DMITRY G. MELNIK AND TERRY A. MILLER The Ohio State University, Dept. of Chemistry, Laser Spectroscopy Facility, 120 W. 18th Avenue, Columbus, Ohio PRECISE MEASUREMENTS OF ABSORPTION CROSS-SECTIONS OF PEROXY RADICALS BY DUAL WAVELENGTH CAVITY RING-DOWN SPECTROSCOPY

Research Motivation Peroxy radicals (RO 2 ) are important reactive intermediates in atmospheric and combustion chemistry. Quantitative monitoring of radical abundance can provide valuable information on ongoing chemical processes Frequency measurement of transitions of RO 2 : allows for species specific monitoring of the radicals in studies of complex chemical systems allows for structural analysis of both electronic states thus providing spectroscopic and quantum chemical information. Intensity measurement of absorption of RO 2 : provides basis for using transition as versatile diagnostic tool for measuring of concentrations allows to calculate the absorption cross-section and derive electronic transition moment

Experimental challenge Quantitative measurement of the fractional absorption (Beer’s Law): Average number density is not known a priori, needs to be measured independently. Experimental strategy: measure the absorption of chemically stable and well-studied co-product of the radical’s synthesis (“reporter” molecule) to obtain its number density 1.RO 2 and “reporter” need to be formed in known stochiometric ratio, e.g. 2.Absorption cross-section of reporter molecule  REP must be known to high accuracy 3. RO 2 and “reporter” generally don’t absorb in the same region: probing with the second radiation source along optically equivalent path is required, i.e. two different paths A, B must be used:

RO 2 and reporter balance Synthesis route through hydrogen abstraction by chlorine atoms (R=C 2 H 5 ): R. Atkinson, J. Phys Chem. Ref. Data, 26, 215, (1997) D. B. Atkinson, J. W. Hudgens, J. Phys. Chem. A, 101, 3901 (1997) 1% measurement window  s [Cl] 0 = 2*10 15 [O 2 ]=2*10 18 [C 2 H 6 ]=10 16 Est. systematic error ~ 0.5% Radical-to-reporter concentration ratio ratio

Principal design of the dual-CRDS setup YAG 532 nm H 2 Raman Cell (200 psi) Sirah Dye Laser PD Dye Laser system (0.06 cm -1 ) YAG 532 nm (Injection seeded) Reaction region Idler output (1.8  m) for HCl overtone,  HCl =0.02 – cm -1 Second Stokes (1.3  m) for A-X of RO 2 LP filter Pinhole filter Excimer Laser 193 nm ADC Pulse/Delay Generator Gate Photolysis control ( computer) Overall control (to computer) Arm “A” Arm “B” OPO (single mode, 0.01 cm -1 ) RS-232

Equivalence of the optical paths

Ethyl peroxy radical [a] 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)  EP p =  EP ( max ) max =7596 cm -1

Precursors: 500 mTorr of (COCl) 2, 250 mTorr of C 2 H 6 Buffer: 300 Torr of synthetic air Dual wavelength scan of C 2 H 5 O 2 /HCl P[(COCl) 2 ] =750 mTorr P[C 2 H 6 ]=250 mTorr P(air) = 300 Torr Avg=20 H 37 Cl, 2, cm -1 Time max = 7596 cm -1

Calculations of  P from dual spectra From the Beer’s law for optically thin media: Reporter Line S HCl,, cm 1/N l, cm  l /  HCl Reporter line traces  EP P, cm 2 P(1), H 37 Cl (25) P(5), H 37 Cl (19) P(6), H 37 Cl (40)

Time, ms 1/  ln I, ppm -1 Kinetic measurements of  EP p Second order decay law: k obs, cm 3 s -1 Ref.  EP p, cm 2 1.0(1)a4.27(50) 1.30(16)b5.56(76) 0.987(74)c4.21(49) 1.1(1)d4.70(52) 0.95(7)e4.06(39) 1.29(7)f5.51(46) 1.24(40)g5.3(17) 1.42(7)h6.07(49) 1.1(3)i4.7(14) [a] H. Adachi et al, J. Chem. Kinet, XI, 1211 (1979) [b] F.C.Cattell et al, J. Chem. Soc. Faraday Trans.2, 86, 1999 (1986) [c] T. J. Wallington et al, J. Photochem. Photobiol. A, 42, 173 (1988) [d] D. Bauer et al, J. Photochem. Photobiol. A, 65, 329 (1992) [e] C. Anastasi et al, J. Chem. Soc. Faraday Trans.1, 79, 505 (1983) [f] F.F.Fenter et al, J. Phys. Chem., 97, 3530 (1993) [g]. D. B. Atkinson and J. W. Hudgens, J. Phys. Chem. A, 101, 3901 (1997) [h]A. C. Noel et al, J. Phys. Chem A, to be published (2010) [i]. P. D. Lightfoot et al, Atmos. Environ, 26A, 1805 (1992)

I0I0 I1I1 I 2 (I ’ 2 ) I0I0 I’2I’2 I2I2 I1I1 T1T1 T2T2 Measurements of  EP p calibrated with photolysis absorption x UV IR

Summary of measured values of  EP p ab c a – dual CRDS measurement b – kinetic measurements c – photolysis absorption calibration Comparison to previously reported values: j [j] D. B. Atkinson and J. L Spillman, J. Chem. Phys. A, 106, 889 (2002) k [k] P. Rupper et al, J. Phys. Chem. A, 111, 832 (2007)

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 proposed method eliminates the need to measure the intensity of the probing beam (i.e. insensitive to the power fluctuation of the source and detector calibration issues) Independent of the previously measured values of the reaction constants that have intrinsically large error bars The accuracy of the measurements are limited to that of the which are determined to substantially higher accuracy.

Acknowledgements Colleagues: Dr. Phillip Thomas Rabi Chhantyal Pun Dr. Gabriel Just, Ming-Wei Chen Terrance Codd, Neal Kline

Precursors: 500 mTorr of (COCl) 2, 250 mTorr of C 2 H 6 Buffer: 300 Torr of synthetic air Dual wavelength scan of C 2 H 5 O 2 /HCl P[(COCl) 2 ] =750 mTorr P[C 2 H 6 ]=250 mTorr P(air) = 300 Torr Avg=20 H 37 Cl, 2, cm -1 Exp Sim T-conf,