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Gabriel M. P. Just, Patrick Rupper, Dmitry G. Melnik and Terry A. Miller ANALYSIS OF THE CAVITY RINGDOWN SPECTRA OF THE SMALLEST JET- COOLED ALKYL PEROXY.

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Presentation on theme: "Gabriel M. P. Just, Patrick Rupper, Dmitry G. Melnik and Terry A. Miller ANALYSIS OF THE CAVITY RINGDOWN SPECTRA OF THE SMALLEST JET- COOLED ALKYL PEROXY."— Presentation transcript:

1 Gabriel M. P. Just, Patrick Rupper, Dmitry G. Melnik and Terry A. Miller ANALYSIS OF THE CAVITY RINGDOWN SPECTRA OF THE SMALLEST JET- COOLED ALKYL PEROXY RADICALS

2 Alkyl peroxy radicals play a key role as intermediates in the oxidation of hydrocarbons (atmospheric as well as combustion chemistry) Methyl peroxy is smallest alkyl peroxy radical → starting point for spectroscopic characterization Ambient cell cavity ring-down spectroscopy (CRDS)  Several peroxy radicals have been studied in our lab → near IR electronic transition is sensitive, species-specific diagnostic  Rotational structure is only partially resolved (congestion due to overlap of different rotational lines and different conformers) High resolution, rotationally resolved IR CRDS of alkyl peroxy radicals under jet-cooled conditions would be of great value  provide molecular parameters to characterize radicals and benchmark quantum chemistry calculations  identify directly spectra of different isomers and conformers Peroxy Radicals: Motivations

3 Ti:Sa ring cw laser Ti:Sa Amplifier (2 crystals) Nd:YAG pulse laser Raman Cell PD InGaAs Detector Ring-down cavity with slit-jet (absorption length ℓ = 5 cm) L = 135 cm Vacuum Pump 1 m single pass, 13 atm H nm,  ~ 1 MHz mJ  ~ MHz (FT limited) ℓ Nd:YAG cw laser 1 st Stokes, ~ 1.3  m (NIR), ~ 2 mJ  SRS ~ 200 MHz (limited by power and pressure broadening in H 2 ) R ~ – 1.3  m SRS (stimulated Raman scattering) 20 Hz, ns, 350 mJ slit-jet: longer absorption path-length less divergence of molecular density in the optical cavity S. Wu, P. Dupré and T. A. Miller, Phys. Chem. Chem. Phys. 8 (2006) 1682 P. Dupré and T. A. Miller, Rev. Sci. Instrum. 78 (2007) Experimental Setup Nd:YAG pulse laser 20 Hz, ns, 150 mJ BBO BBO, ~ 1.3  m (NIR), ~ mJ  BBO < 100 MHz (specification of the laser)

4 IR Beam 9 mm -HV radical densities of molecules/cm 3 (10 mm downstream, probed) rotational temperature of K plasma voltage ~ 500 V, I  1 A (~ 400 mA typical), 220 µs length dc and/or rf discharge, discharge localized between electrode plates, increased signal compared to longitudinal geometry Previous similar slit-jet designs: D.J. Nesbitt group, Chem. Phys. Lett. 258, 207 (1996) R.J. Saykally group, Rev. Sci. Instrum. 67, 410 (1996) Pulsed Supersonic Slit-jet and Discharge Expansion 5 cm 5 mm 10 mm Electrode carrier gas (300 – 700 Torr Ne) + precursor RI (1%) and O 2 (10%) Viton Poppet

5 CH 3 O 2 (Methyl Peroxy Radical) In the NIR

6 CH 3 O 2 a) Jafri et al., J. Am. Chem. Soc. 112, 2586 (1990). b) O. J. Nielsen and T. J. Wallington, in Peroxyl Radicals, (John Wiley and Sons, New York, 1997). a b ~ ~ ~ weak, σ ~ cm 2 /molecule A state - bound selective ~

7 Exp Sim CRDS Spectroscopy of CD 3 O 2 at RT Predicted tunneling (A/E) splittings (strong dependence upon the mass) for the vibrationless band for CH 3 O 2 : 2 – 3 GHz for CD 3 O 2 : 100 – 200 MHz G.M.P.Just, A.B.McCoy, and T.A.Miller JCP 127, (2007) C.-Y.Chung, C.-W.Cheng, Y.-P.Lee, H.-S.Liao, E.N.Sharp, P.Rupper, and T.A.Miller, JCP 127, (2007) wave numbers / cm Experimental Data S X = 1.1, S A =

8 Jet-cooled CRDS Spectrum of CD 3 O 2 10 % O 2 and ~ 1% CD 3 I in Ne dc discharge: 350 mA stepsize: 50 MHz RD time average: 4 A 2 A’ ← X 2 A”, vibrationless band ~~ r 0 Q p 1 Q C s symmetry → pure c-type transition moment close to a prolate symmetric top ( ΔK ΔJ) spread out over ~ 30 cm -1 > 1000 lines, 350 of which due to single transition K” S. Wu, P. Dupre, P. Rupper and T. A. Miller, J. Chem. Phys., (2007)

