Ultrafast electron diffraction: Excited state structures and chemistries of aromatic carbonyls

Journal Article · Sun May 07 04:00:00 EDT 2006 · Journal of Chemical Physics

DOI:https://doi.org/10.1063/1.2194017· OSTI ID:20783271

Park, Sang Tae; Feenstra, Jonathan S; Zewail, Ahmed H ^[1]

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1. Laboratory for Molecular Science, and Physical Biology Center for Ultrafast Science and Technology, California Institute of Technology, Pasadena, California 91125 (United States)

The photophysics and photochemistry of molecules with complex electronic structures, such as aromatic carbonyls, involve dark structures of radiationless processes. With ultrafast electron diffraction (UED) of isolated molecular beams it is possible to determine these transient structures, and in this contribution we examine the nature of structural dynamics in two systems, benzaldehyde and acetophenone. Both molecules are seen to undergo a bifurcation upon excitation (S{sub 2}). Following femtosecond conversion to S{sub 1}, the bifurcation leads to the formation of molecular dissociation products, benzene and carbon monoxide for benzaldehyde, and benzoyl and methyl radicals for acetophenone, as well as intersystem crossing to the triplet state in both cases. The structure of the triplet state was determined to be 'quinoidlike' of {pi}{pi}* character with the excitation being localized in the phenyl ring. For the chemical channels, the product structures were also determined. The difference in photochemistry between the two species is discussed with respect to the change in large amplitude motion caused by the added methyl group in acetophenone. This discussion is also expanded to compare these results with the prototypical aliphatic carbonyl compounds, acetaldehyde and acetone. From these studies of structural dynamics, experimental and theoretical, we provide a landscape picture for, and the structures involved in, the radiationless pathways which determine the fate of molecules following excitation. For completeness, the UED methodology and the theoretical framework for structure determination are described in this full account of an earlier communication [J. S. Feenstra et al., J. Chem. Phys. 123, 221104 (2005)].

• OSTI ID: 20783271
• Journal Information: Journal of Chemical Physics, Journal Name: Journal of Chemical Physics Journal Issue: 17 Vol. 124; ISSN JCPSA6; ISSN 0021-9606
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
ACETALDEHYDE
ACETONE
ACETOPHENONE
BENZALDEHYDE
BENZENE
BIFURCATION
CARBONYLS
DISSOCIATION
ELECTRON DIFFRACTION
ELECTRONIC STRUCTURE
EXCITATION
EXCITED STATES
METHYL RADICALS
MOLECULAR BEAMS
MOLECULAR STRUCTURE
PHOTOCHEMISTRY
REACTION KINETICS
TRIPLETS