LINE MIXING EFFECTS OF O2 A-BAND WITH
PHOTOACOUSTIC SPECTROSCOPY IN
SUPPORT OF REMOTE SENSING
Thinh Bui1, Daniel Hogan1, Priyanka M. Rupasinghe1, Mitchio
Okumura1
David A. Long2, Joseph T. Hodges2
Charles E. Miller3
1California
Institute of Technology, Division of Chemistry, MC 127-72,
Pasadena, CA 91125
2NIST, 100 Bureau Drive, Stop 1070, Gaithersburg, MD 20899-1070
3Jet Propulsion Laboratory, California Institute of Technology, MS 183-901,
Pasadena, CA 91109
OUTLINE
 Motivation
 Collisional line mixing
 Photoacoustic spectroscopy (PAS)
 Data/Analysis
 Summary
MOTIVATION
http://oco.jpl.nasa.gov/
 In order to control rapidly growing atmospheric CO2
concentration around the world NASA’s future OCO-2 mission
will monitor global atmospheric CO2 from space.
 As a precession requirement, this space based remote sensing
mission must monitor the global atmospheric CO2 with an
unprecedented accuracy of 0.25%.
MOTIVATION CONT.
http://oco.jpl.nasa.gov/
 To minimize systematic uncertainties OCO-2 will measure
column averaged CO2 dry-air mole fraction (XCO2) as defined by,
X CO 2 
N CO 2
N O2
[O 2 ]
where , [ O 2 ]  0 . 2095
 This requires absorption line shape parameters of O2 and CO2 to
be known with 0.1% accuracy.
MOTIVATION CONT.
 However, as of today, the current knowledge of these parameters
(widths, shifts…) with this accuracy is very limited or not known.
 In order to achieve this goal an accurate modeling of data will be
required including line mixing as well as other non-Voigt
features, collision induced absorption (CIA), Dicke narrowing
and speed dependence (SD).
In this talk we will present our effort to measure line mixing
effect in O2 A-band using photoacoustic spectroscopy.
OUTLINE
 Motivation
 Collisional line mixing
COLLISIONAL LINE MIXING
(A) Low pressure regime
At low pressures, lines can be considered as isolated each other.
COLLISIONAL LINE MIXING
(B) High pressure regime
The resulting spectrum can not be considered as a superposition of two
isolated lines due the population exchange.
COLLISIONAL LINE MIXING
(
): Predictions made with the inclusion of line mixing
( ---- ): Predictions made without the line mixing
Pressure
Typical evolution of the line shape of a doublet of collisionally
coupled transitions with increasing pressure.
Collisional effects on molecular spectra: laboratory experiments and models, consequences for applications / Jean-Michel Hartmann,
Christian Boulet, Daniel Robert. Amsterdam; Boston: Elsevier Science, 2008
OUTLINE
 Motivation
 Collisional line mixing
 Photoacoustic spectroscopy (PAS)
PHOTOACOUSTIC SPECTROSCOPY (PAS)
Why PAS?
 Compared to direct absorption spectroscopy, this method is
benefited by zero background measurement.
 Optically broadband devices (acoustic properties of the system do
not depend on the spectral distribution of the absorbed radiation).
 High precision (0.01%), high resolution (200 MHz) and a large
dynamic range (Unsaturated lines for (0.1 – 5 atm)).
PHOTOACOUSTIC SPECTROMETER
f
Optical
transition
vfi + vmod
P
i
 A non-radiative de-excitation mechanism is considered. Absorbed
photon energy is released via translational energy by increasing the
temperature (heating).
 The heat dissipation to the surrounding environment creates a
pressure wave.
 When the laser is modulated, this pressure variation is also periodic
and results a acoustic wave.
 This acoustic wave can be detected by using a sensitive microphone.
PAS SPECTROMETER AT CALTECH
K.A.Gillis et al. Rev. Sci. Instrum. 81, 064902 (2010)
In order to reduce background noise the spectrometer is
located at sound proof environment.
EXPERIMENTAL SETUP
Preamp
Lock-in Amp
PD
Mic.
PAS cell
400
mW
50 mW
Diode Laser
FG
800 mW
AOM
Amp.
WM
AOM Driver
1.6 kHz
OUTLINE
 Motivation
 Collisional line mixing
 Photoacoustic spectroscopy
 Data/Analysis
A P-BRANCH DOUBLET FITTED WITH GALATRY PROFILE
(NO LINE MIXING)
Spectrum S/N > 10 000, Fitted S/N ~ 600
SAME DOUBLET FITTED WITH GALATRY PROFILE
(PAIRWISE LINE MIXING IS INCLUDED)
A.U
Pressure = 4 atm
Spectrum S/N > 10 000, Fitted S/N ~ 1325
Still not close to spectrum S/N ??
A CLOSER LOOK:
Pairwise line mixing approximation is
no longer valid.
This is a limitation of our current
data analysis software
Solution:
Multi-spectrum fit with
line mixing is required!
RECENT DATA: O2 A-BAND @ 4 ATM

( a  g  X  g ( 0 , 0 ))
1
3
R-branch
A.U
P-branch
Not only line mixing but also CIA is clearly visible.
OUTLINE
 Motivation
 Collisional line mixing
 Photoacoustic spectroscopy (PAS)
 Data/Analysis
 Summary
SUMMARY
 We have utilized a photoacoustic spectrometer for O2 A-band at 760 nm
to meet precision requirements of current (ACOS/GOSAT/TCCON) and
future (OCO-2/OCO-3/ASCENDS) remote sensing applications.
 This can be used to study line mixing effect as well as collision induced
absorption for remote sensing relevant pressures (0.1-5 atm).
 In order to fit the spectra at relatively high pressures, more complete line
mixing model must be included in the fitting procedure.
ACKNOWLEDGMENT
This work was performed at California Institute of Technology, and was
supported by NASA’s Jet Propulsion Laboratory at Caltech and NSF
Graduate Fellowship program.