brian siller, andrew mills, michael porambo & benjamin mccall chemistry department, university...
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Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy
Brian Siller, Andrew Mills, Michael Porambo & Benjamin McCallChemistry Department, 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
Ions & Astrochemistry
H2+
H3+
CH+
CH2+
CH3+
CH5+
CH4
C2H3+
C2H2
C3H+
C3H3+
C4H2+
C4H3+
C6H5+
C6H7+ C6H6
H2
H2
H2
H2
H2
C
e
C+
e
C+
C
H
C2H2
H2
e
OH+H2O+
H3O+H2O
OHe
O
H2
H2
HCO+
CO
HCNCH3NH2
CH3CN
C2H5CN
N, e
NH3, e
HCN, eCH3
CN, e
eCO, e
H2O, e
CH3OH, e
CHCH2CO
CH3OH
CH3OCH3
CH3+
C2H5+e
C2H4
e
C3H2
eC3H
eC2H
Ion Spectroscopy Techniques
Ion-neutral discrimination
Low rotational temperature
Narrow linewidth
Compatible with cavity-enhanced spectroscopy
VelocityModulation
Supersonic
Expansion
Hollow Cathode
High ion column density
Positive column discharge cell◦ High ion density, rich chemistry◦ Cations move toward the cathode
Velocity Modulation Spectroscopy
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
Velocity Modulation Spectroscopy
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
Velocity Modulation Spectroscopy
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
Velocity Modulation Spectroscopy
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
Velocity Modulation Spectroscopy
Plasma Discharge Cell
Laser
Detector
Also want lowest noise possible, so combine with heterodyne spectroscopy
Doppler-broadened lines◦ Blended lines◦ Limited determination of line centers
Sensitivity◦ Limited path length through plasma
Velocity Modulation Limitations
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
Cavity Enhanced Absorption Spectroscopy (CEAS)
Laser
CavityDetector
CavityTransmission
Error Signal
Pound-Drever-Hall Locking
Ti:Sapph Laser
EOM
PZT
Lock Box
30MHz
Detector
Detector
AOM
PolarizingBeamsplitter
QuarterWave Plate
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
Extracting N2+ Absorption Signal
Abso
rpti
on S
trength
(A
rb.
Unit
s)
Relative Frequency (GHz)
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
Extracting N2+ Absorption Signal
Typical Scan of Nitrogen Plasma
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)
B. M. Siller, A. A. Mills and B. J. McCall, Opt. Lett., 35, 1266-1268. (2010)
Line centers determined to within 1 MHz with optical frequency comb
Sensitivity limited by plasma noise
Precision & Accuracy0
1
2
A. A. Mills, B. M. Siller, and B. J. McCall, Chem. Phys. Lett., 501, 1-5. (2010)
Noise Immune Cavity Enhanced Optical Heterodyne Molecular Spectroscopy
NICE-OHMS
Large Signal Small Noise
Cavity Enhancement
Heterodyne Spectroscopy
NICE-OHMS
Noise Immune Cavity Enhanced Optical Heterodyne Molecular Spectroscopy
NICE-OHMS
Cavity Modes
Laser Spectrum
Experimental Setup
Ti:Sapph Laser
EOM
PZT
Lock Box
30MHz
Detector
Detector
AOM
PolarizingBeamsplitter
QuarterWave Plate
Lock-In Amplifier
Signal
40 kHzPlasma
Frequency
Experimental SetupTi:Sapph Laser
EOM
PZT Detector
EOM
N × Cavity FSR(113 MHz) N oise
I mmuneC avityE nhanced-O pticalH eterodyneV elocityM odulationS pectroscopy
NICE-OHVMS
• Sidebands spaced at 9 cavity FSRs (1 GHz)• 3rd 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
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
NICE-OHVMS Summary
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