1. 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

2. 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

3. Photodissociation of Alkyl Radicals  Prototypical organic radicals  Important intermediates in combustion  Photodissociation via Rydberg states  Our group’s previous work: methyl, ethyl, propyl

4. 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

5. 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

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

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

8. 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

9. 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

10. H-atom TOF Spectra  check precursors

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

12. CM Product Translational Energy Distribution

13. Average E T Release

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

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

16. Photodissociation Mechanism I Conical intersection

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

18. 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)

19. Acknowledgements  Prof. Jingsong Zhang  Yanlin Liu  Zhang Group