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

2.  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

3. Ion Spectroscopy Techniques    Ion-neutral discrimination Low rotational temperature Narrow linewidth Compatible with cavity-enhanced spectroscopy Velocity Modulation Supersonic Expansion Hollow Cathode  High ion column density  

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

5.  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

6.  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

7.  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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9.  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

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

11.  Doppler-broadened lines ◦ Blended lines ◦ Limited determination of line centers  Sensitivity ◦ Limited path length through plasma

12.  Optical cavity acts as a multipass cell ◦ Number of passes = ◦ For finesse of 300, get ~200 passes  Must actively lock laser wavelength/cavity length to be in resonance with one another  DC signal on detector is extremely noisy ◦ Velocity modulation with lock-in amplifier minimizes effect of noise on signal detection Laser Cavity Detector

13. Cavity Transmission Error Signal Ti:Sapph Laser EOM PZT Lock Box 30MHz Detector AOM Polarizing Beamsplitter Quarter Wave Plate

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

16.  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)

17. V (kV) t (μs) Absorption Relative Frequency

18.  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

19.  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)

20.  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)

21.  Noise Immune Cavity Enhanced Optical Heterodyne Molecular Spectroscopy Large Signal Small Noise Cavity Enhancement Heterodyne Spectroscopy NICE-OHMS

22.  Noise Immune Cavity Enhanced Optical Heterodyne Molecular Spectroscopy Cavity Modes Laser Spectrum

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

24. Ti:Sapph Laser EOM PZT Detector

25. Lock-In Amplifier Signal 40 kHz Plasma Frequency Ti:Sapph Laser EOM PZT Detector EOM N × Cavity FSR (113 MHz) N oise I mmune C avity E nhanced - O ptical H eterodyne V elocity M odulation S pectroscopy

26. Sidebands spaced at 9 cavity FSRs (1 GHz) 3 rd derivative-like Doppler lineshape Lamb dips from each laser frequency and combination of laser frequencies 0 1 2 3 See talk MI10 for more thorough analysis

27. Retain ion-neutral discrimination N2*N2* N2+N2+

29.  Increased path length through plasma  Better sensitivity due to heterodyne modulation  Retained ion-neutral discrimination  Sub-Doppler resolution due to optical saturation ◦ 50 MHz Lamb dip widths ◦ Resolve blended lines ◦ Better precision & absolute accuracy with comb

30.  McCall Group ◦ Ben McCall ◦ Andrew Mills ◦ Michael Porambo  Funding