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Cavity-Enhanced Velocity Modulation Spectroscopy

This study presents Cavity-Enhanced Velocity Modulation Spectroscopy (CEVMS) as a technique for investigating molecular ions critical to interstellar chemistry. The authors, Brian Siller, Andrew Mills, Michael Porambo, and Benjamin McCall from the University of Illinois at Urbana-Champaign, explore the role of ions as reaction intermediates, highlighting over 150 molecules observed in the interstellar medium, of which about 20 are ions. Laboratory data generated through this method aims to support astronomers by providing precise spectral targets for molecular identification in space.

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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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