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

 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 Cell Detector 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

 Optical cavity acts as a multipass cell ◦ Number of passes = ◦ For finesse of 150, get ~100 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 14MHz Detector

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)  Hottest time of day is 5pm (peak N 2 * density)  5 hour time delay in 24 hour day = 75° phase shift

 Potentially orders of magnitude more sensitivity  Allows for saturation spectroscopy ◦ Resolve Doppler-blended lines ◦ Better line center determination ◦ Width of Lamb dip allows more information to be extracted from spectra  (See talk FD02 for much more detail)

 Better isolation of plasma noise ◦ Faraday cage ◦ Dedicated grounding  Improved cavity locking ◦ Less noise induced by lock to cavity ◦ Allow for locking to a higher finesse cavity  Heterodyne Detection

 Improves upon absorption strength of velocity modulation experiments  Allows for Doppler-free spectroscopic studies of molecular ions  Very general technique  To unlock full potential, must first minimize plasma noise with careful experimental setup

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