1. Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy
Brian Siller, Andrew Mills, Michael Porambo & Benjamin McCall
Chemistry Department, University of Illinois at Urbana-Champaign

2. Ions & Astrochemistry
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
H2+
H3+
CH+
CH2+
CH3+
CH5+
CH4
C2H3+
C2H2
C3H+
C3H3+
C4H2+
C4H3+
C6H5+
C6H7+
C6H6 H2 C e C+ H
OH+
H2O+
H3O+
H2O OH O
HCO+ CO
HCN
CH3NH2
CH3CN
C2H5CN
N, e
NH3, e
HCN, e
CH3CN, e
CO, e
H2O, e
CH3OH, e CH
CH2CO
CH3OH
CH3OCH3
C2H5+
C2H4
C3H2
C3H
C2H

3. Ion Spectroscopy Techniques
Velocity
Modulation
Hollow Cathode
Supersonic
Expansion   
High ion column density   
Ion-neutral discrimination    
Low rotational temperature    
Narrow linewidth 
Compatible with
cavity-enhanced spectroscopy  

4. Velocity Modulation Spectroscopy
Positive column discharge cell
High ion density, rich chemistry
Cations move toward the cathode
+1kV
-1kV
Plasma Discharge Cell

5. Velocity Modulation Spectroscopy
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

6. Velocity Modulation Spectroscopy
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

7. Velocity Modulation Spectroscopy
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
Laser
Plasma Discharge Cell
Detector

8. Velocity Modulation Spectroscopy1

9. 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
Laser
Plasma Discharge Cell
Detector
Also want lowest noise possible, so combine with heterodyne spectroscopy

10. Velocity Modulation of N2+
Single-pass direct absorption
Single-pass 1GHz 1 2

11. Velocity Modulation Limitations
Doppler-broadened lines
Blended lines
Limited determination of line centers
Sensitivity
Limited path length through plasma

12. Cavity Enhanced Absorption Spectroscopy (CEAS)
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
Detector
Laser

13. Pound-Drever-Hall Locking
Ti:Sapph Laser
PZT
Detector
Polarizing
Beamsplitter
EOM
Detector
Cavity
Transmission
Error Signal
AOM
30MHz
Quarter
Wave Plate
Lock Box

14. CEVMS Setup Audio Amplifier Lock-In Amplifier Transformer Laser
40 kHz
Lock-In Amplifier
Transformer
Laser
Cavity Mirror Mounts

15. CEVMS Setup

16. Extracting N2+ Absorption Signal
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)

17. Extracting N2+ Absorption Signal
V (kV)
t (μs)
Absorption
Relative Frequency

18. Extracting N2+ Absorption Signal
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

19. 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*
B. M. Siller, A. A. Mills and B. J. McCall, Opt. Lett., 35, (2010)
1M. Havenith, M. Schneider, W. Bohle, and W. Urban; Mol. Phys. 72, 1149 (1991)

20. Precision & Accuracy 1 2
Line centers determined to within 1 MHz with optical frequency comb
Sensitivity limited by plasma noise
A. A. Mills, B. M. Siller, and B. J. McCall, Chem. Phys. Lett., 501, 1-5. (2010)

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

22. NICE-OHMS
Noise Immune Cavity Enhanced Optical Heterodyne Molecular Spectroscopy
Cavity Modes
Laser Spectrum

23. Experimental Setup Ti:Sapph Laser PZT Lock Box Detector Polarizing EOM
Beamsplitter
EOM
Detector
AOM
30MHz
Quarter
Wave Plate
Lock Box

24. Experimental Setup
Ti:Sapph Laser
EOM
PZT
Detector

25. Experimental Setup Ti:Sapph Laser N oise I mmune C avity E nhanced -
PZT
Detector
Ti:Sapph Laser
EOM
EOM
N × Cavity FSR
(113 MHz)
N oise
I mmune
C avity
E nhanced -
O ptical
H eterodyne
V elocity
M odulation
S pectroscopy
Lock-In Amplifier
Signal
40 kHz
Plasma
Frequency

26. NICE-OHVMS See talk MI10 for more thorough analysis1 2 3
See talk MI10 for more thorough analysis
Sidebands spaced at 9 cavity FSRs (1 GHz)
3rd derivative-like Doppler lineshape
Lamb dips from each laser frequency and combination of laser frequencies

27. NICE-OHVMS
N2*
N2+
Retain ion-neutral discrimination

28. Velocity Modulation Techniques

29. NICE-OHVMS Summary 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

30. Acknowledgements McCall Group Funding Ben McCall Andrew Mills
Michael Porambo
Funding