1. 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

2. 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 cm 2 ) Dissociative transition, lacks selectivity A-X transition: Weak (  ~ 10 -20 cm 2 ) Selective J.A. Jafri and D.H. Phillips, J. Am. Chem. Soc. 112, 2586 (1990)

3. 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.

4. 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 I0I0 I0-II0-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 [RO 2 ] = [HCl] The spectra of peroxy radicals and HCl are typically separated in frequency scale. Solution: use dual-wavelength absorption technique measures (  I/I 0 ) of RO 2 measures [HCl]=[RO 2 ] 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)

5. Principal design of the dual-CRDS setup Sirah Dye LaserH 2 Raman Cell (300 psi) YAG 532 nm H 2 Raman Cell (200 psi) Sirah Dye Laser 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) for A-X of RO 2 LP filter Excimer Laser 193 nm ADC Pulse/Delay Generator Gate Photolysis control ( computer) Dye laser control (computer) Arm “A” Arm “B”

6. Equivalence of the optical paths CH 3 O 2 O-O stretch band 10 ppm vertical offset

7. 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) G conformer T conformer Ethyl peroxy radical: the simplest RO 2 that could be obtained using H-abstraction well characterized (a)

8. Dual wavelength scan of C 2 H 5 O 2 /HCl

9. Calculation of [HCl] from the absorption spectra I i – laser line profile  i – absorption profile “Response curve” for multiexponential decay (nonlinear LSF) Tran (H 37 Cl) S n’n x10 21  HCl,cm -1 P11.5410.071 P22.5240.064 P32.8020.058 P42.5010.052 P51.8930.044 P61.2450.036

10. Calculation of  P of G-conformer of C 2 H 5 O 2 P(N 2 )=216 Torr P(O 2 )=84 Torr P(C 2 H 6 )=0.1Torr C2H5O2C2H5O2 HCl 0.743(47)

11. Measurement of self-reaction decay of C 2 H 5 O 2 Self-reaction decay : (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[C 2 H 5 O 2 ] 0 k self =226(32)s -1 From HCl absorption measurements : a,b

12. Comparison with previously measured values of  p of Ethyl Peroxy radical Source  p x10 -21 cm 2 Method of concentration measurements P.Rupper et.al(a)4.4(11) (c) (COCl) 2 absorption of photolysis beam D. B. Atkinson and J. L. Spillman (b) 3.0(15)Self-reaction decay of peroxy radicals This work6.3(4)HCl absorption This work4.1(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

13. 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)

14. Acknowledgements Colleagues: Gabriel Just, Ming-Wei Chen Terrance Codd, Neal Kline OSU DOE