Laboratory Studies of VUV CH 4 Photolysis and Reactions of the Resulting Radicals Robin Shannon, Mark Blitz, Mike Pilling, Dwayne Heard, Paul Seakins University of Leeds, UK

Background to Leeds Leeds has long background in Laboratory Reaction Kinetics with applications to: – Combustion – Pyrolysis – Atmospheric Chemistry Additionally field work on OH and HO 2 detection (spectroscopic) and hydrocarbons (chromatography) Development of large models (MCM) Theory on pressure dependent reactions New STFC grant on methane photolysis and benzene formation on Titan

Outline 1.Methane Photolysis – Previous work – Possible approaches 2.Reactions of 1 CH 2 – Rare gas collisions – Reaction vs relaxation 3.Reactions of CH 4.Recent studies with Laval expansion system (Heard)

1. Methane Photolysis Gans et al. PCCP Front cover

CH 4 Photolysis – Background Product Channels: CH 3 + H 1 CH 2 + H 2 3 CH 2 + 2H CH + H + H 2 Smith and Nash, Icarus, 2006

CH 4 Photolysis – Previous Work C Gans et al. PCCP 2011

CH 4 Photolysis – Previous Work ReferenceGans et al. Park et al. Mordaunt et al. Heck et al. Brownswo rd et al. Wang et al. Lodriguito et al. Method Direct determination of CH 2 and CH 3 Simultaneous photolysis and detection of H atoms by LIF ToF H atom kinetic energy spectroscopy Photofragmen t imaging Photolysis and H atom detection (vuvLIF) at Lyman α Determination of H and molecular products Trajectory calculations Date λ/nm nm H atom nm H and CH 3 + H 0.26 ± ± ± ± ± 0.03 CH 2 (a 1 A 1 ) + H ± ± ± ± 0.06 CH 2 (X 3 B 1 ) + 2H 0.48 ± ± ± ± 0.02 CH + H + H ± ± 0.01 Total H1.31 ± ± ± ± ± ± ± 0.10 Total H ± ± ± ± 0.06 Summary of Previous Results

CH 4 Photolysis – Possible approaches Repeat of Gans et al. approach (synchrotron photolysis source?) Direct detection of CH via laser induced fluorescence Enhanced end product analysis studies – Excimer lamps (e.g. 126 nm) as strong sources (>50 mW cm -2 ) – Chemical conversion ( 3 CH 2 particularly difficult to detect via optical spectroscopy) – Use of PTR-MS for sensitive end-product analysis, H 3 O + + RH → RH + + H 2 O (soft ionization)

2. 1 CH 2 Reactions – Temperature Dependence

Importance of 1 CH 2 reactions Wilson and Atreya, JGR 108, E2 5014, 2003

1 CH 2 + rare gas 1 CH 2 + RG → 3 CH 2 + RG Gannon et al. JCP

Temperature Dependence of 1 CH 2 removal by C 2 H 2 Gannon et al. JPCA Monitor removal of 1 CH 2 by LIF 1 CH 2 + C 2 H 2 → C 3 H 3 + H 1 CH 2 + C 2 H 2 + M → C 3 H 4 + M 1 CH 2 + C 2 H 2 → 3 CH 2 + C 2 H 2 Monitor calibrated production of H by LIF

Product Temperature Dependence Temperature k k overall k relaxation reaction relaxation

H Atom Yields 1 CH 2 + ΓHΓH 195 K250 K298 K398 K498 K C2H2C2H ± ± ± ± ± 0.42 C2H4C2H ± ± ± ± ± 0.19 Relaxation increases with decreasing temperature Opposite of rare gas behaviour Relaxation will be more important for planetary atmospheres – more focus on 3 CH 2 chemistry ?

PES showing surface crossing Crossing is below entrance channel Gannon et al. Faraday Discussions (Glowacki and Harvey, Bristol)

3. CH Reactions

CH Chemistry Reactivity very high – capable of reacting with N 2 Important intermediate for increasing carbon number CH + CH 4 → H + C 2 H 4 Single channel so useful calibration reaction More usually several open channels CH + CH 3 OH → HCHO + CH 3 CH + CH 3 OH → H + CH 3 CHO

4. Product Studies from Laval Reactor (Blitz, Shannon and Heard)

Low temperature kinetics of abstraction Reactions OH + CH 3 COCH 3 → H 2 O + CH 2 COCH 3 Barrier, so activated process – what is happening at low T? Shannon et al. PCCP

Product Formation OH + CH 3 OH → CH 3 O + H 2 O Shannon et al. Nature Chem

5. Summary CH 4 photolysis yields are important Currently uncertainty on CH 4 photochemistry New experiments to be undertaken as part of STFC project building on expertise in atmospheric and combustion studies 1 CH 2 chemistry shows interesting T dependence, not always taken into account in models. More focus on 3 CH 2 ? Acceleration in loss rates at low temperatures associated with chemical reaction. Further experiments in Laval systems in progress

Reagent and product time profiles 1 CH 2 H

Experimental Generate 1 CH 2 by pulsed photolysis of ketene Monitor removal of 1 CH 2 by LIF 1 CH 2 + C 2 H 2 → C 3 H 3 + H 1 CH 2 + C 2 H 2 + M → C 3 H 4 + M 1 CH 2 + C 2 H 2 → 3 CH 2 + C 2 H 2 Monitor calibrated production of H by LIF

Master Equation Calculations MESMER (Master Equation Solver for Multi Energy-well Reactions) A + B k Ri k ji k ij k Pj source term n j (E) n i ) Products (infinite sink)  E i En)(  E j En)( K(E)’s calculated from RRKM theory. Energy transfer calculated an exponential down model ~ cm -1

Master Equation Results Modelling shows no stabilization below 50 Torr Balance of reaction is relaxation Experimental Pressure

Experimental James Lockhart

Flash Photolysis LIF Detection C Gas mixing manifold MFC N 2 C2H2C2H2 (CH 3 ) 3 COOH O2O2 Reaction Cell Exhaust Line / Needle valve Rotary Pump Gas mixture flows in towards the cell Photolysis laser pulse 248 nm Rhodamine 6G Dye Laser Nd: YAG Laser Probe Laser Pulse 282 nm Photodiode PMT Boxcar Averager Excimer Laser

k = (7.59 ± 0.31) s -1 cm -3 Gas phase oxidation will compete with aerosol uptake Onel, L; Blitz, M. A; Seakins, P. W J.Phys.Chem.Lett 2012, 3, 853−856 II - OH + MEA (monoethanolamine) OH uptake ? PM

II - Recycling OH with Excess Oxygen OH Decay in N 2 Zero OH Yield 100% OH Yield Experimental OH Yield

MESMER Master Equation Solver for Multi Energy-well Reactions MESMER 3.0 Released 24 th Feb Contact Robin Shannon for more