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

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.