Transcript columbus2005intensitiesMF01.ppt

DO INTEGRATED INFRARED BAND
STRENGTHS CHANGE WITH
TEMPERATURE IN THE GAS-PHASE ?
Robert L. Sams
Steven W. Sharpe
Timothy J. Johnson
Pacific Northwest National Laboratory
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This talk will be divided into 3 parts
One:
A brief introduction of the PNNL ir database.
Two:
.
What does theory says about the temperature
dependence of integrated band strengths?
Three: What we have observed and conclusions?
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Requirements of this spectral library
• Comprehensive collection of signatures -
Content
• Suitable for use with active and passive sensors -
Resolution
• Passive (thermal) spectral region - Coverage
• Applicable to a variety of “climates” • Quantitative -
Temperature
Procedure based
• Rigorous error analysis- Data Analysis
• Validation to others -
NIST comparison
• Documentation - Metadata
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Spectral resolution
User driven requirements–
• Tropospheric, near sea-level  760 Torr  0.1 cm-1
• Active sensors (lidar)  MHz/KHz
• Passive sensors (FTS & dispersive)  =0.5 to 8 cm-1
Practical consideration–
• Best available system (Bruker-66V)
• Time and cost (75 chemicals per year)
• Signal-to-noise (as high as possible)
 = 0.112 cm-1
(spacing is 0.06 cm-1)
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Measured spectral resolution
Lord Rayleigh
0.5
Rayleigh criterion: Two lines, as seen
convolved with the point spread function of
an instrument, are resolved if the maximum
of one line falls on the first null of the second
SINC2 Calculated
Observed
0.4
0.062 cm-1
0.3
0.2
0.1
fwhm = 0.09 cm-1
SINC
Calculated
Observed
0.0
2102.6
Absorbance
0.30
2102.8
2103.0
2103.2
2103.4
2103.6
P(10) of CO
2103.8
2104.0
~2 Torr neat CO
0.20
0.10
0.00
2050
2100
2150
Wavenumbers (cm-1)
2200
2250
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Positional accuracy
(after linear correction)
CO 1st-overtone
N2O
CO fundamental
N2O
Use ~160 CO and N2O lines from NIST (Maki and Wells)
rms= 0.0013 cm-1
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Data analysis
August
Beer
Pierre
Bouguer
Johann
Lambert
In order to derive statistically meaningful values for the extinction
coefficients, one must apply Beer-Bouguer-Lambert’s law
Absorbance10 = concentration x path-length x (abs. coeff.)10 = CL
() = 1/CL = (mol/mol)-1 m-1
Weighted linear least squares fit for A()i = b+ ()CiL, making certain to
vary concentration
Weighting scheme:
W()i = (Ti)2 for A<1.6 (Ti<0.025)
W()i = 0
for A≥1.6 (Ti≥0.025)
were Ti = -log(Ii/I0)
*P.M.
Chu, F.R. Guenther, G.C. Rhoderick and W.J. Lafferty, “The NIST
Quantitative Infrared Database”, J. of Res. Of the NIST, 104 pp. 59-81 1999.
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Data acquisition
15 samples at pressures of:
104.89, 10.1940 , 6.9897 , 5.2855, 4.1265, 2.9744,
2.5300, 1.4745, 0.95546, 0.73963, 0.45713, 0.23808,
0.17109, 0.06291, 0.04708 Torr
SO2F2
Run absorbance features through
dynamic range of instrument
Curve of Growth: The slope is equal to
the absorption coefficient. Note nonlinear behavior at higher burdens.
– Weighted fit
– Unweighted fit
1.6
All points above red
line are weighted as
zero
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Bryce Crawford Jr. and co-workers in the
late 1950’s using the harmonic-oscillator
expression finds the integrated band
strengths of fundamental bands for diatomic
molecules are invariant to temperature.
They find that overtones do have a small
temperature dependence (increase band
strength with increasing temperature)
because the induced emission term does not
cancel.
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Scientist at General Dynamics in the mid
1960’s expand on Bryce Crawford’s earlier
findings to small polyatomic molecules. That
is, the band strength of fundamental
vibrations are invariant to temperature and
that overtones and combination bands have a
small increase with increasing temperature.
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John Overend and students in the mid
1970’s using anharmonicity in the
harmonic-oscillator expression finds the
integrated band strengths of fundamental
bands for diatomic and small polyatomic
molecules can have a small temperature
dependence (10% increase at a
temperature of 5,000-60,000 K). They also
suggest that molecules with lots of low
frequency fundamentals may exhibit a
larger temperature dependence.
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Lets see if we can detect any of these
temperature effects. First are the spectra
in the PNNL database precise enough?
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Second: Is there a temperature effect for
most fundamental bands?
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Systematic study of Freons w/r temperature
AX  A (5,25,50˚C )
% 
100
A (5,25,50˚C )
HCFC RANGE
F10
700-1700
F11
650-1800
F12
620-1700
F13
725-1500
F14
580-1980
F20
590-1850
F21
610-1740
F23
630-1900
F30
620-1700
F32
620-1700
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Next: Do combinations and overtones
have a temperature dependence in the
278-323 K temperature range?
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molecule
band
calc increase obs increase
CS2
22+3
7.3 %
6.1 %
N2O
22
5.5 %
4.7 %
OCS
22
6.7 %
6.5 %
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Last: Do the fundamental bands in a
polyatomic molecule with lots of low
frequency fundamentals like acetone
have any temperature dependence in the
233-323 K temperature range?
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Some integrated band strengths of acetone
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Additional bonus : Peak heights of
fundamental bands that have Q-branches
are not invariant to temperatures, but if
we do a linear fit of these Q-branches
using the three temperatures of the
database then the peak height can be
estimated to about 4% at stratospheric
temperatures.
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-6
-1
Absorption coefficient (µmole/mole) m
-1
200x10
Q-branch peak increase of an acetone band
with temperature 233-323 K
150
100
50
0
2976
2974
2972
2970
2968
-1
Wavenumber (cm )
2966
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180x10
* Observed peak height 2970 band in acetone
-6
-1
Absorption coefficient (µmole/mole) m
-1
--- linear fit of peak heights to the 3 highest temperatures
170
160
150
140
130
240
260
280
Temperature (K)
300
320
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CONCLUSION: A selection of molecules
from the PNNL database corroborates the
theoretical findings that there are small
temperature dependences of overtone and
combination bands but fundamentals (which
have most of the integrated intensity) will
have no measurable (< 0.2%) temperature
dependence in the temperature range 233323 K.
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The authors
Steven Sharpe
Timothy Johnson
Robert
Sams
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