1 / 31

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

aqua
Télécharger la présentation

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

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. 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

More Related