1 / 30

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

elisha
Download Presentation

Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

More Related