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

 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

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  

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

 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

 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

 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

 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

Laser RF FM Laser + -  Most environmental noise scales like 1/f  Velocity modulation is ~40kHz ◦ Much better than direct DC detection ◦ Still room for improvement  Frequency Modulation (FM) ◦ Modulate laser frequency at RF (≳100MHz) ◦ Demodulate detector signal RF Carrier Audio FM Signal

 Single-pass direct absorption  Single-pass 1GHz

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

 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

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

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

 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)

V (kV) t (μs) Absorption Relative Frequency

 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

 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).

N2+N2+ ◦ Velocity directly dependent on voltage ◦ No significant phase shift with respect to voltage N2*N2* ◦ 78° phase shift with respect to N 2 + signal ◦ Peak N 2 * density occurs when rate of formation equals rate of destruction V (kV) t (μs) Peak N 2 * Density

 N 2 + ◦ Velocity directly dependent on voltage ◦ No significant phase shift with respect to voltage  N 2 * ◦ 78° phase shift with respect to N 2 + signal ◦ Peak N 2 * density occurs when rate of formation equals rate of destruction ◦ Analogous to Earth’s heating/cooling cycle with the sun  Sun is brightest at noon (peak voltage and N 2 + velocity)  Hottest time of day is 5pm (peak N 2 * density)  5 hour time delay in 24 hour day = 75° phase shift

 Line centers determined to within 1 MHz with optical frequency comb

 Combination differences to compute THz transitions by observing rovibrational transitions in the mid-IR  Support for Herschel & Sofia THz observatories

J’ cm -1 J” IR Transitions Even Combination differences Odd Combination Differences 1-0 Rotational Transition Reconstructed Rotational Transitions

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

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

3 rd derivative Doppler lineshape Lamb dips from each laser frequency

Direct Absorption Heterodyne Single Pass Cavity Enhanced

 McCall Group  Funding ◦ Air Force ◦ NASA ◦ Dreyfus ◦ Packard ◦ NSF ◦ Sloan