1. Brian Siller, Michael Porambo & Benjamin McCall Chemistry Department University of Illinois at Urbana-Champaign

2.  Applications ◦ Astrochemistry ◦ Fundamental physics  Goals ◦ Completely general (direct absorption) ◦ High resolution

3.  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

4.  Combination differences to compute THz transitions by observing rovibrational transitions in the mid-IR  Support for Herschel, SOFIA, and ALMA THz observatories 60-670 µm0.3-1600 µm3-400 µm

5. 0 1 2 3 4 5 6 0 1 2 3 4 J’ cm -1 J” IR Transitions Even Combination differences Odd Combination Differences 1-0 Rotational Transition Reconstructed Rotational Transitions

6.  CH 5 + is a prototypical carbocation ◦ S N 1 reaction intermediates ◦ Highly fluctional structure ◦ Spectrum completely unassigned E.T. White, J. Tang, and T. Oka, “CH5+: The Infrared Spectrum Observed”, Science, 284, 135-137 (1999). Animation from Joel Bowman, Emory University

7.  Positive Column ◦ High ion density ◦ Simple setup  Ion Beam ◦ Rigorous ion-neutral discrimination ◦ Mass-dependent Doppler shift

8.  Positive column discharge cell ◦ High ion density, rich chemistry ◦ Cations move toward the cathode Plasma Discharge Cell +1kV-1kV

9.  Positive column discharge cell ◦ High ion density, rich chemistry ◦ Cations move toward the cathode ◦ Ions absorption profile is Doppler-shifted Plasma Discharge Cell +1kV-1kV Laser Detector

10.  Positive column discharge cell ◦ High ion density, rich chemistry ◦ Cations move toward the cathode ◦ Ions absorption profile is Doppler-shifted Plasma Discharge Cell -1kV+1kV Laser Detector

11.  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 Plasma Discharge CellDetector Laser

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13.  Want strongest absorption possible  Signal enhanced by modified White cell ◦ Laser passes through cell unidirectionally ◦ Can get up to ~8 passes through cell Plasma Discharge Cell Laser Detector  Also want lowest noise possible, so combine with heterodyne spectroscopy

14.  Single-pass direct absorption  Single-pass Heterodyne @ 1GHz 0 1 2

15.  Doppler-broadened lines ◦ Blended lines ◦ Limited determination of line centers  Sensitivity ◦ Limited path length through plasma  Improve by combining with cavity enhanced absorption spectroscopy

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

17. Lock-In Amplifier Transformer Cavity Mirror Mounts Audio Amplifier Laser 40 kHz

19.  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)

20.  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

21.  Cavity Finesse 150  30mW laser power  N 2 + Meinel Band  N 2 * 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 N 2 + /N 2 * 1 M. Havenith, M. Schneider, W. Bohle, and W. Urban; Mol. Phys. 72, 1149 (1991) B. M. Siller, A. A. Mills and B. J. McCall, Opt. Lett., 35, 1266-1268. (2010)

22.  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)

23.  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)

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

25. Ti:Sapph Laser EOM PZT Detector

26. Absorption Signal Lock-In Amplifier 40 kHz Plasma Frequency Ti:Sapph Laser EOM PZT Detector EOM 113 MHz Cavity FSR Dispersion Signal Lock-In Amplifier 90° Phase Shift XYXY

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

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

29. 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! Absorption Dispersion 113MHz Ultra-High Resolution Spectroscopy

30. VMSOHVMS CEVMS NICE-OHVMS

31.  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

32. Ion Beam Instrument Absorption Signal Lock-In Amplifier 40 kHz Plasma Frequency Ti:Sapph Laser EOM PZT Detector EOM Dispersion Signal Lock-In Amplifier XYXY

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

36.  Ion density ~5×10 6 cm -3  Cavity finesse ~450  Lock-in τ=10s  4kV float voltage  ±5V modulation  ~120MHz linewidth Ion mass Float voltage

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

38.  Positive Column ◦ Mid-IR OPO system  ~1W mid-IR idler power  Pump and signal lasers referenced to optical frequency comb ◦ Liquid-N 2 cooled discharge cell  Ion Beam ◦ Mid-IR DFG laser  Ti:Sapph referenced to comb  Nd:YAG locked to I 2 hyperfine transition ◦ Supersonic expansion discharge source

39.  McCall Group ◦ Ben McCall ◦ Michael Porambo  Funding