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