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Miscellaneous studies of transient processes in kinetics apparatus

Miscellaneous studies of transient processes in kinetics apparatus and some ideas that came out of it. Dmitry Melnik . Nov 25 2013. Kinetics measurements of ethyl peroxy , re-visited. n = 7594cm-1, not calibrated, P=300 Torr , compressed air 3-pentnone, ~ 1 Torr Flow rate not measured.

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Miscellaneous studies of transient processes in kinetics apparatus

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  1. Miscellaneous studies of transient processes in kinetics apparatus and some ideas that came out of it. Dmitry Melnik. Nov 25 2013

  2. Kinetics measurements of ethyl peroxy, re-visited n= 7594cm-1, not calibrated, P=300 Torr, compressed air 3-pentnone, ~ 1 Torr Flow rate not measured Excimer laser pulse N0=1.6*1015

  3. Laser pseudo-locking scheme n V (a) Td DVe Servo electronics (DAC) Response delay, 8 ms T (b) Td/4 (c) Td/4 + dTe

  4. Electronics response, short scale

  5. Electronics response, long scale

  6. Experimental setup ECDL AOM 1.3 mm, P=3.5 mW OI M3, 1.5 mm, F*=100 PD Additional cavity serves dual purpose: Provides an independent frequency scale 2. ECDL can be locked to it PD PD M2, 1.2 mm, F*=103 Reactive sample M1, 1.3 mm, F*=105

  7. Observed signal Frequency Frequency Frequency sweep turning point M1,M2: L=72 cm, FSR=208 MHz, Df= 32.8 MHz M3: L=21.5 cm, FSR=695 MHz, Df = 59.5 MHz TEM00 TEM01 TEM10 TEM02 TEM11 TEM20 Df M3 (reference) cavity TEM00 tR M2 cavity df

  8. Data Avg=25 Conditions: P=300 Torr, 3-pentanone, P~ 1 Torr. T=298K, 193 nm photolysis (total power 80 mJ/pulse) He N2 Ar Air

  9. FP resonance frequency. In the present experiments, the frequency difference between the selected resonances of two cavities is followed, on time scale ~ 0.1 s. For a Fabry-Perot etalon, n is a function of T, P and sample content, Therefore, for small resonance shifts caused by any of these parameters, For gases at near-ambient conditions in IR, n is very close to unity. L(t) “drifts” with ambient temperature, on the time scale of tens of minutes. L can be considered constant on the time scale of the experiment.

  10. FP resonance frequency shift. Heating of the sample Changes caused by chemical removal of precursors and additions of products. Pressure equilibrates on sub-ms time scale chemical dispersion effects Estimate of chemical dispersion effects (assuming products are optically dense): df ~ 3 MHz.

  11. Heat effects. For dry air: For average refractive index, as in case of chemical dispersion, J.C.Owens, Appl. Opt. 6, p51, (1967)

  12. PI Cell geometry MASK CL SL EXCIMER L0=60 mm 175 mm 40 cm PI 20 cm 8 mm 12.5 mm 25 mm

  13. Heat balance. Evacuation of heated sample Losses through heat conductivity “leftover” power dissipation Absorption of photolysis radiation Heat generated by chemical reactions At this point we consider the combined effect of these two processes and ignore absorption of photolysis radiation by bath gas.

  14. Heat dissipation effects. Present experiments are conducted at the same flow conditions as reported in the recent kinetic paper, at the low flow limit. Flow rate v=1.9 cm/s, L/v = 3 s. (in experiments with air) S • 2. Heat conductivity. • Fourier law: • Q -- amount of heat transferred • – heat conductivity coefficient • (pressure-independent) • S – surface area. • For the constant pressure regime, x For the present apparatus configuration. This is comparable to the evacuation rate ( 3 1/s) and about 5 times slower than overall 1st order removal (incl. radical diffusion)

  15. Data, revisited 1 Raw data To a crude approximation, on < 100 ms time scale we can ignore transport effects (heat transfer, diffusion, evacuation), with a possible exception of He bath. He N2 Ar Air chemical dispersion + diffusion? Sensitivity of frequency shift to the released heat. N2 Ar Air Data normalized to Cp This quantity does not depend on bath gas!

  16. Thermodynamics and thermochemistry 3-pentanone + 193.3nm = 148 kCal/mol 55.1 kCal/mol 67.8 kCal/mol 2C2H5 + CO C2H5 + C2H5CO 142.3 kCal/mol 87.2 kCal/mol n-C4H10 DHtotal = 99 kCal/mol

  17. Preliminary conclusions 1. The CRDS resonance shift observed in earlier kinetic measurements and which (possibly) caused excessive noise at 1-2 ms after the photolysis pulse is caused almost entirely by the heat released into system as a result of dissipation of the photolysis pulse energy and chemical reactions. 2. It takes about ~ 5 ms for this shift to reach its maximum, which appears to be independent of reaction chemistry, as a result of expansion of the sample. This means that a decent laser locking technique will probably be able to handle this shift. 3. The amount of deposited heat does depend on the reaction chemistry, which indicates a possibility of direct measurements of the thermodynamics of gas-phase chemical reactions involving radicals. 4. The present noise level (at avg=25 shots) corresponds to the temperature shift sensitivity of better than 100 mK. The present arrangement is far from optimal for these kinds of measurements. By simple engineering means the sensitivity can be improved by almost an order of magnitude, using the present experimental algorithm. For general application, sensitivity of about ~1mK is desired. 5. The transient (< 10 ms) fluctuations of the frequency shift are well-reproduced and need to be understood. For accurate measurements of thermodynamic quantities (e.g., DH298) of reactions, a more thorough and accurate thermophysical modeling of transport processes is required.

  18. Possible directions in the instrumental development Monitor shifts at higher frequencies Fq (e.g. visible), which will increase both Fq and h. Redesign the photolysis scheme to improve l/L factor. It is feasible to increase it from the present 8% to 50-60%, which alone will gain a factor of up 8 to the sensitivity. Use interferometric techniques, similar to Michelson interferometer, modulated laser light (e.g. using low-frequency EOM) and fast detector (avalanche photodiode, ~1 ns) to detect weak shifts. This will allow to accurately measure shifts comparable to the width of the resonance (with potentially another factor of 6-8 increase of sensitivity.) Use a supercontinuum light source? Stabilize cavities. Eliminate vibrations. Use various precursors to initiate similar reaction mechanism. This will allow to separate heat dissipation effects due to laser heating of the sample from the effect of chemical reactions. Study related reaction mechanisms with common elementary steps to deconvolute data which is essentially a combined heat from the entire reaction mechanism. Reference to quantum chemistry calculations and data obtained from earlier measurements. Measurement strategies

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