RABI CHHANTYAL PUN, PHILLIP THOMAS, DMITRY G. MELNIK AND TERRY A. MILLER The Ohio State University,...
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Transcript of RABI CHHANTYAL PUN, PHILLIP THOMAS, DMITRY G. MELNIK AND TERRY A. MILLER The Ohio State University,...
RABI CHHANTYAL PUN, PHILLIP THOMAS, DMITRY G. MELNIK AND TERRY A. MILLER
The Ohio State University, Dept. of Chemistry, Laser Spectroscopy Facility, 120 W. 18th Avenue, Columbus, Ohio 43210
QUANTITATIVE MEASUREMENTS OF ABSORPTION CROSS-SECTIONS BY DUAL WAVELENGTH CAVITY RING-DOWN SPECTROSCOPY
Characterization and monitoring peroxy radicals
Combustion chemistry: • Key intermediate in low-temperature (<1000 K) combustion. Atmospheric chemistry:
• Hydrocarbon/VOC oxidation• Ozone production• Acid rains
O. J. Nielsen and T. J. Wallington, in Peroxyl Radicals, (John Wiley and Sons, New York, 1997), pp. 72-73.
B-X transition:
• Strong ( 10-17 cm2)• Dissociative transition, lacks selectivity
A-X transition:
• Weak ( ~ 10-20 cm2)• Selective
J.A. Jafri and D.H. Phillips, J. Am. Chem. Soc. 112, 2586 (1990)
Problem formulation
• A number of research problems require quantitative measurements of the concentration of the reactive species.
• Absorption spectroscopy is the most straightforward (and in certain cases, the only) method of such a measurement
Beer’s law for optically thin media:
• In general,
The goal:
• Calculate the |e|2 from the measurements of p
(i) enable the calculation of at the variety of conditions(ii) provide the benchmark for quantum chemical calculations of
|e|2 which is difficult to calculate.
0
( )( )
( )
In l
I
2( ) , , ;
max( ( ))
( )
i
p
e
I
G T P a
d
Experimental method
• Approach to determining p: obtain the absorption cross-section and transition strength from the fractional absorption of the sample of known concentration
• The concentration of the reactive species is not known a priori and needs to be measured independently
I0 I0-I
l
n?
• Determining n: Use a well-studied synthesis co-product as a “reporter” molecule (i.e. a molecule whose absorbing properties are well-known).
Example: HCl
[RO2] = [HCl]
1932
2 2 2 2
( ) 2 2nmCOCl CO Cl
RH Cl R HCl
R O N RO N
• The spectra of peroxy radicals and HCl are typically separated in frequency scale.• Solution: use dual-wavelength absorption technique
measures (I/I0) of RO2
measures [HCl]=[RO2]
A.V.Baklanov, L.V. Krasnoperov, J. Phys. Chem. A, 105, 97, (2001)M. Bartel, K. Hoyermann, U.Lange, Ber. Bunsen-Ges.Phys.Chem., 93, 423, (1989)
Principal design of the dual-CRDS setup
Sirah Dye LaserH2 Raman Cell (300 psi)
YAG 532 nmH2 Raman Cell (200 psi) Sirah Dye Laser
PD
PD
Laser system 1
YAG 532 nm
Laser system 2
Reaction region
Second Stokes (1.8 m) for HCl overtone
Second Stokes(1.3 m) forA-X of RO2
LP filter
LP filter
ExcimerLaser
193 nm
AD
C
Pulse/DelayGenerator
GatePhotolysis control ( computer)
Dye laser control(computer)
Arm “A”
Arm “B”
Equivalence of the optical paths
F r e q u e n c y , c m - 1
8 3 2 0 8 3 4 0 8 3 6 0 8 3 8 0 8 4 0 0 8 4 2 0 8 4 4 0 8 4 6 0 8 4 8 0
Abs
orpt
ion,
ppm
/pas
s
0
1 0
2 0
3 0
4 0
5 0
6 0
7 0
A r m A
A r m B
CH3O2 O-O stretch band
10 ppmvertical offset
cm-1
7200 7300 7400 7500 7600 7700 7800
ppm
0
20
40
60
80
100
120
140
160
180
Ethyl peroxy radical as a test system
P.Rupper , E.N. Sharp, G. Tarczay and T.A.Miller, J. Phys. Chem. A, 111, 832 (2007)
000
000
2 5 2 2 5 2
2 5 2 2 5 2
[ ] :[ ] 3 :1 at T=293 K
[ ] 0.7 [ ]5