9 Jet-cooled CRDS Spectrum of CD 3 O 2 - Q branch - simulation 1 using 15 fitted parameters (350 lines have been used in the fit, N up to 10, K up to 4) H=H rot +H SR +T 00 T = 15.5 K linewidth (Voigt profile) 300 MHz Lorentzian → finite lifetime ~ 1.5 ns of electronic transition 300 MHz Gaussian → Doppler plus source linewidth 1 SpecView simulation package, V.L.Stakhursky, T.A.Miller, 56th MSS Symposium, 2001 Q Q J”= J”= A 2 A’ ← X 2 A” ~ ~ J”=N”+1/2 J”=N”-1/2 p1p1 r0r ppm

10 J’’=1.5 J”=N”-1/2 J”=N”+1/2 A 2 A’ ← X 2 A”, p1 P band ~ ~ two other branches in this region (not labelled) p2 Q around 7370 cm -1 r0 P around cm -1 Jet-cooled CRDS Spectrum of CD 3 O 2 - P branch ppm

11 Tunneling splitting H=H rot +H SR +H TR +T 00

12 Jet-cooled CRDS Spectrum of CH 3 O 2

13 Jet-cooled CRDS Spectrum of CH 3 O 2 - Q branch - More complicated Spectrum due to the fact that the tunneling effect is of the same order of magnitude that the spin-rotation

14 C 2 H 5 O 2 (Ethyl Peroxy Radical)

15 P.Rupper, E.N.Sharp, G.Tarczay, and T.A.Miller, JPCA 111, 832 (2007) CsCs C1C1 T conformerG conformer ΔE X (T-G) = 81 cm -1  N T /N G =e -6 at 20K CRDS Spectrum of C 2 H 5 O 2 at RT wave numbers / cm -1 absorption / ppm

16 Jet-cooled CRDS Spectrum of C 2 H 5 O 2 - G Conformer - ~ 90 K ~20 K

17 able to vary/control the rotational temperature in the jet non-thermalized conformer population in the jet (T conformer < 0.2 % at 20 K) → T rot  T population → conformer not at eq and not at the statistical limit. → conformers are not relaxed T conf ~ 78 K ~ 20 K ~ 90 K Jet-cooled CRDS Spectrum of C 2 H 5 O 2 - T Conformer - 3 point smoothing was applied

18 C 2 H 5 O 2 T Conformer Experiment Simulation with T rot = 90 K K” = rQrQ pQpQ - asymmetric rotor with spin-rotation interaction - similarity of rotational constants in ground and excited states - upper and lower spin rotation components (J = N ± 1/2) group together for several N XA A1.0988(5)1.0666(4) B0.1473(2)0.1477(2) C0.1365(2)0.1364(2) ~ J=N -1/2J=N +1/2

19 C 3 H 7 O 2 (Propyl Peroxy Radical)

20 Propyl Peroxy Assignments at RT T 1 G 2 (90 cm -1 ) 1-propyl peroxy 2-propyl peroxy T 1 T 2 (104 cm -1 ) G 1 T 2 (27 cm -1 ) G 1 G 2 (0 cm -1 ) G 1 ’G 2 (147 cm -1 ) G (0 cm -1 )T (149 cm -1 ) Tarczay et al., Chem. Phys. Lett. 406, 81 (2005). * * CH 3 O 2 → generality to observe peroxy radicals of this size jet-cooled with high resolution using our CRDS setup

21 C 6 H 5 O 2 (Phenyl Peroxy Radical)

22

23 CH 3 O 2 (Methyl Peroxy Radical) In the MIR

24 Going to the MIR At the end of 2007 Y.P. Lee group published the observation at room temperature of the fundamental C-H stretch of the methyl peroxy radical using a step FTIR 1 We are looking now to obtain the first jet- cooled High Res. Spectrum in the MIR of CH 3 O 2. 1 Huang et al. JCP 127, (2007)

25

26 ν 9 CH 3 as ν 1 CH 3 ss

27 Conclusion and Future Work We have successfully observed and start to analyze C(H/D) 3 O 2 C 2 (H/D) 5 O2 C 3 H 7 O 2 and C 6 H 5 O 2. We have obtain preliminary spectra in the MIR (79 lines) that might belong to CH 3 O 2 and need to be pursued as well as the vibrational transition of other radicals

28 Aknowledgment Dr Miller The Miller group:  Dr Patrick Rupper (Switzerland)  Dr Erin Sharp (JILA)  Ming-Wei Chen  Dr Dmitry Melnik  Dr Philip Thomas  Dr Linsen Pei  Rabi ChhantyalPun  Dr Shenghai Wu (U. of Minnesota) NSF $$$


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