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Ultraviolet Photodissociation Dynamics of the Cyclohexyl Radical Michael Lucas, Yanlin Liu, Jingsong Zhang Department of Chemistry University of California, Riverside 69 th International Symposium on Molecular Spectroscopy 6/17/2014

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Photodissociation of Free Radicals Free radicals Open shell Highly reactive Important in many areas of chemistry Combustion, atmospheric, plasma, interstellar Dissociation depends on potential energy surfaces Provide benchmarks for theory

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Photodissociation of Alkyl Radicals Prototypical organic radicals Important intermediates in combustion Photodissociation via Rydberg states Our groups previous work: methyl, ethyl, propyl

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Photodissociation of Ethyl via 3s Rydberg State Bimodal H-atom distribution Fast Pathway: anisotropic ( = 0.5), high f T , direct H-atom scission via nonclassical H-bridged structure from the 3s state to yield H + C 2 H 4 (X 1 A g ). Slow Pathway: isotropic, modest f T , unimolecular dissociation after internal conversion. G. Amaral et al. J. Chem. Phys. 114 (2001) 5164 M. Steinbauer et al. J. Chem. Phys. 137 (2012) 014303 Conical intersection

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Photodissociation of Aromatic Radicals Our recent work: phenyl, benzyl, o-pyridyl, m- pyridyl Important intermediates in combustion and soot formation Photodissociation mechanisms unimolecular dissociation following internal conversion; statistical product energy distribution I.C. Y. Song et al. J. Chem. Phys. 136 (2012) 044308

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Cyclohexyl Radical Cycloalkanes are important component of conventional fuels Cyclohexane model cycloalkane Major producer of benzene No previous photodissociation studies of cyclohexyl

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Potential Energy Diagram of c-C 6 H 11 C. Franklin Goldsmith et al. J. Phys. Chem. 113 (2009) 13357 ~ ~

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High-n Rydberg H-atom Time-of-Flight (HRTOF) H Lyman- Probe 121.6 nm Photolysis Pulsed Valve Rydberg Probe 366.2 nm Detector Skimmer 193 nm H transitions 1 2 nH+H+ H (n) H (2 2 P) 121.6 nm Lyman- 366.2 nm K. Welge and co-workers, J Chem Phys 92 (1990) 7027 Chlorocyclohexane Bromocyclohexane

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Production of Cyclohexyl Radical Beam 121.6-nm VUV photoionization mass spectrometry Net mass spectrum: 193-nm radical generation radiation on minus off Radical production Precursor depletion Cl-C 6 H 11 + hv Cl + C 6 H 11

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H-atom TOF Spectra check precursors

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H-atom Product Action Spectrum compare with absorption spectrum J. Platz et al. J. Phys. Chem. A 103 (1999) 2688

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CM Product Translational Energy Distribution

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Average E T Release

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H-atom Product Angular Distribution ~ 0.3 - 1.0 Anisotropic distribution Dissociation time faster than 1 rotation period E v

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Photodissociation Mechanism I ~ ~ x x Unlikely, Unimolecular dissociation, Cannot compete with c-C 6 H 10 channel Repulsive dissociation; Similar to ethyl x

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Photodissociation Mechanism I Conical intersection

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Photodissociation Mechanism II http://webbook.nist.gov v

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Summary UV photodissociation dynamics of cyclohexyl was studied in 232-262 nm for the first time Observed: cyclohexyl cyclohexene + H Large translational energy release, f T 0.45-0.55 Anisotropic distribution Non-statistical distribution Dissociation mechanism: direct dissociation from the excited state and/or on the repulsive part of the ground state (possibly via conical intersection)

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Acknowledgements Prof. Jingsong Zhang Yanlin Liu Zhang Group