Semiclassical Dynamics Simulation Research Articles

Laser induced C(60) cage opening studied by semiclassical dynamics simulation.

Laser induced opening of the C60 cage is studied by a semiclassical electron-radiation-ion dynamics technique. The simulation results indicate that the C60 cage is abruptly opened immediately after laser excitation. The opening of the C60 cage induces a quick increase in kinetic energy and a sharp decrease in electronic energy, suggesting that the breaking of the C60 cage efficiently heats up the cluster and enhances the thermal fragmentation of C60 fullerene.

Dynamics Simulation of a New Deactivation Pathway for Stacked Adenines

A semiclassical dynamics simulation study was undertaken to investigate the deactivation of the lowest excited state of π-stacked adenines, as induced by a laser pulse. Only one of the adenines was subjected to a laser pulse in this simulation. The simulation results show that the interaction between the excited adenines (A) and their unexcited neighbors (A′) increases significantly, followed by a shortening of the intermolecular distance. The interbases interaction leads to a new deactivated pathway in which atom C2 in molecule A and atom C2′ in molecule A′ are link to each other and form a "bonded excimer" intermediate. The lifetime of the "bonded excimer" intermediate is about 390 fs. The deformation of the pyrimidine ring at the C2 atom and the displacement of the H2′ atom away from the pyrimidine ring play a significant role in the deactivation process of the "bonded excimer" intermediate. After deactivation, the C2-C2′ dissociates and the released bond energy converts to molecular kinetic energy. Both adenine molecules return to the planar geometries of their ground states.

A semiclassical dynamics study of the photoisomerization of methyl-substituted azobenzene

A semiclassical dynamics simulation study is reported for a trans–cis photoisomerization cycle of four azo compounds, including azobenzene, 2-methylazobenzene, 2,6-dimethylazobenzene, and 2,2′-dimethylazobenzene. For both trans → cis and cis → trans isomerization processes, each compound is excited by a 50 fs (fwhm) laser pulse with a photon energy leading to a ππ* excitation. It is found that the compound in both cases follows a rotational path from reactant to product and that the isomerization dynamics is significantly affected by substitution. The relative times for completing a trans–cis isomerization cycle for four compounds, 2,6-dimethylazobenzene > 2,2-dimethylazobenzene and 2,6-dimethylazobenzene > 2-methylazobenzene> azobenzene, follow the same order as for the photoinduced formation of the surface relief grating of polymers based on these four compounds. The simulation results provide a basis for understanding the surface relief grating formation of azobenzene-based materials upon irradiation with laser beams.

Cage opening and fragmentation of C60 fullerene induced by an ultrashort laser pulse

Response of C60 fullerene to a 40 fs full-width at half-maximum laser pulse with a photon energy of 2.0 eV and different laser intensities is studied by semiclassical dynamics simulation technique. The simulation results show that soon after the irradiation with a strong laser pulse, many C–C bonds abruptly break but no fragments are produced. The breaking of multiple C–C bonds induces a quick increase in the kinetic energy and potential energy and a decrease in electronic energy. These results suggest that the opening of the C60 cage is an effective channel for the conversion of electronic energy to kinetic energy for the electronically excited C60 fullerene.

A Straightforward Method of Analysis for Direct Quantum Dynamics: Application to the Photochemistry of a Model Cyanine†

We present a new way of analyzing direct quantum dynamics simulations based on a Mulliken-type population analysis. This provides a straightforward interpretation of the wavepacket in much the same way as semiclassical trajectories are usually analyzed. The result can be seen as a coupled set of quantum trajectories. We apply this to the study of the photochemistry of a 12-atom model cyanine to explore possibilities for intelligent optimal control. The work presented here builds on previous semiclassical dynamics simulations [ Hunt , P. A. ; Robb , M. A. J. Am. Chem. Soc. 2005 , 127 , 5720 ]. Those calculations suggested that, by controlling the distribution of momentum components in the initial wavepacket, it should be possible to drive the system to a specific region of the conical intersection seam and ultimately control the product distribution. This was confirmed experimentally by optimal control methods [ Dietzek , B. ; Bruggemann , B. ; Pascher , T. ; Yartsev , A. J. Am. Chem. Soc. 2007 , 129 , 13014 ]. This paper aims to demonstrate this in a quantum dynamics context and give further insight into the conditions required for control. Our results show that directly addressing the trans-cis torsional modes is not efficient. Instead, one needs to decrease the momentum in the skeletal deformation coordinates to prompt radiationless decay near the minimum conical intersection at large twist angles.

