1. Gabriel M. P. Just, Patrick Rupper, Dmitry G. Melnik and Terry A. Miller EXPERIMENTAL PROGRESS FOR HIGH RESOLUTION CAVITY RINGDOWN SPECTROSCOPY OF JET- COOLED REACTIVE INTERMEDIATES

2. Alkyl peroxy radicals play a key role as intermediates in the oxidation of hydrocarbons (atmospheric as well as combustion chemistry) Peroxy Radicals: Motivations

3. Atmospheric and Combustion interest The low temperature combustion of hydrocarbons is a critical process in the overall degradation of our atmosphere quality leading to the formation of the peroxy radicals which, by reacting with the NO radical upset the NO  NO 2 balance and leads to the formation of O 3 in the troposhere. The formation of peroxy radicals is believed to be partially responsible for the negative temperature coefficient (NTC) behavior of hydrocarbon combustion observed from 550-700 K.

4. Alkyl peroxy radicals play a key role as intermediates in the oxidation of hydrocarbons (atmospheric as well as combustion chemistry) 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) Peroxy Radicals: Motivations

5. Alkyl peroxy radicals play a key role as intermediates in the oxidation of hydrocarbons (atmospheric as well as combustion chemistry) 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

6. Cavity Ringdown Spectroscopy R L A = σ Nl A = L/c τ absorber - L/c τ 0 Time Intensity 00  absorber Sensitivity of Technique: If R = 99.999% and L = 135 cm then τ 0 = 550 µs L eff = 165.0 km ~ 100 Miles ~ Columbus – Cleveland l = 5 cm l eff = 6.1 km τ abs σ Nl + = cL)/( R1 - ( ) τ0τ0 c L )/ ( R1 - = l

7. 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 2 730 - 930 nm,  ~ 1 MHz 50 - 100 mJ  ~ 8 - 30 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 ~ 99.995 – 99.999% @ 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) 033102 Experimental Setup Nd:YAG pulse laser 20 Hz, ns, 150 mJ BBO BBO, ~ 1.3  m (NIR), ~ 2 - 3 mJ  BBO < 100 MHz (specification of the laser)

8. IR Beam 9 mm -HV radical densities of 10 12 - 10 13 molecules/cm 3 (10 mm downstream, probed) rotational temperature of 15 - 30 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

9. Spectra improvement It is known that the methyl peroxy radical (CH 3 O 2 ) has a tunneling splitting which is due to the methyl torsion 1. This tunneling splitting was estimated to be about 2-3 GHz for CH 3 O 2 and about 200 MHz for CD 3 O 2 1 G.M.P.Just, A.B.McCoy, and T.A.Miller JCP 127, 044310 (2007)

10. CRDS Spectroscopy of CD 3 O 2 at RT C.-Y.Chung, C.-W.Cheng, Y.-P.Lee, H.-S.Liao, E.N.Sharp, P.Rupper, and T.A.Miller, JCP 127, 044311 (2007) 12 2 2 12 3 3 12 1 1 7000720074007600 78008000 wave numbers / cm -1 0 100 200 300 400 600 8 0 1 12 1 1 8 0 1 12 2 2 801801 000000

11. CD 3 O 2 using DFM

13. More characterization of the laser source For characterization purposes and more importantly spectroscopic purposes, we decided to change frequency range in order to go to the MIR using DFM by using not a BBO crystal but a LiNbO 3 crystal and the fundamental of a Nd:YAG laser

14. MIR Linewidth CH 3 I Absorption

15. 2 12  1 11 2 21  1 10

16. Estimating the source linewidth NIRMIR Δν Doppler 128 MHz53 MHz Δν Source 69 MHz49 MHz

17. Conclusion and Future Work We can obtain an experimantal linewidth of about 145 MHz in the NIR and of about 70 MHz in the MIR (nearly Doppler limited). The improvement in linewidth (from 250 MHz for SRS to 145 MHz width DFM in the NIR) allowed us to resolve the tunneling splitting in CD 3 O 2 which wasn’t the case using SRS. From these investigation, we can estimate that our source linewidth is about 69 MHz in the NIR and 49 MHz in the MIR

18. 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 $$$