Ab initio nonadiabatic molecular dynamics of the ultrafast electron injection across the alizarin-TiO2 interface

The observed 6-fs photoinduced electron transfer (ET) from the alizarin chromophore into the TiO2 surface is investigated by ab initio nonadiabatic (NA) molecular dynamics in real time and at the atomistic level of detail. The system derives from the dye-sensitized semiconductor Gr?tzel cell and addresses the problems of an organic/inorganic interface that are commonly encountered in photovoltaics, photochemistry, and molecular electronics. In contrast to the typical Gr?tzel cell systems, where molecular donors are in resonance with a high density of semiconductor acceptor states, TiO2 sensitized with alizarin presents a novel case in which the molecular photoexcited state is at the edge of the conduction band (CB). The high level ab initio analysis of the optical absorption spectrum supports this observation. Thermal fluctuations of atomic coordinates are particularly important both in generating a nonuniform distribution of photoexcited states and in driving the ET process. The NA simulation resolves the controversy regarding the origin of the ultrafast ET by showing that although ultrafast transfer is possible with the NA mechanism, it proceeds mostly adiabatically in the alizarin-TiO2 system. The simulation indicates that the electron is injected into a localized surface state within 8 fs and spreads into the bulk on a 100-fs or longer time scale. The molecular architecture seen in the alizarin-TiO2 system permits efficient electron injection into the edge of the CB by an adiabatic mechanism without the energy loss associated with injection high into the CB by a NA process.     

Ab initio quantum dynamics using coupled-cluster

The curse of dimensionality (COD) limits the current state-of-the-art ab initio propagation methods for non-relativistic quantum mechanics to relatively few particles. For stationary structure calculations, the coupled-cluster (CC) method overcomes the COD in the sense that the method scales polynomially with the number of particles while still being size-consistent and extensive. We generalize the CC method to the time domain while allowing the single-particle functions to vary in an adaptive fashion as well, thereby creating a highly flexible, polynomially scaling approximation to the time-dependent Schro?dinger equation. The method inherits size-consistency and extensivity from the CC method. The method is dubbed orbital-adaptive time-dependent coupled-cluster, and is a hierarchy of approximations to the now standard multi-configurational time-dependent Hartree method for fermions. A numerical experiment is also given.     

Ab initio nonadiabatic dynamics study of ultrafast radiationless decay over conical intersections illustrated on the Na3F cluster

We present a theoretical approach for the ultrafast nonadiabatic dynamics based on the ab initio molecular dynamics carried out "on the fly" in the framework of the configuration interaction method combined with Tully's surface hopping algorithm for nonadiabatic transitions. This approach combined with our Wigner distribution approach allows us to perform accurate simulations of femtosecond pump-probe spectra in the systems where radiationless transitions among electronic states take place. In this paper we illustrate this by theoretical simulation of ultrafast processes and nonradiative relaxation in the Na(3)F cluster, involving three excited states and the ground electronic state. Furthermore, we show that our accurate simulation of the photoionization pump-probe spectrum is in full agreement with the experimental signal. Based on the nonadiabatic dynamics at high level of accuracy and taking into account all degrees of freedom, the nonradiative lifetime for the 1 (1)B(1) excited state of Na(3)F has been determined to be approximately 900 fs.     

Ab initio quantum dynamical study of the multi-state nonadiabatic photodissociation of pyrrole

There has been a substantial amount of theoretical investigations on the photodynamics of pyrrole, often relying on surface hopping techniques or, if fully quantal, confining the study to the lowest two or three singlet states. In this study we extend ab initio based quantum dynamical investigations to cover simultaneously the lowest five singlet states, two π-σ? and two π-π? excited states. The underlying potential energy surfaces are obtained from large-scale MRCI ab initio computations. These are used to extract linear and quadratic vibronic coupling constants employing the corresponding coupling models. For the N-H stretching mode Q(24) an anharmonic treatment is necessary and also adopted. The results reveal a sub-picosecond internal conversion from the S(4) (π-π?) state, corresponding to the strongly dipole-allowed transition, to the S(1) and S(2) (π-σ?) states and, hence, to the ground state of pyrrole. The significance of the various vibrational modes and coupling terms is assessed. Results are also presented for the dissociation probabilities on the three lowest electronic states.     

Ab initio study of ultrafast photochemical reaction dynamics of phenol blue

Phenol blue (PB) is a primary skeletal structure part of indoaniline dyes and well-known as a solvatochromic dye. It has been recently observed by pump-probe (PP) transient absorption measurements that PB shows ultrafast ground state recovery within a few hundred femtoseconds after photoexcitation. In this work, the ultrafast photochemical reaction mechanism of PB has been investigated using direct ab initio (CASSCF) nonadiabatic molecular dynamics with the trajectory surface hopping (TSH) method. The swarm of trajectories starting from the S1 Franck-Condon (FC) point has mostly shown surface hops (nonadiabatic transitions) from the S1 state to the S0 state at 110-120 fs in the vicinity of an S1/S0 conical intersection and after decay to the S0 state bifurcated into two (Reverse and Forward) directions with almost the same branching ratio and reached the vicinity of the S0 minimum energy point at 200-300 fs, which is in good agreement with the fast time component of the ground state recovery in the PP measurements. After reaching the vicinity of the S0 minimum energy point, the trajectories showed a coherent vibration of bending motion between quinoneimine and aniline rings with a low frequency of 43 cm-1, which presumably corresponds to a coherently photoexcitation-induced vibrational mode with a low frequency recently observed by the PP measurements.     