Photoinduced dissociation of cyclobutane thymine dimer studied by semiclassical dynamics simulation

Photoinduced dissociation of the thymine dimer is studied in a semiclassical dynamics simulation. The simulation follows excitation of an isolated thymine dimer by a 25 fs fwhm laser pulse, and finds that dissociation proceeds via an asynchronously concerted mechanism, in which the C 5– C 5 ′ bond breaks soon after application of the laser pulse, followed by cleavage of the C 6– C 6 ′ bond. The dissociation results in two thymine monomers, one in an electronically excited state and the other in the ground state. The former decays to the electronic ground state through an avoided crossing induced by deformation of the pyrimidine ring at the C 5 ′ and C 6 ′ sites.

TORSIONAL POTENTIALS AND FULL-DIMENSIONAL SIMULATION OF ELECTRONIC ABSORPTION SPECTRA OF para-PHENYLENEVINYLENE OLIGOMERS USING SEMIEMPIRICAL HAMILTONIANS

A systematic study of torsional potential curves in electronic ground state based on second-order Møller–Plesset energy (MP2), density functional theory (DFT), and Austin mode 1 (AM1) methods is presented for para-phenylenevinylene oligomers constructed from two to four aromatic rings. The semiempirical AM1 approach gives the correct location of potential energy minima in comparison with the reference MP2 calculations and literature data. However, the semiempirical AM1 energy barriers at perpendicular orientation are ca. 30% smaller than the MP2 ones. The DFT calculations indicate optimal planar structures and the barriers are three times higher than MP2 values. Excited-state potential energy curves evaluated from vertical excitations at the time-dependent density functional theory (TD-DFT) and ab initio CI levels exhibit much steeper increase of values in the vicinity of perpendicular orientation than in the semiempirical Zerner's intermediate neglect of differential overlap (ZINDO) and ab initio RI-CC2 cases. The effects of vibrational motion of phenylene rings on the torsional broadening of absorption spectra were estimated from semi-classical molecular dynamics simulations and harmonic oscillator sampling. The simulated spectra agree well with the experiment and allow estimating the conformer distribution of the molecules.

Photoinduced Thymine Dimerization Studied by Semiclassical Dynamics Simulation

Response to ultrashort laser pulses of two stacked thymine molecules has been studied by semiclassical dynamics simulation with laser radiation explicitly incorporated. The laser pulses used to excite the thymine molecule have a 25 (fwhm) fs Gaussian shape with a photon energy of 4.0 eV. Simulation follows two different reaction paths produced by the laser pulses with two different fluences. In one reaction, the stacked thymine molecules form a cyclobutane pyrimidine dimer, which is the main course of photoinduced DNA damage, and the formation of two chemical bonds linking two thymines occurs nonsynchronously after the excimer decays to electronic ground state. In the other reaction, only one bond is formed between the two thymine molecules. In the second reaction, the bond breaks about 50 fs after formation, and then the two molecules move away from each other. This reaction leads to the DNA damage repair. The simulation finds that the deformation of the pyrimidine ring plays an important role in cleaving this bond.

Laser-induced nonthermal fragmentation of C60 studied by semiclassical dynamics simulation

Photofragmentation of the C60 fullerene induced by ultrafast laser pulses is studied by semiclassical dynamics simulation. The simulation study is focused on the excitation below the continuum levels. A laser pulse of 40 fs (FWHM) with an effective photon energy of 2.0 eV and different intensities was selected to interact with the C60 fullerene. The simulation results show that averaged fragmentation size distribution over groups of initial geometries selected at random exhibits a power law pattern with the peak at C2 at high laser pulse intensities. The threshold for the C60 fragmentation was determined. The simulation finds that as many as 55 electrons are excited from the occupied molecular orbitals to unoccupied molecular orbitals upon the laser irradiation and that the number of the fragments significantly depends on the number of electrons excited. Finally, the temperature examination seems to suggest that the nonthermal effect may play a significant role in laser fragmentation of the C60 fullerene.

