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Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy

Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy. Brian Siller , Andrew Mills, Michael Porambo & Benjamin McCall Chemistry Department, University of Illinois at Urbana-Champaign. Ions & Astrochemistry. Molecular ions are important to interstellar chemistry

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Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy

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  1. Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy Brian Siller, Andrew Mills, Michael Porambo & Benjamin McCall Chemistry Department, 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 0 1

  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, so combine with heterodyne spectroscopy

  10. Velocity Modulation of N2+ • Single-pass direct absorption • Single-pass Heterodyne @ 1GHz 0 1 2

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

  12. 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

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

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

  15. CEVMS Setup

  16. 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)

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

  18. 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 • Ions and excited neutrals are observed to be ~75° out of phase with one another

  19. 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* B. M. Siller, A. A. Mills and B. J. McCall, Opt. Lett., 35, 1266-1268. (2010) 1M. Havenith, M. Schneider, W. Bohle, and W. Urban; Mol. Phys. 72, 1149 (1991)

  20. Precision & Accuracy • Line centers determined to within 1 MHz with optical frequency comb • Sensitivity limited by plasma noise 0 1 2 A. A. Mills, B. M. Siller, and B. J. McCall, Chem. Phys. Lett., 501, 1-5. (2010)

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

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

  23. Experimental Setup Ti:Sapph Laser Detector PZT Polarizing Beamsplitter EOM Detector AOM 30MHz Quarter Wave Plate Lock Box

  24. Experimental Setup Detector PZT Ti:Sapph Laser EOM

  25. Experimental Setup Detector PZT Ti:Sapph Laser EOM EOM N × Cavity FSR (113 MHz) N oise Immune C avity Enhanced - O ptical H eterodyne V elocity M odulation S pectroscopy Lock-In Amplifier 40 kHz Plasma Frequency Signal

  26. NICE-OHVMS 0 See talk MI10 for more thorough analysis 1 2 3 • Sidebands spaced at 9 cavity FSRs (1 GHz) • 3rd derivative-like Doppler lineshape • Lamb dips from each laser frequency and combination of laser frequencies

  27. NICE-OHVMS N2+ N2* • Retain ion-neutral discrimination

  28. Velocity Modulation Techniques

  29. NICE-OHVMS Summary • Increased path length through plasma • Better sensitivity due to heterodyne modulation • Retained ion-neutral discrimination • Sub-Doppler resolution due to optical saturation • 50 MHz Lamb dip widths • Resolve blended lines • Better precision & absolute accuracy with comb

  30. Acknowledgements • McCall Group • Ben McCall • Andrew Mills • Michael Porambo • Funding

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