Transcript p007.pptx
EXPERIMENTAL PROGRESS FOR HIGH RESOLUTION CAVITY RINGDOWN SPECTROSCOPY OF JETCOOLED REACTIVE INTERMEDIATES Gabriel M. P. Just, Patrick Rupper, Dmitry G. Melnik and Terry A. Miller Peroxy Radicals: Motivations Alkyl peroxy radicals play a key role as intermediates in the oxidation of hydrocarbons (atmospheric as well as combustion chemistry) 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 NO2 balance and leads to the formation of O3 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. Peroxy Radicals: Motivations 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 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 Cavity Ringdown Spectroscopy A = L/cτabsorber - L/cτ0 Intensity L R l 0 absorber A = σ Nl Time Sensitivity of Technique: If R = 99.999% and L = 135 cm then τ0 = 550 µs Leff = 165.0 km ~ 100 Miles ~ Columbus – Cleveland l = 5 cm leff = 6.1 km ( L / c) τ= 1-R τ = ( L / c) (1 - R)+ σ Nl 0 abs Experimental Setup 20 Hz, ns, 350 mJ 20 Hz, ns, 150 mJ Nd:YAG pulse laser 730 - 930 nm, ~ 1 MHz Nd:YAG cw laser Ti:Sa ring cw laser P. Dupré and T. A. Miller, Rev. Sci. Instrum. 78 (2007) 033102 PD Ti:Sa Amplifier (2 crystals) slit-jet: SRS (stimulated Raman scattering) 1 m single pass, 13 atm H2 Raman Cell BBO BBO, ~ 1.3 ~mm ~2-~ 3 mJ 1st Stokes, 1.3(NIR), mm (NIR), 2 mJ Ring-down cavity with slit-jet BBO < 200 100 MHz SRS ~ (absorption length ℓ = 5 cm) (specification of the laser) (limited by power and pressure L = 135 cm broadening in H2) ℓ R ~ 99.995 – 99.999% @ 1.3 mm InGaAs Detector S. Wu, P. Dupré and T. A. Miller, Phys. Chem. Chem. Phys. 8 (2006) 1682 Nd:YAG pulse laser 50 - 100 mJ ~ 8 - 30 MHz (FT limited) Vacuum Pump longer absorption path-length less divergence of molecular density in the optical cavity Pulsed Supersonic Slit-jet and Discharge Expansion carrier gas (300 – 700 Torr Ne) + precursor RI (1%) and O2 (10%) Viton Poppet 9 mm Electrode Electrode 5 mm 10 mm 5 cm IR Beam 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) • radical densities of 1012 - 1013 molecules/cm3 (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 -HV Spectra improvement It is known that the methyl peroxy radical (CH3O2) has a tunneling splitting which is due to the methyl torsion1. This tunneling splitting was estimated to be about 2-3 GHz for CH3O2 and about 200 MHz for CD3O2 1G.M.P.Just, A.B.McCoy, and T.A.Miller JCP 127, 044310 (2007) CRDS Spectroscopy of CD3O2 at RT 000 600 600 absorption / ppm 500 1211 400 400 300 300 1222 200 200 123 100 100 0 7000 7000 801 7200 7200 7400 7400 8011211 8011222 3 7600 7600 7800 7800 8000 8000 cm-1/ cm-1 wave numbers 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) CD3O2 using DFM 8 ppm per pass 6 4 2 SRS 0 DFM -2 7365 7370 7375 cm-1 7380 8 ppm per pass 6 4 2 SRS 0 DFM -2 7373.4 7373.6 7373.8 cm-1 7374.0 7374.2 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 LiNbO3 crystal and the fundamental of a Nd:YAG laser MIR Linewidth CH3I Absorption 1600 1400 ppm per pass 1200 1000 800 600 400 200 3000 3005 3010 cm-1 3015 3020 cm-1 7287.0 7287.2 7287.4 7287.6 7287.8 7288.0 7288.2 350 1400 212 111 147 MHz 1200 221 110 300 142 MHz 250 200 800 150 600 100 71 MHz 400 82 MHz 75 MHz 67 MHz 62 MHz 50 83 MHz 65 MHz 0 200 3006.8 3007.0 3007.2 cm-1 3007.4 3007.6 3007.8 ppm per pass ppm per pass 1000 Estimating the source linewidth 2 2 2 NIR Doppler 2 ( ) , NIR Source 2 MIR 2 Doppler , NIR 2.42 2 Source 2 NIR MIR ΔνDoppler 128 MHz 53 MHz ΔνSource 69 MHz 49 MHz 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 CD3O2 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 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 $$$