Yusheng Dou

Ultrafast excited-state dynamics of tetraphenylethylene studied by semiclassical simulation

Detailed simulation study is reported for the excited-state dynamics of photoisomerization of cis-tetraphenylethylene (TPE) following excitation by a femtosecond laser pulse. The technique for this investigation is semiclassical dynamics simulation, which is described briefly in the paper. Upon photoexcitation by a femtosecond laser pulse, the stretching motion of the ethylenic bond of TPE is initially excited, leading to a significant lengthening of ethylenic bond in 300 fs. Twisting motion about the ethylenic bond is activated by the energy released from the relaxation of the stretching mode. The 90 degrees twisting about the ethylenic bond from an approximately planar geometry to nearly a perpendicular conformation in the electronically excited state is completed in 600 fs. The torsional dynamics of phenyl rings which is temporally lagging behind occurs at about 5 ps. Finally, the twisted TPE reverts to the initial conformation along the twisting coordinate through nonadiabatic transitions. The simulation results provide a basis for understanding several spectroscopic observations at molecular levels, including ultrafast dynamic Stokes shift, multicomponent fluorescence, viscosity dependence of the fluorescence lifetime, and radiationless decay from electronically excited state to the ground state along the isomerization coordinate.

Detailed dynamics of the nonradiative deactivation of adenine: a semiclassical dynamics study.

A realistic dynamics simulation study is reported for the ultrafast radiationless deactivation of 9H-adenine. The simulation follows two different excitations induced by two 80 fs (fwhm) laser pulses that are different in energy: one has a photon energy of 5.0 eV, and the other has a photon energy of 4.8 eV. The simulation shows that the excited molecule decays to the electronic ground state from the (1)pipi* state in both excitations but through two different radiationless pathways: in the 5.0 eV excitation, the decay channel involves the out-of-plane vibration of the amino group, whereas in the 4.8 eV excitation, the decay strongly associates with the deformation of the pyrimidine at the C 2 atom. The lifetime of the (1) npi* state determined in the simulation study is 630 fs for the 5.0 eV excitation and 1120 fs for the 4.8 eV excitation. These are consistent with the experimental values of 750 and 1000 fs. We conclude that the experimentally observed difference in the lifetime of the (1) npi* state at various excitations results from the different radiationless deactivation pathways of the excited molecule to the electronic ground state.

Energy-efficient migration and consolidation algorithm of virtual machines in data centers for cloud computing

In this paper, we developed a dynamic energy-efficient virtual machine (VM) migration and consolidation algorithm based on a multi-resource energy-efficient model. It can minimize energy consumption with Quality of Service guarantee. In our algorithm, we designed a method of double threshold with multi-resource utilization to trigger the migration of VMs. The Modified Particle Swarm Optimization method is introduced into the consolidation of VMs to avoid falling into local optima which is a common defect in traditional heuristic algorithms. Comparing with the popular traditional heuristic algorithm Modified Best Fit Decrease, our algorithm reduced the number of active physical nodes and the amount of VMs migrations. It shows better energy efficiency in data center for cloud computing.

Detailed Molecular Dynamics of the Photochromic Reaction of Spiropyran: A Semiclassical Dynamics Study

A realistic semiclassical dynamics simulation study is reported for the photoinduced ring-opening reaction of spiropyran. The main simulation results show that one pathway involves hydrogen out-of-plane (HOOP) torsion of phenyl ring nearby N atom in 254 fs on the excited state and the isomerization from cis- to trans-SP that is complete in about 10 ps on the ground state after the electron transition ; the other dominate pathway corresponds to the ring-opening reaction of trans-SP to form the most stable merocyanine (MC) product. Unlike the previous theoretical finding, one C−C bond cleavage on the real molecule rather than the C−N dissociation of the model one is more probable than the ring-opening reaction after the photoexcitation of SP. The simulation findings provide more important complementarity for interpreting experimental observations, confirming the previously theoretical studies of photochromic ring-opening process and even supplying other possible reaction mechanisms.

