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Enhancing Ion Spectroscopy Techniques for Astrochemistry Research

This research outlines the use of Cavity Enhanced Velocity Modulation Spectroscopy to study molecular ions in interstellar chemistry. It focuses on improving sensitivity by combining techniques and discusses future work in ion spectroscopy.

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Enhancing Ion Spectroscopy Techniques for Astrochemistry Research

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

  2. Outline • Motivation • Velocity Modulation Spectroscopy • Cavity Enhancement • NICE-OHMS • Ion Beam Spectroscopy • Comparison of Techniques • Future Work

  3. 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+

  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 • Improve by combining with cavity enhanced absorption spectroscopy

  12. Pound-Drever-Hall Locking Cavity Transmission Ti:Sapph Laser Error Signal Detector PZT Polarizing Beamsplitter EOM Detector AOM 30MHz Quarter Wave Plate Lock Box 0.1-60kHz <100Hz

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

  14. CEVMS Setup

  15. 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)

  16. 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

  17. 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)

  18. 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)

  19. NICE-OHMS • Noise Immune Cavity Enhanced Optical Heterodyne Molecular Spectroscopy Cavity Modes Laser Spectrum J. Ye, L. S. Ma, and J. L. Hall, JOSA B, 15, 6-15 (1998)

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

  21. Experimental Setup Detector PZT Ti:Sapph Laser EOM

  22. Experimental Setup Detector PZT Ti:Sapph Laser EOM EOM 90° Phase Shift 113 MHz Cavity FSR Lock-In Amplifier Lock-In Amplifier 40 kHz Plasma Frequency X Y X Y Absorption Signal Dispersion Signal

  23. Results 113 MHz Sidebands 1 Cavity FSR Dispersion Absorption Lock-In X Lock-In Y

  24. Dispersion Absorption Lock-In X Lock-In Y No center Lamb dip in absorption Spectra calibrated with optical frequency comb Frequency precision to <1 MHz!

  25. Ultra-High Resolution Spectroscopy Absorption Dispersion 113MHz Sub-Doppler fit based on pseudo-Voigt absorption and dispersion profiles (6 absorption, 7 dispersion) Line center from fit: 326,187,572.2 ± 0.1 MHz After accounting for systematic problems, line center measured to within uncertainty of ~300 kHz!

  26. Technique Comparison VMS OHVMS NICE-OHVMS CEVMS NICE-OHVMS

  27. NICE-OHVMS Summary • Better sensitivity than traditional VMS • Increased path length through plasma • Decreased noise from heterodyne modulation • Retained ion-neutral discrimination • Sub-Doppler resolution • Better precision & absolute accuracy with comb • Resolve blended lines • Can use same optical setup for ion beam spectroscopy

  28. Experimental Setup Ion Beam Instrument Detector PZT Ti:Sapph Laser EOM EOM Lock-In Amplifier Lock-In Amplifier 40 kHz Plasma Frequency X Y X Y Absorption Signal Dispersion Signal

  29. Ion Beam Spectrometer S _ R I Be S Laser retractable Faraday cup Brewster window Einzel lens 2 TOF beam modulation electrodes electrostatic deflector 2 wire beam profile monitors electron multiplier TOF detector drift tube (overlap) variable apertures Ion source electrostatic deflector 1 Ion optics steerers ion source Current measurements Co-linearity with laser Mass spectrometer Laser coupling Velocity modulation ±5V ~ ±100MHz Einzel lens 1 Faraday cup Brewster window Ground 4kV 2kV

  30. Ion Beam Results • 4kV float voltage • ±5V modulation • ~120MHz linewidth • Ion density ~5×106 cm-3 • Cavity finesse ~450 • Lock-in τ=10s Float voltage Ion mass

  31. Unique Advantages • Ion Beam • Rigorous ion-neutral discrimination • Simultaneous mass spectroscopy • Mass identification of each spectral line • No Doppler-broadened component of lineshape • Positive Column • High ion density • Simpler setup • Direct measurement of transition rest frequency

  32. Future Work • Positive Column • Mid-IR OPO system • ~1W mid-IR idler power • Pump and signal lasers referenced to optical frequency comb • Liquid-N2 cooled discharge cell • Ion Beam • Mid-IR DFG laser • Ti:Sapph referenced to comb • Nd:YAG locked to I2 hyperfine transition • Supersonic expansion discharge source

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

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