Semiclassical dynamic simulation of photon induced ring-opening of cyclohexadiene to hexatriene

A dynamical picture and the detailed changes in the structure for the ring opening reaction of cyclohexadiene (CHD) to form hexatriene (HT) are studied by a semiclassical electron radiation-ion dynamics (SERID) simulation. Following the simulated trajectory, the potential energy curves (PECs) of three lowest singlet states (S 0, S 1 and S 2) have been calculated at the CASSCF/MRPT2 level with 6-31G* basis sets. Two non-adiabatic state-to-state transitions are found at 112 fs and 129 fs, which confirms the mechanism suggested in the Ref. [M. Garavelli, C.S. Page, P. Celani, M. Olivucci, W.E. Schmid, S.A. Trushin, W. Fuss, J. Phys. Chem. A 105 (2001) 4458].

Molecular Response to Ultrashort Laser Pulses

Experimental methods and static quantum calculations can't always give the most realistic picture of the reaction dynamics that laser pulse irradiation induces. Semiclassical dynamics simulations offer an alternative.

Detailed mechanism of trans–cis photoisomerization of azobenzene studied by semiclassical dynamics simulation

A realistic dynamics simulation study is reported for the trans–cis photoisomerization of azobenzene. The simulation follows both nπ* and ππ* excitations and each excitation is induced by a 50 fs (FWHM) laser pulse. The simulation results show that, for both excitations, the reaction path is predominated by the rotation coordinate of the NN bond. The simulation finds that the CNN inversion angles expand as soon as the rotation starts. The expansion of the CNN bond angles permits the molecule to rotate efficiently. It is therefore suggested that the photoisomerization of trans-azobenzene follows an inversion-assisted rotation path. These simulation results are significant for understanding the mechanism of this important process.

Shear viscosity and thermal conductivity of nuclear “pasta”

We calculate the shear viscosity \ensuremath{\eta} and thermal conductivity \ensuremath{\kappa} of a nuclear pasta phase in neutron star crusts. This involves complex nonspherical shapes. We use semiclassical molecular dynamics simulations involving 40, 000 to 100, 000 nucleons. The viscosity \ensuremath{\eta} can be simply expressed in terms of the height ${Z}^{*}$ and width $\ensuremath{\Delta}q$ of the peak in the static structure factor ${S}_{p}(q)$. We find that \ensuremath{\eta} increases somewhat, compared to a lower density phase involving spherical nuclei, because ${Z}^{*}$ decreases from form factor and ion screening effects. However, we do not find a dramatic increase in \ensuremath{\eta} from nonspherical shapes, as may occur in conventional complex fluids.

Involvement of excited triplet state in the photodissociation of cyclobutane

AbstractThe potential energy curves (PECs) of the ground state and the low‐lying excited states for the photodissociation of cyclobutane have been calculated at the multi‐reference configuration interaction with singlet and doublet excitation (MRCISD) and the multi‐reference second order perturbation theory (MRPT2). Firstly, the PECs are constructed following a reaction path determined by semiclassical dynamics simulation, which suggests that the lowest triplet state of tetramethylene is involved in the photodissociation of cyclobutane. Then, the adiabatic PECs are calculated for the breaking processes of C1C3 and C2C4 bond respectively. The singlet‐triplet PECs' intersections have been found in the two breaking CC bond processes. During the breaking process of the second C2C4 bond, a local minimum has been found on the PEC of the lowest triplet state, which gives us some insight to reinterpret the experimental observed diradical intermediate as being trapped in its triplet state. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

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.

A Diabatic Representation Including Both Valence Nonadiabatic Interactions and Spin−Orbit Effects for Reaction Dynamics

