Cy. Ruan et al., Ultrafast diffraction and structural dynamics: The nature of complex molecules far from equilibrium, P NAS US, 98(13), 2001, pp. 7117-7122
Citations number
34
Categorie Soggetti
Multidisciplinary
Journal title
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Studies of molecular structures at or near their equilibrium configurations
have long provided information on their geometry in terms of bond distance
s and angles. Far-from-equilibrium structures are relatively unknown-especi
ally for complex systems-and generally, neither their dynamics nor their av
erage geometries can be extrapolated from equilibrium values. For such none
quilibrium structures, vibrational amplitudes and bond distances play a cen
tral role in phenomena such as energy redistribution and chemical reactivit
y. Ultrafast electron diffraction, which was developed to study transient m
olecular structures, provides a direct method for probing the nature of com
plex molecules far from equilibrium. Here we present our ultrafast electron
diffraction observations of transient structures for two cyclic hydrocarbo
ns. At high internal energies of approximate to4 eV, these molecules displa
y markedly different behavior. For 1,3,5-cycloheptatriene, excitation resul
ts in the formation of hot ground-state structures with bond distances simi
lar to those of the initial structure, but with nearly three times the aver
age vibrational amplitude. Energy is redistributed within 5 ps, but with a
negative temperature characterizing the nonequilibrium population. In contr
ast, 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 Angstrom, which is 0.2
Angstrom away from any expected equilibrium value. Even up to 400 ps, energ
y remains trapped in large-amplitude motions comprised of torsion and asymm
etric stretching. These studies promise a new direction for studying struct
ural dynamics in nonequilibrium complex systems.