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DMITRY G. MELNIK The Ohio State University, Dept. of Chemistry, Laser Spectroscopy Facility, 120 W. 18th Avenue, Columbus, Ohio 43210 QUANTITATIVE ABSORPTION.

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Presentation on theme: "DMITRY G. MELNIK The Ohio State University, Dept. of Chemistry, Laser Spectroscopy Facility, 120 W. 18th Avenue, Columbus, Ohio 43210 QUANTITATIVE ABSORPTION."— Presentation transcript:

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

2 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

3 Signatures and reporters Precursors Products Chemical signature: 2 -CRDS: used HCl absorption as reference : Thermal signature: n ph N i,j (t) Chemical mechanism  Q(t) Thermo chemical data Closing the loop,  Q(t)  n ph ; N j (t)

4 How do we measure heat? 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,  f, of resonance frequencies. Q TT  T  Q ff  f  Q l L n0n0

5 Heat balance and timing of gas-phase reaction. Absorption of photolysis radiation laser power dissipation Heat generated by chemical reactions Heat Losses through thermal conductivity Evacuation of heated sample Sample expansion 0 10 ns 1-10 ms 100 ms1s … … …

6 Thermodynamic processes in CRDS reaction cell From flow mass controllers operating at ~ 5% capacity To pump Needle valve This part of the system is essentially sealed on 100ms time scale Buffer contracts The setup is configured for kinetic measurements in slow flow regime Sample expands Buffer cycle V V1V1 V2V2 P V P V 0 -V 2 V 0 -V 1 adiabatic limit Heat deposition from photolysis (isochore) Sample cycle

7 Frequency shift and concentrations V0V0 V1V1 l L probe beam Properties of the gas “Instrumental” parameter (independent of gas composition) Heat deposited by photolysis laser and reactions Heat transfer effects, calculated numerically  f(t) Q(t) Thermodynamic and optical analysis Thermochemical data and photochemical mechanism

8 Experimental setup PD Reactive sample ECDL PD AOM OI 1.3  m, P=3.5 mW M1, 1.3  m, F*=10 5 M2, F*=10 2 M3, F*=10 2 Additional cavity serves dual purpose: 1.Provides an independent frequency scale 2. ECDL can be locked to it Kinetic measurements Calorimetric measurements

9 Measurement of frequency shift with reference cavity Frequency Frequency sweep turning point TEM 00 TEM 01 TEM 10 TEM 02 TEM 11 TEM 20 Fine frequency scale,  f mode =58.7(5)MHz  f(t)

10 Optical calorimetry data: ethyl peroxy radical Noise ~ 70 kHz RMS “Acoustic” oscillations adiabatic calculations calculations including heat transfer 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.

11 Time resolved CW-CRDS measurements of C 2 H 5 O 2 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

12 Rate constant from OC/CW-CRDS measurements

13 METHYL PEROXY MEASUREMENTS M. Pushkarsky, S. Zalyubovsky, T. A. Miller, J. Chem. Phys., 112, 10695, (2001) Spectral range accessible for diode laser 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/  ). The A 0 (k/  ) data obtained in the vicinity of the peak absorption are combined to derive N 0 kL 0.

14 Summary of the results on the two radicals Referencek obs, cm 3 /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 measurements5.14(48) Referencek obs, cm 3 /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 (1997) 1.24(41) D. Melnik and T.A.Miller, JCP, 139, (2013) 0.966(44) These measurements (avg) 1.09(9) EthO 2 MethO 2

15 Summary 1.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. 2.The developed optical calorimetry (OC) technique is a non-resonant technique which does not require accurate frequency monitoring or stabilization of the probe lasers. 3.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. 4.The OC technique requires careful monitoring of relatively large number of parameters, however is potentially useful for measurement of broad range of species

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

17 References, x cm 2 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 work1.20(11) Methyl peroxy peak absorption cross-section for band


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