QUANTITATIVE ABSORPTION AND KINETIC STUDIES OF TRANSIENT SPECIES USING GAS PHASE OPTICAL CALORIMETRY DMITRY G. MELNIK The Ohio State University, Dept. of Chemistry, Laser Spectroscopy Facility, 120 W. 18th Avenue, Columbus, Ohio 43210

Realistic chemical processes (e.g., hydrocarbon oxidation): Problem overview Realistic chemical processes (e.g., hydrocarbon oxidation): Complex mechanism with thousands of elementary steps Most reactions involve radicals Mostly second or pseudo-second order – require independent concentration determination Accurate quantitative data required for better understanding and management Experimental method sought: Based on simple and well understood principles Broadly applicable (i.e. diverse chemical systems) Robust Cheap

Signatures and reporters Chemical signature: 2l-CRDS: used HCl absorption as reference : Precursors Products Chemical mechanism Thermal signature: nph Ni,j(t) Thermo chemical data Closing the loop, dQ(t)nph; Nj(t) dQ(t)

How do we measure heat? dT Q l dT Q n0 L df df Q Calorimetry: Heat from the target is transferred to a body whose thermal properties are well known, and whose change of state is used to characterize the target. Gas phase implementation: When a region within a gas sample is heated, it expands and its density (and the refractive index) is lowered. Variation of refractive index causes phase shift of the transmitted light. In Fabry-Perot interferometer, such a phase shift results in shift, df, of resonance frequencies. n0 l L df df Q

… … … Heat balance and timing of gas-phase reaction. 10 ns 1-10 ms 10 ns 1-10 ms 100 ms 1s … Evacuation of heated sample … … Absorption of photolysis radiation Heat Losses through thermal conductivity Sample expansion Heat generated by chemical reactions laser power dissipation

Thermodynamic processes in CRDS reaction cell The setup is configured for kinetic measurements in slow flow regime From flow mass controllers operating at ~ 5% capacity Buffer contracts Needle valve To pump Sample expands This part of the system is essentially sealed on 100ms time scale Sample cycle V V1 V2 P Heat deposition from photolysis (isochore) V P V0-V2 V0-V1 Buffer cycle adiabatic limit

Frequency shift and concentrations probe beam V0 V1 l L Properties of the gas “Instrumental” parameter (independent of gas composition) Heat deposited by photolysis laser and reactions Heat transfer effects , calculated numerically df(t) Q(t) Thermodynamic and optical analysis Thermochemical data and photochemical mechanism

PD PD PD ECDL Experimental setup Kinetic measurements Calorimetric AOM OI 1.3 mm, P=3.5 mW M3, F*=102 PD Additional cavity serves dual purpose: Provides an independent frequency scale 2. ECDL can be locked to it PD PD M2, F*=102 Reactive sample M1, 1.3 mm, F*=105

Measurement of frequency shift with reference cavity Frequency sweep turning point TEM01 TEM10 TEM00 TEM02 TEM11 TEM20 Fine frequency scale, Dfmode=58.7(5)MHz df(t)

Optical calorimetry data: ethyl peroxy radical adiabatic calculations Precursor: 3-pentanone, f=7522 cm-1 Traces averaged over 100 shots (4 traces taken ca. 1 hr apart are shown). The actual concentration of ethyl peroxy radicals calculated from experimental trace: Uncertainty correspond to variations between traces. calculations including heat transfer “Acoustic” oscillations Noise ~ 70 kHz RMS

Time resolved CW-CRDS measurements of C2H5O2 self-reaction Raw data at 7596 cm-1 (R-branch head of the AX origin band) Absorption decay curve due to ethyl peroxy self-reaction

Rate constant from OC/CW-CRDS measurements

METHYL PEROXY MEASUREMENTS Spectral range accessible for diode laser M. Pushkarsky, S. Zalyubovsky, T. A. Miller, J. Chem. Phys., 112, 10695, (2001) Note: for these measurements, the absolute frequency of the diode laser is not calibrated. Instead, the laser frequency is step-scanned to optimize signal and possibly, minimize (k/s). The A0(k/s) data obtained in the vicinity of the peak absorption are combined to derive N0kL0.

Summary of the results on the two radicals EthO2 Reference kobs, 10-13 cm3/s P.D. Lightfoot, Atmos.Environ., 26A, 10, 1805 (1992) 1.08(34) T.J.Wallington et al, Chem. Rev, 92, 667 (1992) 0.91(23) R.Atkinson, J. Phys. Chem. Ref. Data, 26, 217 (1997) 1.03(29) F.F.Fenter et al, J. Phys. Chem., 97, 3530 (1993) 1.29(7) A.C.Noell et al, J. Phys. Chem. A, 114, 6983, (2010) 1.42(7) D.B.Atkinson and J. W. Hudgens, JPCA. A, 101, p. 3901 (1997) 1.24(41) D. Melnik and T.A.Miller, JCP, 139, 094201 (2013) 0.966(44) These measurements (avg) 1.09(9) Reference kobs, 10-13 cm3/s D. A. Parkes et al, CPL, 23, p425 (1973) 3.3(1.1) D. A. Parkes, IJCK, 9, p 451 (1977) 5.2(9) C.J. Hochanadel et. al., JPC, 81, p3 (1977) 3.8(5) C. K. Kan, IJCK, 11, p921 (1979) 4.2(5) H. Adachi et al., IJCK, 12, p949 (1980) 5.8(5) K. McAdam et al., CPL, 133, p39 (1987) 5.9(10) M. E. Jenkin et al., JCSFT2, 84, p913 (1988) 4.7(5) F.-G. Simon, IJCK, 22, p791 (1990) 4.8(5) R. Atkinson, JPCRD, 26, p215 (1997) 4.9(11) These measurements 5.14(48) MethO2

Summary The use of heat signature for determination of the absolute concentrations has been successfully demonstrated by measuring the effective rate constants of methyl and ethyl peroxy self reaction. The developed optical calorimetry (OC) technique is a non-resonant technique which does not require accurate frequency monitoring or stabilization of the probe lasers. The accuracy and precision of the method is fundamentally limited by those of thermochemical data, which are generally higher than spectroscopic absorption data of potential “reporter” species. The OC technique requires careful monitoring of relatively large number of parameters, however is potentially useful for measurement of broad range of species

Acknowledgements Colleagues: Prof. T. A. Miller Dr. Mourad Roudjane Dr. Rabi Chhantyal Pun Dr. Neal Kline Terrance Codd, Meng Huang Henry Tran OSU DOE

Methyl peroxy peak absorption cross-section for 1211 band Reference s, x 1020 cm2 D. D. Atkinson and J. L. Spillman, JPCA, 106, p8891 (2001) 1.5(8) M. Pushkarsky, S. Zalyubovsky and T. A. Miller, JCP, 112, p10695 (2001) ~ 1.0 (2) E. Farago, C. Fittshen et. al, JPCA, 117, p12802 (2013) 3.40(68) This work 1.20(11)