High-Resolution Spectroscopy of the ν8
Band of Methylene Bromide Using a
Quantum Cascade Laser-Based Cavity
Ringdown Spectrometer
Jacob T. Stewart and Brian E. Brumfield, Department
of Chemistry, University of Illinois at Urbana-Champaign
Matthew D. Escarra and Claire F. Gmachl,
Department of Electrical Engineering, Princeton University
Benjamin J. McCall, Departments of Chemistry and
Astronomy, University of Illinois at Urbana-Champaign
Why a Quantum Cascade Laser-Based
Spectrometer?
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
C60 spectroscopy at
~8.5 μm in order to
perform an
astronomical search
Quantum cascade
lasers (QCLs) offer
performance necessary
for high-resolution
spectroscopy at this
wavelength
How do QCLs work?




Semiconductor laser
based on stacks of
quantum wells
Lasing occurs through
transitions within the
conduction band
Different frequencies
possible by changing
thickness of quantum
wells
Each QCL has limited
tunability
Testing our QCL Spectrometer



Decided on methylene
bromide as a test
molecule
No previous highresolution work in IR
Probe of temperature
conditions in
supersonic jet
•ν8 band of CH2Br2 ~1197 cm-1
•Tuning range of our QCL ~1182-1200
cm-1
Initial Layout of QCL Spectrometer
800 μm pinhole
PC-MCT
Experimental Spectra and Spectrometer
Performance
Reference (SO2) Spectrum
Wavemeter
CH2Br2 Spectrum
Noise equivalent absorption = 1.4×10-8 cm-1
Linewidth = 40 - 60 MHz
Step size = ~21 MHz (0.0007 cm-1)
Sensitivity = 5×10-8 cm-1Hz-1/2
Assigning the Spectrum
79

79
79
81
81

81
Three isotopologues:

Abundance
•CH279Br2
1
•CH279Br81Br
2
•CH281Br2
1

Two Br isotopes with
almost equal
abundance (79Br & 81Br)
Near-prolate top
Ground state rotational
constants known from
microwave
Fitting done using
PGOPHER
PGOPHER, a Program for Simulating Rotational Structure, C. M. Western, University of
Bristol, http://pgopher.chm.bris.ac.uk
Room Temperature Spectra
Room Temperature Spectra
Room Temperature Spectra
Jet-Cooled Spectra
Experimental Spectrum of Jet-Cooled Sample
Combination
Trot = 300 K
Simulated spectrum Trot = 7 K
B. E. Brumfield, J. T. Stewart, S. L. Widicus Weaver, M. D. Escarra, S. S. Howard, C. F. Gmachl, B. J. McCall,
Rev. Sci. Instrum. (2010), 81, 063102.
Molecular Constants
Molecule
ν0 (cm-1)
A’ (cm-1)
CH279Br2
1196.98363(99)
0.8634519(22) 0.00035
20
CH279Br81Br 1196.95797(12)
0.8626649(28) 0.00045
22
CH281Br2
0.8619108(23) 0.00044
20
1196.93206(12)
Avg. |o-c|
(cm-1)
# lines
assigned
Improvements to the Spectrometer

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PC-MCT
Faster detector (PVMCT from Kolmar)
Using a 150 μm×12
mm slit instead of 800
μm pinhole
Ten times smaller
current step size
New piezo driver to
increase ringdowns per
second
New mirror mounts with
flexible
bellows
PV-MCT
New and Improved Spectra
Not Noise!
Noise equivalent absorption = ~4×10-9 cm-1
Linewidth = ~10 MHz
Step size = ~2.4 MHz (0.00008 cm-1)
Sensitivity = ~8×10-9 cm-1 Hz-1/2
Conclusions
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Quantum cascade lasers are useful light
sources for high-resolution infrared
spectroscopy
We have constructed the first QCL-based cwringdown spectrometer coupled with a
supersonic expansion source
We have obtained and assigned the
previously unobserved rotational structure of
the ν8 band of methylene bromide
What’s Next?
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
Collect high-resolution
spectrum of pyrene (C16H10)
Collect high-resolution
spectrum of C60
Dealing with Back-Reflection
Ringdown Cavity
Fresnel rhomb uses
total internal
reflections to act like
a quarter wave plate
Laser

Polarizer
ZnSe Frsenel Rhomb
Acknowledgments
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Brian Siller
Andrew Mills
Richard Saykally
Kevin Lehmann
Funding
•NASA
•Packard Foundation
•Dreyfus Foundation
•University of Illinois