Ab initio based surface-hopping dynamics study on ultrafast internal conversion in cyclopropanone

Cyclopropanone exhibits an intriguing phenomenon that the fluorescence from the S(1) state disappears below 365 nm. This is ascribed to the ultrafast S(1) → S(0) internal conversion process via conical intersection, which deprives opportunity of the fluorescence emission. In this work, we have used ab initio based surface hopping dynamics method to study vibrational-mode-dependent S(1) → S(0) internal conversion of cyclopropanone. A new conical intersection between the S(1) and S(0) states is determined by the state-averaged CASSCF/cc-pVDZ calculations, which is confirmed to play a critical role in the ultrafast S(1) → S(0) internal conversion by the nonadiabatic dynamics simulations. It is found that the internal conversion occurs more efficiently when the initial kinetic energies are distributed in the four vibrational modes related to the C═O group, especially in the C-O stretching and the O-C-C-C out-of-plane torsional modes. Meanwhile, the S(1) lifetime and the time scale of the S(1) → S(0) internal conversion are estimated by the ab initio based dynamics simulations, which is consistent with the ultrafast S(1) → S(0) internal conversion and provides further evidence that the ultrafast internal conversion is responsible for the fluorescence disappearance of cyclopropanone.     

Environment-dependent ultrafast photoisomerization dynamics in azo dye

Azoaromatic dyes have been extensively investigated over the past decade due to their potential use in a variety of optical devices that exploit their ultrafast photoisomerization processes. Among the azoaromatic dyes, Disperse Red 19 is a commercially available azobenzene nonlinear optical chromophore with a relatively high ground-state dipole moment. In the present study, we used ultrafast time-resolved spectroscopy to clarify the dynamics of a push-pull substituted azobenzene dye. Solution and film samples exhibited different ultrafast dynamics, indicating that the molecular environment affects the photoisomerization dynamics of the dye.     

Ab initio Ehrenfest dynamics

We present an ab initio direct Ehrenfest dynamics scheme using a three time-step integrator. The three different time steps are implemented with nuclear velocity Verlet, nuclear-position-coupled midpoint Fock integrator, and time-dependent Hartree-Fock with a modified midpoint and unitary transformation algorithm. The computational cost of the ab initio direct Ehrenfest dynamics presented here is found to be only a factor of 2-4 larger than that of Born-Oppenheimer (BO) dynamics. As an example, we compute the vibration of the NaCl molecule and the intramolecular torsional motion of H2C=NH2+ by Ehrenfest dynamics compared with BO dynamics. For the vibration of NaCl with an initial kinetic energy of 1.16 eV, Ehrenfest dynamics converges to BO dynamics with the same vibrational frequency. The intramolecular rotation of H2C=NH2+ produces significant electronic excitation in the Ehrenfest trajectory. The amount of nonadiabaticity, suggested by the amplitude of the coherent progression of the excited and ground electronic states, is observed to be directly related to the strength of the electron-nuclear coupling. Such nonadiabaticity is seen to have a significant effect on the dynamics compared with the adiabatic approximation.     

Matching-pursuit/split-operator Fourier-transform simulations of nonadiabatic quantum dynamics

A rigorous and practical approach for simulations of nonadiabatic quantum dynamics is introduced. The algorithm involves a natural extension of the matching-pursuitsplit-operator Fourier-transform (MPSOFT) method [Y. Wu and V. S. Batista, J. Chem. Phys. 121, 1676 (2004)] recently developed for simulations of adiabatic quantum dynamics in multidimensional systems. The MPSOFT propagation scheme, extended to nonadiabatic dynamics, recursively applies the time-evolution operator as defined by the standard perturbation expansion to first-, or second-order, accuracy. The expansion is implemented in dynamically adaptive coherent-state representations, generated by an approach that combines the matching-pursuit algorithm with a gradient-based optimization method. The accuracy and efficiency of the resulting propagation method are demonstrated as applied to the canonical model systems introduced by Tully for testing simulations of dual curve-crossing nonadiabatic dynamics.     

Spatial confinement alters the ultrafast photoisomerization dynamics of azobenzenes

Ultrafast transient absorption spectroscopy reveals new excited-state dynamics following excitation of trans-azobenzene (t-Az) and several alkyl-substituted t-Az derivatives encapsulated in a water-soluble supramolecular host–guest complex. Encapsulation increases the excited-state lifetimes and alters the yields of the trans → cis photoisomerization reaction compared with solution. Kinetic modeling of the transient spectra for unsubstituted t-Az following nπ* and ππ* excitation reveals steric trapping of excited-state species, as well as an adiabatic excited-state trans → cis isomerization pathway for confined molecules that is not observed in solution. Analysis of the transient spectra following ππ* excitation for a series of 4-alkyl and 4,4′-dialkyl substituted t-Az molecules suggests that additional crowding due to lengthening of the alkyl tails results in deeper trapping of the excited-state species, including distorted trans and cis structures. The variation of the dynamics due to crowding in the confined environment provides new evidence to explain the violation of Kasha''s rule for nπ* and ππ* excitation of azobenzenes based on competition between in-plane inversion and out-of-plane rotation channels.

Ultrafast transient absorption spectroscopy reveals new excited-state dynamics following excitation of trans-azobenzene (t-Az) and several alkyl-substituted t-Az derivatives encapsulated in a water-soluble supramolecular host–guest complex.     

Ab initio photoionization dynamics of beta-alanine

Photoionization dynamics of beta-alanine is studied by the trajectory simulations using the ab initio potential energy surface. