Cavity-Enhanced Velocity Modulation Spectroscopy
description
Transcript of Cavity-Enhanced Velocity Modulation Spectroscopy
Cavity-Enhanced Velocity Modulation Spectroscopy
Brian Siller, Andrew Mills, Michael Porambo & Benjamin McCallUniversity 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
Velocity Modulation Spectroscopy
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
Heterodyne (FM) Spectroscopy
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 Heterodyne @ 1GHz
Velocity Modulation of N2+
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
EOMPZT
Lock Box
30MHz
Detector
Detector
AOM
PolarizingBeamsplitter
QuarterWave Plate
CEVMS Setup
Lock-In Amplifier
Transformer
Cavity Mirror Mounts
Audio Amplifier
Laser
40 kHz
CEVMS Setup
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)
V (kV)
t (μs)
Extracting N2+ Absorption Signal
Abso
rpti
on
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
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).
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
Phase Analysis
V (kV)
t (μs)
Peak N2*
Density
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 and N2
+ velocity) Hottest time of day is 5pm (peak N2
* density) 5 hour time delay in 24 hour day = 75° phase shift
Phase Analysis
Line centers determined to within 1 MHz with optical frequency comb
Precision & Accuracy
Combination differences to compute THz transitions by observing rovibrational transitions in the mid-IR
Support for Herschel & Sofia THz observatories
Indirect Terahertz Spectroscopy
0
80
60
40
20
0
2.01.51.00.50.0-0.5-1.0
80
60
40
20
0
2.01.51.00.50.0-0.5-1.0
3320
3300
3280
3260
3240
2.01.51.00.50.0-0.5-1.0
3320
3300
3280
3260
3240
2.01.51.00.50.0-0.5-1.0
12
3
4
5
6
0
1
2
3
4J’
cm-1
cm-1
J”
IR Transitions
Even Combination differences
Odd Combination Differences
1-0 Rotational Transition
Reconstructed Rotational Transitions
Indirect Terahertz Spectroscopy
Sensitivity Limited by Plasma Noise
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
NICE-OHMS
• 3rd derivative Doppler lineshape• Lamb dips from each laser frequency
Direct Absorption Heterodyne
SinglePass
CavityEnhanced
Velocity Modulation Techniques
McCall Group Funding
◦ Air Force◦ NASA◦ Dreyfus◦ Packard◦ NSF◦ Sloan
Acknowledgements