1. 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

2. Outline  Motivation  Spectroscopic Techniques for Ions: N 2 +  Mid-IR Instrument  H 3 + Spectroscopy  Conclusions

3. Astrochemistry Ions reactive intermediates in ISM ~ 20 ions have been observed Many carbo-cations have transitions in mid-IR Lab spectra help observations B.J. McCall. Ph.D. Thesis, U. Chicago, 2001.

4. Indirect THz Spectroscopy Combination differences extract energy spacings for rotational levels. Useful for ions with transitions in the THz region - Herschel, SOFIA

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

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 Plasma Discharge Cell +HV-HV S.K. Stephenson and R. J. Saykally. Chem. Rev., 105, 3220-3234, (2005).

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

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

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

11. Velocity Modulation of N 2 +

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

13. Velocity Modulation of N 2 + Velocity Modulation & Heterodyne at 1 GHz

14. Cavity Enhancement Laser Cavity Detector 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 Ion Signal Encoded at 2x the Plasma Frequency Velocity Modulation Provides Ion-Neutral Discrimination

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

18. NICE-OHVMS Large Signal Small Noise Cavity Enhancement Heterodyne Spectroscopy NICE-OHVMS N oise I mmune C avity E nhanced - O ptical H eterodyne V elocity M odulation S pectroscopy Velocity Modulation Sensitivity to Ions B. M. Siller et 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 Lock-In Amplifier Absorption Signal Plasma Frequency Detector Lock-In Amplifier Dispersion Signal 90° Phase Shift 1 × Cavity FSR Laser EOM 2f

21. Comparison of Techniques on N 2 + NICE- OHVMS

22. Mid-IR Instrument Optical Parametric Oscillator (OPO) High optical power Saturation of rovibrational transitions Spans 3.2 – 3.9 μ m range

23. OPO Light Generation Yb Doped Fiber Laser OPO EOM Amp 1064 nm

24. OPO Light Generation Signal 1.5-1.6  m Pump 1064 nm Idler 3.2-3.9  m Periodically Poled Li:NbO 3

25. Ion Production/Velocity Modulation ~ AC HV 40 kHz L-N 2 In L-N 2 Out Gas In Liquid Nitrogen Cooled Positive Column Discharge Cell- ”Black Widow”

26. Ion Production

27. Mid-IR Instrument OPO YDFL EOM Lock-In Amplifier Absorption Signal Lock-In Amplifier Dispersion Signal Wave- meter 40 kHz Plasma Frequency 80 MHz 1 × Cavity FSR 90 o Phase Shift IPSIPS 2f i  p  s

28. H 3 + Spectra Sensitivity = 2 x 10 -9 cm -1 Hz -1/2 Shot Noise Limit = 8 x 10 -11 cm -1 Hz -1/2 Signal

29. H 3 + Spectra S/N ~ 500 Precision of Line Center ~ 300 kHz Signal

30. Summary & Conclusions Constructed a general high precision mid-IR spectrometer Demonstrated the first NICE-OHVMS spectra of H 3 + 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