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  Conclusions

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.

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

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

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

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

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, , (2005).

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, , (2005).

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, , (2005).

Velocity Modulation of N 2 +

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

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

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

Velocity Modulation in a Cavity Velocity Modulation Provides Ion-Neutral Discrimination

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

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

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, (2011)

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

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

Comparison of Techniques on N 2 + NICE- OHVMS

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

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

OPO Light Generation Signal  m Pump 1064 nm Idler  m Periodically Poled Li:NbO 3

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”

Ion Production

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

H 3 + Spectra Sensitivity = 2 x cm -1 Hz -1/2 Shot Noise Limit = 8 x cm -1 Hz -1/2 Signal

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

Summary & Conclusions Constructed a general high precision mid-IR spectrometer Demonstrated the first NICE-OHVMS spectra of H orders of magnitude from the shot noise limit

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