G T
G
C H O C H O
C H O C H O
G conformerT conformer
Ethyl peroxy radical: • the simplest RO2 that could be obtained using H-abstraction• well characterized(a)
Frequency (cm-1)
5526 5528 5530 7550 7600 7650
Abs
orba
nce
(ppm
/pas
s)
0
20
40
60
C2H5O2
G conformer origin
H35Cl
H37Cl
P(6) transition(first overtone)
Dual wavelength scan of C2H5O2/HCl
Calculation of [HCl] from the absorption spectra
HCl L
det expii i
tI A I
Ii – laser line profile i – absorption profile
P e a k m o l e c u l a r a b s o r p t i o n , ppm
0 50 100 150 200 250 300
C a
l c u
l a t
e d
e f
f e c
t i v e
a b
s o
r p t
i o n
, p
p m
0
50
100
150
200
0.66
1
0.33
0.16
mol
L
“Response curve” for multiexponential decay(nonlinear LSF)
Tran (H37Cl) Sn’n x1021 HCl,cm-1
P1 1.541 0.071
P2 2.524 0.064
P3 2.802 0.058
P4 2.501 0.052
P5 1.893 0.044
P6 1.245 0.036
-10.076 cmL
1
Calculation of P of G-conformer of C2H5O2
P[(COCl)2], Torr
0.000 0.057 0.114 0.170 0.227
Abs
, ppm
/pas
s
0
10
20
30
40
50
60
P[(COCl)2], Torr
0.045 0.068 0.091 0.114 0.136 0.159 0.182 0.205 0.227 0.250
abs(
HC
l)/ab
s(C
2H5O
2)
0.0
0.2
0.4
0.6
0.8
1.0
P(N2)=216 TorrP(O2)=84 TorrP(C2H6)=0.1Torr
C2H5O2
HCl
2 5 2
[ ]
0.75 [ ]
p
HCl
HCl
p
pC H O
IHCl
I S l
IHCl l
I
0.743(47)
2 5 2
2 2
1
16.3(4)
0.7
m
5
10 c
pHCl C H OHCl
p
S I II I
Measurement of self-reaction decay of C2H5O2
Time, s
0 2000 4000 6000 8000 10000 12000
abs
0/a
bs(t)
0
1
2
3
4
Self-reaction decay:
22 5 22 5 2
2 5 2 0
2 52 5
2
14 1
2
3
0
2
1( )
1.1(3) 10 cm
2 se
se
lf
lf
self
k C H O
d C H Ok C H O
dtC H O
tC H O t
k s
(a) Lightfoot et. al. Atmos. Envir. 26A, 1805 (1992)(b) D.B.Atkinson and J.L.Spillman, J.Phys.Chem.A 106, 8891 (2002)
2[C2H5O2]0kself=226(32)s-1
From HCl absorption measurements:
a,b
21 24.1(11) 10 cmp
21 26.3(4) 10 cmp
Comparison with previously measured valuesof p of Ethyl Peroxy radical
Source p x10-21 cm2 Method of concentration measurements
P .Rupper et.al(a) 4.4(11)(c) (COCl)2 absorption ofphotolysis beam
D. B. Atkinson andJ. L. Spillman (b)
3.0(15) Self-reaction decay of peroxy radicals
This work 6.3(4) HCl absorption
This work 4.1(11) Self-reaction decay of peroxy radicals
[HCl] absorption method:
• Low random error• Potentially affected by a systematic error from correction factor due to nonlinear response curve. Solution: use narrow line source
(a) P.Rupper , E.N. Sharp, G. Tarczay and T.A.Miller, J. Phys. Chem. A, 111, 832 (2007)(b)D.B. Atkinson and J.L.Spillman, J.Phys.Chem.A, 106, 8891, (2002) (c)Our estimate
Conclusion
We have developed a novel method of measurement of the absorption cross-sections of the transient reactive species.
The intrinsic advantages of this method:
• The method does not rely on the absolute power measurements (i.e. insensitive to the power fluctuation of the source and detector calibration issues)
• Does not rely on the previously measured values of the reaction constants that have intrinsically large error bars
• The method uses the previously determined transition strength of the stable species (e.g., HCl) which are determined to substantially higher precision.
• The method relies upon the equivalence of the optical paths of the interrogating beams which has been successfully demonstrated
• The major source of the systematic error is straightforwardly eliminated by using a narrow light source for [HCl] measurements (in progress)
Acknowledgements
• Colleagues:
Gabriel Just, Ming-Wei Chen Terrance Codd, Neal Kline
•OSU
•DOE