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Sub-Doppler Spectroscopy of Molecular Ions in the Mid-IR

Sub-Doppler Spectroscopy of Molecular Ions in the Mid-IR. James N. Hodges, Kyle N. Crabtree, & Benjamin J. McCall WI06 – June 20, 2012 University of Illinois at Urbana-Champaign. Outline. Motivation Spectroscopic Techniques for Ions: N 2 + Mid-IR Instrument H 3 + Spectroscopy

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Sub-Doppler Spectroscopy of Molecular Ions in the Mid-IR

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  1. Sub-Doppler Spectroscopy of Molecular Ions in the Mid-IR James N. Hodges, Kyle N. Crabtree, & Benjamin J. McCallWI06 – June 20, 2012 University of Illinois at Urbana-Champaign

  2. Outline • Motivation • Spectroscopic Techniques for Ions: N2+ • Mid-IR Instrument • H3+ Spectroscopy • Conclusions

  3. Astrochemistry • Ions reactive intermediates in ISM • ~20 ions have been observed • Many carbo-cationshave transitions in mid-IR • Lab spectra help observations B.J. McCall. Ph.D. Thesis, U. Chicago, 2001. 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. Indirect THz Spectroscopy • Combination differences extract energy spacingsfor rotational levels. • Useful for ions with transitions in the THz region - Herschel, SOFIA

  5. Fundamental Science • Fluxional species • Spectrum remains unassigned • WI07 up next! CH5+ White et al. Science, 284, 135 (1999).

  6. Motivation • General, Sensitive, High Precision, Mid-IR Spectrometer for Molecular Ions • General – Multiple Ions of Interest • Sensitive – Weak Transitions & Trace Detection • High Precision – Reduced Uncertainty in Combination Differences

  7. Velocity Modulation Spectroscopy • Cations go to cathode +HV -HV Plasma Discharge Cell S.K. Stephenson and R. J. Saykally. Chem. Rev., 105, 3220-3234, (2005).

  8. Velocity Modulation Spectroscopy • Cationsgo to cathode • Doppler Shifted +HV +HV -HV -HV Laser Detector Plasma Discharge Cell Plasma Discharge Cell S.K. Stephenson and R. J. Saykally. Chem. Rev., 105, 3220-3234, (2005).

  9. Velocity Modulation Spectroscopy • Cationsgo to cathode • Doppler Shifted -HV +HV Laser Laser Detector Detector Plasma Discharge Cell Plasma Discharge Cell Plasma Discharge Cell S.K. Stephenson and R. J. Saykally. Chem. Rev., 105, 3220-3234, (2005).

  10. Velocity Modulation Spectroscopy • Cationsgo to cathode • Doppler Shifted • AC Driven – Absorption Profile Modulated • Velocity Modulation Provides Ion-Neutral Discrimination Laser Laser Detector Detector Plasma Discharge Cell Plasma Discharge Cell Plasma Discharge Cell S.K. Stephenson and R. J. Saykally. Chem. Rev., 105, 3220-3234, (2005).

  11. Velocity Modulation of N2+

  12. Heterodyne Spectroscopy Detector Laser EOM Signal • Creates fm-triplet with spacing typically in the rf • Mixers demodulate rf signal • Sensitive to relative sizes/phases of sidebands • Absorption/Dispersion - 90o Phase Separation • “Zero background” • Operation at rf frequencies reduces 1/f noise

  13. Velocity Modulation of N2+ Velocity Modulation & Heterodyne at 1 GHz

  14. Cavity Enhancement Cavity Detector Laser • Enhances Pathlength • Increases Intracavity Power • Allows saturation of rovibrational transitions – sub-Doppler features • Requires active locking to maintain resonance – PDH locking

  15. Velocity Modulation in a Cavity • Velocity Modulation Provides Ion-Neutral Discrimination

  16. Velocity Modulation • Velocity Modulation Provides Ion-Neutral Discrimination Ion Signal Encoded at 2x the Plasma Frequency

  17. Cavity Enhanced Velocity Modulation Spectroscopy of N2+ PZT Detector Lock-In Amplifier Laser EOM 2f Detector B. M. Silleret al., Opt. Lett., 35, 1266-1268. (2010)

  18. NICE-OHVMS Large Signal Sensitivity to Ions Small Noise Noise Immune C avity Enhanced - Optical Heterodyne Velocity M odulation Spectroscopy Velocity Modulation Cavity Enhancement Heterodyne Spectroscopy NICE-OHVMS B. M. Silleret al., Opt. Exp., 19, 24822-24827. (2011)

  19. NICE-OHVMS • Heterodyne sidebands at the cavity FSR allows the combination of heterodyne spectroscopy with a cavity. Cavity Modes Laser

  20. NICE-OHVMS Detector Laser EOM 90° Phase Shift 1 × Cavity FSR Plasma Frequency 2f Lock-In Amplifier Lock-In Amplifier Absorption Signal Dispersion Signal

  21. Comparison of Techniques on N2+ NICE-OHVMS

  22. Mid-IR Instrument Optical Parametric Oscillator (OPO) High optical power Saturation of rovibrationaltransitions Spans 3.2 – 3.9 μm range

  23. OPO Light Generation Amp YbDoped Fiber Laser OPO EOM 1064 nm

  24. OPO Light Generation Signal 1.5-1.6 m Pump 1064 nm Idler 3.2-3.9 m Periodically Poled Li:NbO3

  25. Ion Production/Velocity Modulation AC HV 40 kHz ~ Gas In L-N2 Out L-N2 In Liquid Nitrogen Cooled Positive Column Discharge Cell- ”Black Widow”

  26. Ion Production

  27. Mid-IR Instrument I P S OPO ni = np - ns 90o Phase Shift Wave-meter 40 kHz Plasma Frequency 2f Lock-In Amplifier EOM Lock-In Amplifier 80 MHz 1 × Cavity FSR YDFL Absorption Signal Dispersion Signal

  28. H3+Spectra Signal Sensitivity = 2 x 10-9 cm-1Hz-1/2 Shot Noise Limit = 8 x 10-11 cm-1 Hz-1/2

  29. H3+Spectra Signal S/N ~500 Precision of Line Center ~300 kHz

  30. Summary & Conclusions • Constructed a general high precision mid-IR spectrometer • Demonstrated the first NICE-OHVMS spectra of H3+ • 1.5 orders of magnitude from the shot noise limit

  31. Acknowledgements McCall Group with Special thanks to: Brian Siller & Joseph Kelly NSF GRF#  DGE 11-44245 FLLW Springborn Endowment

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