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The ultrafast laser-induced isomerization dynamics of gas phase stilbene is studied in detail using quasiclassical molecular dynamics methods. To model the dynamics, empirical potential surfaces for both the ground and the first excited electronic states of stilbene are constructed using molecular modeling-type potentials which are fit to available spectroscopic measurements, ultrafast dynamics observations, and theoretical structural information. An algorithm for creating quasiclassical initial conditions that simulate the nonstationary state prepared by an ultrashort (100 fs) laser pulse is presented. This algorithm ultilizes a quantum mechancial formulation of the excitation based on harmonic approximations for the potential surfaces which is then adapted to give initial conditions for an ensemble of trajectories. Using these methods, we recently (Chem.Phys. Lett. 1993,215,306) found evidence for the existence of a small barrier along the cis-trans isomerization (ethylenic torsion) coordinate from the cis-stilbene Franck-Condon region of the excited state and here we improve the estimate of that barrier to between 260 and 400 cm-I. Detailed examination of the excited state dynamics of cis-stilbene reveals that the isomerization process proceeds via multiple pathways to a twisted potential minimum on the excited state from which internal conversion to the ground state occurs. All three photoproducts of cis-stilbene cross to the ground state from the same general region of the excited state and are differentiated during the radiationless transition and subsequent dynamics on the ground state. Nonadiabatic coupling between the electronic states is included in our studies using a semiclassical technique, and this allows the identification of correlations between the excited state dynamics and the branching between final products. The photoexcitation of trans-stilbene is also examined, and our calculations suggest that this isomerization process proceeds through a different minimum on the excited state surface which also corresponds to a 90" twisted configuration. This is a departure from the standard view that both cis-and trans-stilbene proceed through the same intermediate in their respective cis-trans photoisomerization dynamics.

We explore the possibilities of creating radiatively stable entangled states of two three-level dipole-interacting atoms in a Λ configuration by means of laser biharmonic continuous driving or pulses. We propose three novel schemes for generation of entangled states which involve only the lower states of the Λ system, not vulnerable to radiative decay. Two of them employ coherent dynamics to achieve entanglement in the system, whereas the third one uses optical pumping, i.e., an essentially incoherent process. Typeset using REVT E X 1The concept of quantum entanglement, one of the most intriguing properties of multipartite quantum systems [1], has been intensively exploited over the last decade in connection with quantum information processing. It has been shown that the use of entangled states opens new horizons in such practical fields as cryptography [2], computing [3], information transmission [4], and precision measurement [5]. However, all of these applications become possible only with a reliable source of entanglement. Traditionally, entangled particles have been generated in the down-conversion nonlinear process [6,7], but this method is in some cases disadvantageous due to the speedy nature of the produced particles (photons) and the intrinsic randomness of their appearance times. That is why efforts are now being made to find ways for controlled production of entangled states of less volatile massive particles [8].During the last few years, various methods for creation of entangled states of atoms, ranging from continuous observation of radiative decay [9,10] to controlled cold collisions [11], have been proposed and some of them experimentally demonstrated [12,13].Though the resonant dipole-dipole interaction (RDDI) was suggested for realization of entangling dynamics as early as 1995 [14], it was only recently that several authors [15,16,17] investigated this interaction in more detail as a method for entangling neutral atoms in optical traps (neutral atom realizations benefit from the fact that neutral atoms are less sensitive to stray EM fields-a major source of decoherence in ions [18]). While the authors of Ref.[15] offered qualitative arguments for realization of this idea in dipole traps [19], the papers [16,17] considered quantitative models of creation of maximally entangled states of two-level atoms. Unfortunately, such entangled states of two-level atoms have short lifetimes due to radiative decay. Obviously, since radiative decay and the RDDI have the same physical nature, we cannot avoid the former while making use of the latter. In this paper we solve this conceptual problem by presenting new methods for creation of radiatively stable entanglement in a system of dipole-interacting three-level atoms. Though the model considered here is still far from representing the real situations (see [20] for details of a possible experimental realization), it offers new insights into how the RDDI can be used to entangle real, multilevel atoms. I. THE MODELExtending the model described in [16...

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