser passes through cell unidirectionally • Can get up to ~8 passes through cell Laser Plasma Discharge Cell Detector • Also want lowest noise possible

  10. Heterodyne (FM) Spectroscopy • Most environmental noise scales like 1/f • Velocity modulation is ~40kHz • Much better than direct DC detection • Still room for improvement • Frequency Modulation (FM) • Modulate laser frequency at RF (≳100MHz) • Demodulate detector signal - + RF Carrier Laser Audio RF FM Signal FM Laser

  11. Velocity Modulation of N2+ • Single-pass direct absorption • Single-pass Heterodyne @ 1GHz

  12. Velocity Modulation Limitations • Doppler-broadened lines • Blended lines • Limited determination of line centers • Sensitivity • Limited path length through plasma

  13. Cavity Enhanced Absorption Spectroscopy (CEAS) • Optical cavity acts as a multipass cell • Number of passes = • For finesse of 300, get ~200 passes • Must actively lock laser wavelength/cavity length to be in resonance with one another • DC signal on detector is extremely noisy • Velocity modulation with lock-in amplifier minimizes effect of noise on signal detection Cavity Detector Laser

  14. Pound-Drever-Hall Locking Cavity Transmission Ti:Sapph Laser Error Signal Detector PZT Polarizing Beamsplitter EOM Detector AOM 30MHz Quarter Wave Plate Lock Box

  15. CEVMS Setup Audio Amplifier 40 kHz Lock-In Amplifier Transformer Laser Cavity Mirror Mounts

  16. CEVMS Setup

  17. Extracting N2+ Absorption Signal • Doppler profile shifts back and forth • Red-shift with respect to one direction of the laser corresponds to blue shift with respect to the other direction • Net absorption is the sum of the absorption in each direction Absorption Strength (Arb. Units) Relative Frequency (GHz)

  18. Extracting N2+ Absorption Signal V (kV) t (μs) Absorption Relative Frequency

  19. Extracting N2+ Absorption Signal • Demodulate detected signal at twice the modulation frequency (2f) • Can observe and distinguish ions and neutrals • Ions are velocity modulated • Excited neutrals are concentration modulated • Ground state neutrals are not modulated at all

  20. Typical Scan of Nitrogen Plasma • Cavity Finesse 150 • 30mW laser power • N2+ Meinel Band • N2* first positive band • Second time a Lamb dip of a molecular ion has been observed (first was DBr+ in laser magnetic resonance technique)1 • Used 2 lock-in amplifiers for N2+/N2* 1M. Havenith, M. Schneider, W. Bohle, and W. Urban; Mol. Phys. 72, 1149 (1991).

  21. Phase Analysis V (kV) • N2+ • Velocity directly dependent on voltage • No significant phase shift with respect to voltage • N2* • 78° phase shift with respect to N2+ signal • Peak N2* density occurs when rate of formation equals rate of destruction t (μs) Peak N2* Density

  22. Phase Analysis • N2+ • Velocity directly dependent on voltage • No significant phase shift with respect to voltage • N2* • 78° phase shift with respect to N2+ signal • Peak N2* density occurs when rate of formation equals rate of destruction • Analogous to Earth’s heating/cooling cycle with the sun • Sun is brightest at noon (peak voltage and N2+ velocity) • Hottest time of day is 5pm (peak N2* density) • 5 hour time delay in 24 hour day = 75° phase shift

  23. Precision & Accuracy • Line centers determined to within 1 MHz with optical frequency comb

  24. Indirect Terahertz Spectroscopy • Combination differences to compute THz transitions by observing rovibrational transitions in the mid-IR • Support for Herschel & Sofia THz observatories

  25. Indirect Terahertz Spectroscopy J’ 4 3 cm-1 IR Transitions Even Combination differences Odd Combination Differences 1-0 Rotational Transition Reconstructed Rotational Transitions 2 1 0 6 5 cm-1 4 3 2 1 0 J”

  26. Sensitivity Limited by Plasma Noise

  27. NICE-OHMS • Noise Immune Cavity Enhanced Optical Heterodyne Molecular Spectroscopy Large Signal Small Noise Cavity Enhancement Heterodyne Spectroscopy NICE-OHMS

  28. NICE-OHMS • Noise Immune Cavity Enhanced Optical Heterodyne Molecular Spectroscopy Cavity Modes Laser Spectrum

  29. NICE-OHMS • 3rd derivative Doppler lineshape • Lamb dips from each laser frequency

  30. Velocity Modulation Techniques Heterodyne Direct Absorption Single Pass Cavity Enhanced

  31. Acknowledgements • McCall Group • Funding • Air Force • NASA • Dreyfus • Packard • NSF • Sloan

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