ANALYSIS OF THE CAVITY RINGDOWN SPECTRA OF THE SMALLEST JET-COOLED ALKYL PEROXY RADICALS 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) 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

Ring-down cavity with slit-jet Experimental Setup 20 Hz, ns, 150 mJ Nd:YAG pulse laser 50 - 100 mJ Dn ~ 8 - 30 MHz (FT limited) 20 Hz, ns, 350 mJ Nd:YAG pulse laser Ti:Sa Amplifier (2 crystals) SRS (stimulated Raman scattering) 730 - 930 nm,  ~ 1 MHz 1 m single pass, 13 atm H2 Nd:YAG cw laser Ti:Sa ring cw laser Raman Cell BBO P. Dupré and T. A. Miller, Rev. Sci. Instrum. 78 (2007) 033102 BBO, ~ 1.3 mm (NIR), ~ 2 - 3 mJ DnBBO < 100 MHz (specification of the laser) Ring-down cavity with slit-jet (absorption length ℓ = 5 cm) L = 135 cm 1st Stokes, ~ 1.3 mm (NIR), ~ 2 mJ DnSRS ~ 200 MHz (limited by power and pressure broadening in H2) ℓ PD InGaAs Detector R ~ 99.995 – 99.999% @ 1.3 mm Vacuum Pump S. Wu, P. Dupré and T. A. Miller, Phys. Chem. Chem. Phys. 8 (2006) 1682 slit-jet: 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 5 mm Electrode Electrode -HV 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

CH3O2 (Methyl Peroxy Radical) In the NIR

CH3O2 weak, σ ~ 10-20 cm2/molecule A state - bound selective ~ ~ b ~ ~ 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).

CRDS Spectroscopy of CD3O2 at RT 100 200 300 400 600 Experimental Data SX = 1.1, SA = 1.0 Predicted tunneling (A/E) splittings (strong dependence upon the mass) for the vibrationless band for CH3O2: 2 – 3 GHz for CD3O2: 100 – 200 MHz 1211 1222 8011211 801 Exp 8011222 1233 Sim 7000 7200 7400 7600 7800 8000 wave numbers / cm-1 G.M.P.Just, A.B.McCoy, and T.A.Miller JCP 127, 044310 (2007) 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)

Jet-cooled CRDS Spectrum of CD3O2 A 2A’ ← X 2A”, vibrationless band 000 ~ r0 Q Cs 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” p1 Q 10 % O2 and ~ 1% CD3I in Ne dc discharge: 350 mA stepsize: 50 MHz RD time average: 4 S. Wu, P. Dupre, P. Rupper and T. A. Miller, J. Chem. Phys., 127 224305 (2007)

Jet-cooled CRDS Spectrum of CD3O2 - Q branch - A 2A’ ← X 2A” ~ simulation1 using 15 fitted parameters (350 lines have been used in the fit, N up to 10, K up to 4) H=Hrot+HSR+T00 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 p1 r0 Q Q J”=N”-1/2 J”= 0.5 5.5 10.5 10.5 5.5 0.5 J”=1.5 5.5 10.5 10.5 5.5 1.5 J”=N”+1/2 +1.5 ppm 1SpecView simulation package, V.L.Stakhursky, T.A.Miller, 56th MSS Symposium, 2001

Jet-cooled CRDS Spectrum of CD3O2 - P branch - J”=N”+1/2 J”=N”-1/2 J’’=1.5 J’’=1.5 two other branches in this region (not labelled) p2Q around 7370 cm-1 r0P around 7371.5 cm-1 +1.5 ppm A 2A’ ← X 2A”, p1P band ~

Tunneling splitting H=Hrot+HSR+HTR+T00

Jet-cooled CRDS Spectrum of CH3O2

Jet-cooled CRDS Spectrum of CH3O2 - Q branch - More complicated Spectrum due to the fact that the tunneling effect is of the same order of magnitude that the spin-rotation

C2H5O2 (Ethyl Peroxy Radical)

CRDS Spectrum of C2H5O2 at RT P.Rupper, E.N.Sharp, G.Tarczay, and T.A.Miller, JPCA 111, 832 (2007) Cs C1 T conformer G conformer absorption / ppm ΔEX(T-G) = 81 cm-1 NT/NG=e-6 at 20K wave numbers / cm-1

Jet-cooled CRDS Spectrum of C2H5O2 - G Conformer - ~ 90 K ~20 K

Jet-cooled CRDS Spectrum of C2H5O2 - T Conformer - able to vary/control the rotational temperature in the jet non-thermalized conformer population in the jet (T conformer < 0.2 % at 20 K) → Trot  Tpopulation → conformer not at eq and not at the statistical limit. → conformers are not relaxed Tconf ~ 78 K ~ 90 K ~ 20 K 3 point smoothing was applied

C2H5O2 T Conformer pQ rQ Experiment Simulation with Trot = 90 K K” = 0 1 J=N -1/2 J=N +1/2 rQ Experiment Simulation with Trot = 90 K ~ ~ X A 1.0988(5) 1.0666(4) B 0.1473(2) 0.1477(2) C 0.1365(2) 0.1364(2) - 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

C3H7O2 (Propyl Peroxy Radical)

Propyl Peroxy Assignments at RT T1G2 (90 cm-1) 1-propyl peroxy T1T2 (104 cm-1) * CH3O2 G1G2 (0 cm-1) * G1T2 (27 cm-1) G1’G2 (147 cm-1) 2-propyl peroxy T (149 cm-1) G (0 cm-1) → generality to observe peroxy radicals of this size jet-cooled with high resolution using our CRDS setup Tarczay et al., Chem. Phys. Lett. 406, 81 (2005).

C6H5O2 (Phenyl Peroxy Radical)

CH3O2 (Methyl Peroxy Radical) In the MIR

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 FTIR1 We are looking now to obtain the first jet-cooled High Res. Spectrum in the MIR of CH3O2 . 1 Huang et al. JCP 127, 234318 (2007)

ν9 CH3 as ν1 CH3 ss

Conclusion and Future Work We have successfully observed and start to analyze C(H/D)3O2 C2(H/D)5O2 C3H7O2 and C6H5O2. We have obtain preliminary spectra in the MIR (79 lines) that might belong to CH3O2 and need to be pursued as well as the vibrational transition of other radicals

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