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

1.Peroxy radicals play central role in combustion chemisrty. 2.For quantitative monitoring, understanding and managing combustion and atmospheric processes one needs information on physical (absorption cross-section) and chemical properties (relevant reaction rate constants) 3. Present day data suffer from large error bars, and possibly substantial systematic errors 4.Cross-species reaction rates are particularly important for understanding chemical processes in complex environment. Motivation

Kinetics of peroxy radicals. Test species: C 2 H 5 O 2 Single peroxy species RO 2 Second-order decay is expected: k obs, cm 3 /sReference 1.08(34) P.D. Lightfoot, Atmos.Environ., 26A, 10, 1805 (1992) 0.91(23) T.J.Wallington, P. Dagaut and M.J. Kurylo, Chem. Rev, 92, 667 (1992) 1.03(29) R.Atkinson, J. Phys. Chem. Ref. Data, 26, 217 (1997) 1.29(7) F.F.Fenter et al, J. Phys. Chem., 97, 3530 (1993) 1.42(7) A.C.Noell et al, J. Phys. Chem. A, 114, 6983, (2010) 1.24(41) D.B.Atkinson and J. W. Hudjens, J. Phys. Chem. A, 101, 3901 (1997) 1.16(32) averaged

Time evolution of concentration and absorption Beer’s law: Kinetic decay: Uniform stationary optically thin sample: Uniform sample of varying length:  = absorption cross-section

Absorption Frequency Time Spectroscopy and kinetics Spectroscopy: absorption vs. frequency Kinetics: absorption vs. time

O. J. Nielsen and T. J. Wallington, in Peroxyl Radicals, (John Wiley and Sons, New York, 1997), pp B̃-X̃ transition: Strong (  cm 2 ) Dissociative transition, lacks selectivity Ã-X̃ transition: Weak (  ~ cm 2 ) Selective B-X vs. A-X spectroscopy M. B. Pushkarsky, S. J Zalyubovsky and T. A. Miller, J.Chem. Phys, (2000) P. Rupper, E. N. Sharp, G. Tarczay and T. A. Miller, J.Phys. Chem. A, 111, 832 (2007)

Width of the “peak” feature at ambient conditions: ~ 1 cm -1 = 30 GHz Mode spacing in CRDS cell: ~ 0.2 GHz Abs Frequency Time Spectroscopy and kinetics: CRDS implementation

ECDL Function generator Delay generator preamp DAC servo mix comm control Analog Digital P.Port AOM Photodiode Opt. isolators 2 x 30 dB ADC Experimental setup diagram To photolysis laser (excimer)

Time, ms Time,  s I t, a.u C 2 H 5 O 2 ringdown curves and kinetic decay

… Time 2. Automated acquisition of a series of “kinetic decays” at the same experimental conditions and storing the traces in a single file. Number of kinetic decays per file: (limited by computer resources). 3. Each series is processed to extract absorption decay rate, (k obs /  ), derive weighted average and calculate statistics. Data acquisition protocol Photolysis laser shots 1. High duty factor allows to rapidly collect large amount of data ms

Avg A(0), ppm k obs / , x 10 7 cm/s No. of decaysSequence ID (6)200# (6)200# (6)199# (5)200# (8)20# (7)250#9 Experimentally measured absorption decay rate Experimental conditions: Absorption monitored at: P (air) = 315 Torr = 7596 cm -1 P (3-pentanone) = 0.2 – 0.6 Torr  = 5.29(20)x cm 2 v = 10.7 cm/s L 0 = 6 cm T=298 K

Results and statistical analysis k obs, cm 3 /s Reference 1.08(34) P.D. Lightfoot, Atmos.Environ., 26A, 10, 1805 (1992) 0.91(23) T.J.Wallington, P. Dagaut and M.J. Kurylo, Chem. Rev, 92, 667 (1992) 1.03(29) R.Atkinson, J. Phys. Chem. Ref. Data, 26, 217 (1997) 1.29(7) F.F.Fenter et al, J. Phys. Chem., 97, 3530 (1993) 1.42(7) A.C.Noell et al, J. Phys. Chem. A, 114, 6983, (2010) 1.24(41) D.B.Atkinson and J. W. Hudjens, J. Phys. Chem. A, 101, 3901 (1997) 1.16(32) averaged 1.101(54) This work

We require that two criteria must be met: 1.Time resolution is sufficient to record at least M data points during  1/2 2.The signal-to-noise ratio at t=0 must be at least S. Suppose that the noise level is A n and time resolution is  t. For L 0 = 10 cm M=10 S=20 A n =  t = 2.5 *10 -4 s Practical limits for measuring k obs

Future extension for studying cross-reactions YAG 532 nm H 2 Raman Cell (200 psi) Sirah Dye Laser PD Laser system 1 Reaction region Mode-matching optics Second Stokes (1.3  m) for A-X of RO 2 Excimer Laser 193 nm ADC Arm “A” Arm “B” ECDL AOM Optical isolators Control Servo

Summary 1.We have built a CW-CRDS based apparatus capable of a. following the kinetic decay of the same set of radicals with time resolution ~ 250  s. b. rapid data acquisition due to high duty factor. c. capable, at present, measuring absorption decay rates, (k obs /  ), up to about 10 8 cm/s which is suitable for studying kinetics of peroxy radiucals. 2.We have measured the absorption decay rate and the rate of removal of ethyl peroxy radicals due to self-reaction, 3.The presented experimental method can be straightforwardly extended to study of the rate constants of the reactions between different radicals.

Colleagues: Dr. Mourad Roudjane Dr. Takashige Fujiwara Dr. Dianping Sun Terrance Codd, Neal Kline Rabi Chhantyal Pun Acknowledgements

Supplementary slides

2.04(6) x (6) x (6) x (5) x (7) x 10 7 Summary of the experimental data. Statistical distribution

Stationary nonuniform sample cannot be solved in closed form for arbitrary N(x,0). Assume that the sample is near- uniform, i.e.: where is variance of the initial radical concentration. Effects of sample non-uniformity