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Ultrafast diffraction and structural dynamics: The nature of complex molecules far from equilibrium

Ruan, Chong-Yu and Lobastov, Vladimir A. and Srinivasan, Ramesh and Goodson, Boyd M. and Ihee, Hyotcherl and Zewail, Ahmed H. (2001) Ultrafast diffraction and structural dynamics: The nature of complex molecules far from equilibrium. Proceedings of the National Academy of Sciences of the United States of America, 98 (13). pp. 7117-7122. ISSN 0027-8424. http://resolver.caltech.edu/CaltechAUTHORS:RUApnas01

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Abstract

Studies of molecular structures at or near their equilibrium configurations have long provided information on their geometry in terms of bond distances and angles. Far-from-equilibrium structures are relatively unknown---especially for complex systems---and generally, neither their dynamics nor their average geometries can be extrapolated from equilibrium values. For such nonequilibrium structures, vibrational amplitudes and bond distances play a central role in phenomena such as energy redistribution and chemical reactivity. Ultrafast electron diffraction, which was developed to study transient molecular structures, provides a direct method for probing the nature of complex molecules far from equilibrium. Here we present our ultrafast electron diffraction observations of transient structures for two cyclic hydrocarbons. At high internal energies of approx 4 eV, these molecules display markedly different behavior. For 1,3,5-cycloheptatriene, excitation results in the formation of hot ground-state structures with bond distances similar to those of the initial structure, but with nearly three times the average vibrational amplitude. Energy is redistributed within 5 ps, but with a negative temperature characterizing the nonequilibrium population. In contrast, the ring-opening reaction of 1,3-cyclohexadiene is shown to result in hot structures with a C---C bond distance of over 1.7 Å, which is 0.2 Å away from any expected equilibrium value. Even up to 400 ps, energy remains trapped in large-amplitude motions comprised of torsion and asymmetric stretching. These studies promise a new direction for studying structural dynamics in nonequilibrium complex systems.


Item Type:Article
Additional Information:Contributed by Ahmed H. Zewail, April 19, 2001. This work was supported by the National Science Foundation and the Air Force Office of Scientific Research. The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Subject Keywords:ultrafast electron diffraction, 1,3,5-cycloheptatriene, 1,3- cyclohexadiene, 1,3,5-hexatriene, di-s-cis-Z-hexatriene, mono-s-cis-Z-hexatriene, di-s-trans-Z-hexatriene
Record Number:CaltechAUTHORS:RUApnas01
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:RUApnas01
Alternative URL:http://dx.doi.org/10.1073/pnas.131192898
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:507
Collection:CaltechAUTHORS
Deposited By: Archive Administrator
Deposited On:11 Jul 2005
Last Modified:26 Dec 2012 08:40

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