Transcript OSU 2013 MJ06.pptx

Examining contributions to line shapes
in the ν1 + ν3 band of acetylene
Matthew Cicha
Damien Forthommeb, Christopher P. McRavenb,
G.V. Lopeza, Salvatore M. Caiolaa, Stephen W Leea,
Trevor J. Searsa,b, Gregory E. Hall, A.W. Mantzc
aStony
Brook University
bBrookhaven National Laboratory
cConnecticut College
June 19, 2013
Motivation
• Atmospheric data is progressing beyond theory
• Need general models that make few assumptions
about the molecules involved
• Speed-dependent Voigt*
• Speed-dependent
Galatry**
• Want lineshape
parameters relevant
at all pressures and
temperatures.
He broadened P(1) regio n
*J. Ward, J. Cooper, E.W. Smith, J. Quant. Spectrosc. Radiat. Transfer 14, 555-590 (1974).
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**R. Ciurylo and J. Szudy, J. Quant. Spectrosc. Radiat. Transfer 57, 411–423 (1997).
High resolution data
Frequency comb-referenced measurements
• Line shifts are easy to measure
• Temperature control
• Direct absorption with 1550 nm laser
C2H2 in the 1.5 μm region
• P(11) line of (10100←00000), high intensity and isolated
• Many series of data at different pressures and temperatures
• Pure acetylene and acetylene in nitrogen
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C2H2- and N2-broadened C2H2 lines
• Speed Dependent Voigt model (SDV)
– Broadening γ(T) and shift δ(T)
– Speed-dependent effects (narrowing and asymmetry), q
• Multispectrum fitting
1.3% C2H2 in N2, Temperature=175 K
39.9 Torr
100.8 Torr
250.3 Torr
402.9 Torr
549.9 Torr
750.3 Torr
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Temperature dependence of broadening γ(T)
Temperature-dependence
follows a power law:
n
 T0  
 (T )   (T0 ) 
T 
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Temperature dependence of shift δ(T)
nδC2H2=0.116(10)
nδ N2=0.592(10)
n
T
 0
 (T )   (T0 ) 
T 
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Example data: 1.3% C2H2 in N2, T=175 K
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Speed dependence: another look
• Speed-dependent Galatry model used as a speed-dependent
Voigt (Q-SDV) and evaluated in the time domain*
– Broadening γ(T) and shift δ(T)
– Dicke narrowing β(T), here set to zero
– Quadratic speed dependence Γ(νa)
• Speed dependence of broadening γ2(T) (narrowing)
• Speed dependence of shift δ2(T) (asymmetry)
   2 3 
( a )    2   a   
 a

2
 0 

νa= velocity
νa0= most probable velocity
*D. Forthomme, FD03, 9:04 am Friday, 1015 McPherson
Γ= γ(T) – iδ(T)
Γ2= γ2(T) – iδ2(T)
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Temperature dependence of narrowing γ2(T)
Temperature-dependence, is
related to speed-dependence
n
 T0   2
 2 (T )   2 (T0 ) 
T 
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Temperature dependence of asymmetry δ2(T)
Temperature dependence is clear but the shape
is currently unmodeled
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Q-SDV fit: small improvements over SDV
Residuals suggest finding a way to account for
β(T) narrowing may improve the fit.
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Parameters determined from multispectrum fits
C2H2-C2H2
(SDV)
nγ
0.68740(8)
γ(T0) 0.152640(4)
nδ
0.116(10)
δ(T0) -0.007684(2)
q
7.352(3)
nγ2
γ2(T0)
δ2(T)
C2H2-C2H2
(QSDV)
0.6871(2)
0.155504(15)
0.1094(9)
-0.007842(3)
0.605(1)
.022109(13)
Varies with T
C2H2-N2
(SDV)
0.7881(2)
0.086366(13)
0.592(10)
-0.009445(2)
6.768(7)
C2H2-N2
units
(QSDV)
0.7378(3)
0.89421(14) cm-1 atm. -1
0.5911(6)
-0.009090(2) cm-1 atm. -1
0.448(2)
0.01391(1)
cm-1 atm. -1
Varies with T
• SDV model has 5 parameters fully describing the line shape
• Q-SDV model has ~8 parameters
• Pending further studies with other molecules these models show
promising behavior for modeling line shapes in atmospheric conditions
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Summary
• Data at all temperatures and pressures can be
modeled with only few effective parameters
• Low T and moderate pressure data illustrates
possiblility for separating dicke narrowing β(T)
from speed-dependent narrowing γ2(T).
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Acknowledgements
Brookhaven National Laboratory
Damien Forthomme
Christopher McRaven
Trevor Sears (also Stony Brook)
Gregory Hall
Connecticut College
Arlan Mantz
Stony Brook University
Gary Lopez
Undergraduates
Salvatore Caiola
Stephen Lee
Work at Brookhaven National Laboratory
was carried out under Contract No. DEAC02-98CH10886 with the
U.S. Department of Energy and
supported by its Office of Basic Energy
Sciences, Division of Chemical Sciences,
Geosciences and
Biosciences.
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Comparison to Voigt
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Nitrogen shift t-dependence
comparison
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Concentration in the SDV
• The total broadening and shift parameters for
any given absorber speed were estimated as
the sum of the pressure-weighted
contributions of each present molecule,
multiplied by a unitless βi function from Ward
et al., Eqn. 7.7.
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What hasn’t been included yet
• High concentration of absorber
• Line mixing and overlapping lines
• Line center measurements
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