Brian Siller, Andrew Mills & Benjamin McCall
University of Illinois at Urbana-Champaign
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Positive column discharge cell
◦ High ion density, rich chemistry
◦ Cations move toward the cathode
+1kV
-1kV
Plasma Discharge Cell

Positive column discharge cell
◦ High ion density, rich chemistry
◦ Cations move toward the cathode
◦ Ions absorption profile is Doppler-shifted
+1kV
-1kV
Laser
Plasma Discharge Cell
Detector

Positive column discharge cell
◦ High ion density, rich chemistry
◦ Cations move toward the cathode
◦ Ions absorption profile is Doppler-shifted
-1kV
+1kV
Laser
Plasma Discharge Cell
Detector

Positive column discharge cell
◦ High ion density, rich chemistry
◦ Cations move toward the cathode
◦ Ions absorption profile is Doppler-shifted
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Drive with AC voltage
◦ Ion Doppler profile alternates red/blue shift
◦ Laser at fixed wavelength
◦ Demodulate detector signal at modulation frequency
Laser
Plasma Discharge Cell
Detector
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Want strongest absorption possible
Signal enhanced by modified White cell
◦ Laser passes through cell unidirectionally
◦ Can get up to ~8 passes through cell
Laser
Plasma Discharge Cell
Detector
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Optical cavity acts as a multipass cell
◦ Number of passes =
◦ For finesse of 150, get ~100 passes
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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
Cavity
Laser
Detector
Detector
Ti:Sapph
Laser
EOM
PZT
Detector
Cavity
Transmission
14MHz
Lock
Box
Error Signal
Audio Amplifier
Transformer
Laser
Cavity Mirror Mounts
40 kHz
Lock-In
Amplifier
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Absorption Strength (Arb. Units)

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
Relative Frequency (GHz)
V (kV)
Absorption
t (μs)
Relative Frequency
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
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
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Cavity Finesse 150
30mW laser power
N2+ Meinel Band
N2* 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 N2+/N2*
1M.
Havenith, M. Schneider, W. Bohle, and W. Urban; Mol. Phys. 72, 1149 (1991).

N2+
◦ Velocity directly dependent on voltage
◦ No significant phase shift with respect to voltage
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N2*
◦ 78° phase shift with respect to N2+ signal
◦ Peak N2* density occurs when rate of formation
equals rate of destruction
V (kV)
Peak N2* Density
t (μs)
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N2+
◦ Velocity directly dependent on voltage
◦ No significant phase shift with respect to voltage

N2*
◦ 78° phase shift with respect to N2+ signal
◦ Peak N2* 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 N2* density)
 5 hour time delay in 24 hour day = 75° phase shift
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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)
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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
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Heterodyne Detection
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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
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McCall Group
Funding
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Air Force
NASA
Dreyfus
Packard
NSF
Sloan