Dynamics of laser-excited stacked adenines: Semiclassical simulations

The nonradiative decay of a π-stacked pair of adenine molecules, following laser excitation, was studied by semiclassical dynamics simulations. Two deactivation pathways were characterized. One pathway involves an ultrafast internal conversion within ~600 fs induced by an out-of-plane vibration of the H atom and deformation of the pyrimidine ring at the C(2) site. A slower process (~2400 fs) involves covalent bond formation between the stacked molecules, which lowers the excimer state energy and inhibits the deformation of the pyrimidine ring; the decay is also induced by an out-of-plane vibration of the H atom at the C(2) site of the pyrimidine ring.

Competing Deactivation Channels for Excited π-Stacked Cytosines

The deactivation of π-stacked cytosine molecules following excitation by ultrashort laser pulses was studied using semiclassical dynamics simulations. Another deactivation channel was found to compete with a previously reported path that led to dimerization. For both pathways, the initial excited state was found to form a charge-separated neutral exciton state, which forms an excimer state by charge transfer. When the interbase distance becomes less than 3 A, charge recombination occurs due to strong intermolecular interaction, ultimately leading to an avoided crossing. Results indicate that the C2–N1–C6–C5 and ––– dihedral angles play a significant role in the vibronic coupling between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Vibrational energy distribution determines the fate of the excimer at the avoided crossing. Higher-amplitude vibration of C5 or C6 atoms leads to a nonadiabatic transition to the electronic ground state (a photophysical pathway); otherwise, a chemical reaction leading to the formation of cyclobutane type dimer occurs as found in earlier studies. The S1 and S0 potential energy surfaces calculated at TD-DFT level and the simulated trajectories were found to be consistent with CASPT2 results.

Femtosecond-scale photodissociation of benzene

Semiclassical electron-radiation-ion dynamics (SERID) has been used to calculate the bond lengths, HOMO–LUMO energy gap and vibrational modes of benzene, and to examine the photodissociation of benzene molecules subjected to fast intense laser pulses. The calculated ground-state properties are in good agreement with experiment, confirming that density-functional-based SERID simulations provide a reliable treatment of bonding. We show results for representative simulations of the response of benzene to femtosecond-scale laser pulses, at various fluences corresponding to no dissociation, partial dissociation and complete dissociation into atomic constituents.

Molecular dynamics simulation of primary detonation process of TATB crystal under shock loading

Abstract Molecular dynamics simulations were performed to gain fundamental insights into the mechanisms for the primary detonation process of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) under shock wave loading using self-consistent charge density-functional tight binding(SCC-DFTB) calculations combined with the multiscale shock technique (MSST). The primary process starts with shock loading and ends with the formation of dynamically stable heterocyclic clusters, which could inhibit the reactivity of TATB. The results show that the initial step of shocked TATB decomposition is the N–O bond cleavage; then carbon rings aggregate and connect by N atoms to form clusters; after the carbon rings open, heterocyclic clusters with nitrogen are formed, and persist throughout the simulation. This is a new mechanism for the primary processes of shocked TATB and this initiation mechanism is independent of the initial shock speeds.

Dynamics in Photoexcited DNA Bases and Related Molecules

1 Institute of Computational Chemistry, Chongqing University of Posts and Telecommunications, Chongqing 400065, China 2Department of Physical Sciences, Nicholls State University, P.O. Box 2022, Thibodaux, LA 70310, USA 3Department of Physics, National University of Singapore, Singapore 117542 4Department of Applied Physics, Xi’an Jiaotong University, Xi’an 710049, China 5Department of Physics and Astronomy, Texas A&M University, College Station, TX 77843-4242, USA

Mechanisms of Laser-Induced Reactions of Stacked Benzene Molecules: A Semiclassical Dynamics Simulation and CASSCF Calculation

The response to ultrashort laser pulses of two stacked benzene molecules has been studied by semiclassical dynamics simulation; two typical pathways were found following excitation of one of the benzene molecules by a 25 fs (FWHM), 4.7 eV photon. With a fluence of 40.49 J/m2, the stacked molecules form a cyclobutane benzene dimer; the formation of the two covalent bonds linking two benzenes occurs asynchronously after the excimer decays to electronic ground state. With a fluence of 43.26 J/m2, only one bond is formed, which breaks about 50 fs after formation, followed by separation into the two molecules. The deformation of benzene ring is found to play an important role in the bond cleavage.