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Summary: Quantum coherent dynamics of molecules:
A simple scenario for ultrafast photoisomerization
Daniel P. Aalberts,1,
* M. S. L. du Croo de Jongh,2
Brian F. Gerke,1,
and Wim van Saarloos2
1
Physics Department, Williams College, Williamstown, Massachusetts 01267
2
Instituut-Lorentz, Universiteit Leiden, P.O. Box 9506, 2300 RA Leiden, The Netherlands
Received 11 November 1999; published 28 February 2000
We analyze the coherent dynamics of optically excited alkenes in a fully correlated 3d tight-binding model
with extended Hubbard interactions. The scenario that emerges is that the steric repulsive interactions are the
driving force behind ultrafast cis-trans photoisomerizations. This resolves the apparent discrepancy between
values for the torsional stiffness obtained from band-structure potentials and from vibrational spectra. The
mechanism is illustrated in quantitative detail for ethylene and is also shown to yield a promising scenario for
the coherent dynamics of molecules like retinal.
PACS number s : 82.50. m, 87.15.He, 82.40.Js, 34.20.Cf
Cis-to-trans isomerizations see Fig. 1 are important pho-
tochemical reactions for polyenes. These reactions were al-
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