Lineshape and Sensitivity of
+
Spectroscopic Signals of N2 in a
Positive Column Collected Using
NICE-OHVMS
Michael Porambo, Andrew Mills, Brian Siller, Benjamin J. McCall
University of Illinois at Urbana-Champaign
20 June 2011
Outline
• Introduction
• Lineshape Description, Analysis, Ultra-high
Resolution Spectroscopy
• Sensitivity Comparison
• Summary, Conclusions, Present and Future
Work
Spectroscopic Techniques
Velocity Modulation
Spectroscopy (VMS)1,2
1Gudeman
and Saykally, Ann. Rev. Phys. Chem. 1984.
and Saykally, Chem. Rev. 2005.
2Stephenson
Optical Heterodyne3
Velocity Modulation
Spectroscopy (OHVMS)4
EOM
3Bjorklund
4Lindsay,
and Levenson, Appl. Phys. B 1983.
Ph.D. Thesis, University of Chicago, 2002.
Cavity Enhanced
Velocity Modulation
Spectroscopy4,5
4Siller
5Mills
EOM
Sample
et al. Optics Lett. 2010.
et al. Chem. Phys. Lett. 2010.
Noise Immune Cavity
Enhanced Optical
Heterodyne Molecular
Velocity
Spectroscopy (NICEModulation
OHMS)6,7
Spectroscopy
(NICEYe et al. J. Opt. Soc. Am. B 1998.
OHVMS)
Foltynowicz et al. Appl. Phys. B, 2008.
6
7
NICE-OHVMS
N2+ Signal with NICE-OHVMS
Lamb dips from optical saturation
Sideband-carrier interaction
A. U.
~1 GHz
~500 MHz
Sideband-sideband interaction
Carrier-carrier interaction
NICE-OHVMS spectrum of Q11(14) of N2+ acquired with 1 GHz
heterodyne detection bandwidth.
Heterodyne Detection Bandwidth
Relative Frequency (MHz)
As cavity length is scanned, FSR changes.
Laser sidebands do not couple into the cavity
as efficiently, noise immunity suffers.
1.02 GHz (9 × FSR) – 9 kHz shift in longitudinal
mode with respect to sideband.
113 MHz (1 × FSR) – 1 kHz shift in longitudinal
mode with respect to sideband.
Absorption and Dispersion
Absorption
Dispersion
-
+
Absorption and Dispersion
Absorption and dispersion related by the KramersKronig relations.
Example for Gaussian
absorption profile:
Heterodyne Detection Bandwidth
Ti:Sapph
Laser
Detector
PZT
EOM
EOM
Absorption
90° Phase
Shift
X
9
1 × Cavity FSR
1.02
113 MHz
GHz
Y
Lock-In
Amplifier
Dispersion
X
Y
Lock-In
Amplifier
Absorption
Signal
Y
Dispersion
Signal
XY
X
40 kHz
Plasma
Frequency
113
MHz
Detection
Dispersion
Absorption
113 MHz Sidebands
1 Cavity FSR
Dispersion
Lock-In X
Lock-In Y
Absorption
Sub-Doppler Spectra
Dispersion
Absorption
Lock-In X
Lock-In Y
No center
Lamb dip in
absorption
Spectra calibrated with optical frequency comb
Frequency precision to ~1 MHz!
Ultra-High Resolution Spectroscopy
Dispersion
Red – Data
Blue - Fit
Absorption
Red – Fit
Blue - Data
113 MHz
Sub-Doppler fitting equation modeled as convolution of Gaussian and
Lorentzian absorption and dispersion profiles (2 absorption/each,
3 dispersion/each)
Line center from fit: 326,187,572.2 ± 0.1 MHz
After correcting for systematic problems, line center measured to within
uncertainty of ~300 kHz!
Signal and Noise Calculations
S/N for Different Techniques
900
NICE-OHVMS (1 GHz)
800
700
S/N
600
500
OHVMS
(1 GHz)
400
300
200
100
0
VMS
CEVMS
Signal-to-noise ratio calculated for different detection
techniques under the same conditions.
NICE-OHVMS S/N factor of 2 greater than the next
sensitive technique!
Technique Comparison
VMS
CEVMS
OHVMS
NICE-OHVMS
Summary and Conclusions
• NICE-OHVMS addresses well challenges in
direct absorption/dispersion spectroscopy of
ions.
• Distinctive, absorption/dispersion lineshape with
Lamb dips.
• Precise line centers obtained using Lamb dips
and calibrating to optical frequency comb (~1
MHz precision).
• S/N greatly improved over VMS, OHVMS, and
CEVMS.
Present and Future Work
Vibrational spectroscopy in the mid-IR
• Positive column discharge setup with CW OPO
(Aculight Argos).
• Study molecular ions of astronomical, fundamental
chemical interest (e.g., CH5+).
Highly sensitive technique for
molecular ion beam detection
McCall group ion beam instrument
Aculight Argos CW OPO
http://www.lockheedmartin.com/data/assets/ms
2/pdf/ArgosSF.pdf
• Direct absorption/dispersion
spectroscopy of N2+ in a fast ion beam.
• Stay tuned for next talk (MI11) on ion
beam.
Acknowledgments
• McCall Research Group
Ben McCall
Andrew Mills
Brian Siller
• Sources of Funding
–
–
–
–
–
–
Air Force – Research Corp.
NASA
– Univ. of Illinois
Dreyfus
Packard
NSF
Sloan