A diabatic representation is convenient in the study of electronically nonadiabatic chemical reactions because the diabatic energies and couplings are smooth functions of the nuclear coordinates and the couplings are scalar quantities. A method called the fourfold way was devised in our group to generate diabatic representations for spin-free electronic states. One drawback of diabatic states computed from the spin-free Hamiltonian, called a valence diabatic representation, for systems in which spin-orbit coupling cannot be ignored is that the couplings between the states are not zero in asymptotic regions, leading to difficulties in the calculation of reaction probabilities and other properties by semiclassical dynamics methods. Here we report an extension of the fourfold way to construct diabatic representations suitable for spin-coupled systems. In this article we formulate the method for the case of even-electron systems that yield pairs of fragments with doublet spin multiplicity. For this type of system, we introduce the further simplification of calculating the triplet diabatic energies in terms of the singlet diabatic energies via Slater's rules and assuming constant ratios of Coulomb to exchange integrals. Furthermore, the valence diabatic couplings in the triplet manifold are taken equal to the singlet ones. An important feature of the method is the introduction of scaling functions, as they allow one to deal with multibond reactions without having to include high-energy diabatic states. The global transformation matrix to the new diabatic representation, called the spin-valence diabatic representation, is constructed as the product of channel-specific transformation matrices, each one taken as the product of an asymptotic transformation matrix and a scaling function that depends on ratios of the spin-orbit splitting and the valence splittings. Thus the underlying basis functions are recoupled into suitable diabatic basis functions in a manner that provides a multibond generalization of the switch between Hund's cases in diatomic spectroscopy. The spin-orbit matrix elements in this representation are taken equal to their atomic values times a scaling function that depends on the internuclear distances. The spin-valence diabatic potential energy matrix is suitable for semiclassical dynamics simulations. Diagonalization of this matrix produces the spin-coupled adiabatic energies. For the sake of illustration, diabatic potential energy matrices are constructed along bond-fission coordinates for the HBr and the BrCH(2)Cl molecules. Comparison of the spin-coupled adiabatic energies obtained from the spin-valence diabatics with those obtained by ab initio calculations with geometry-dependent spin-orbit matrix elements shows that the new method is sufficiently accurate for practical purposes. The method formulated here should be most useful for systems with a large number of atoms, especially heavy atoms, and/or a large number of spin-coupled electronic states.

Ring opening reaction of 1,3-cyclohexadiene studied by semiclassical dynamics simulation

A semiclassical dynamics simulation study is reported for the ring opening reaction of 1,3-cyclohexadiene (CHD) triggered by a femtosecond-scale laser pulse. The results clearly demonstrate that, following the excitation by the laser pulse, the ring opening occurs at ∼110 fs and the molecule decays to the ground electronic state at ∼210 fs due to non-adiabatic transition of electrons from LUMO to HOMO orbitals. Isomerization of the product of the ring opening reaction, 1,3,5-hexatriene (HT), to various stable isomers are also well demonstrated by the simulations.

Effect of C [sbnd]C [sbnd]C bond bending vibration on the photodissociation of cyclobutane

The effect of C C C bond bending on the photodissociation of cyclobutane to form two ethylene molecules was investigated by performing semiclassical electron-radiation-ion dynamics simulations and also by examining the potential energy surfaces of the electronic ground state and lowest excited states. These potential energy surfaces, calculated at the CASSCF/MRPT2 level with 6-31G* basis sets along a reaction path determined by the semiclassical dynamics simulations, show well-defined energy minima and maxima in the intermediate state region. It is found that in addition to rotation of the molecule around the central C C bond, C C C bond bending plays an important role in determining the features of the potential energy surfaces for the intermediate species.

Tailoring model Hamiltonians for dihalogen—rare gas matrix problems

A strategy for reducing the number of electronic states which need to be incorporated into a diatomics-in-molecules description of prototypical condensed phase processes is demonstrated. The selection of a problem-adapted representation is based on symmetry properties of the molecular basis set and its disturbance by the external potential due to the environment. Using the \({B \leftarrow X}\) excitation of Br2 in solid Argon as an example, we show that the energy range accessible via a Franck–Condon transition can approximately be described by 17 instead of all 36 valence states. The approach shall be particularly useful for reducing the numerical effort of semiclassical and quantum dynamics simulations.

Light-harvesting function through one-by-one electron and hole transfer in a methane-lithium system

Carrying out a semiclassical molecular dynamics simulation of a CH4-Li2 system by using the time-dependent local density approximation of the time-dependent density functional theory, we find that one-by-one electron and hole transfer takes place from CH4 to Li2 when an electron is excited in CH4. Probability of the transfer is low when the molecules are fixed, but it increases when the molecules are freely relaxed or Li2 has 1 eV of initial velocity.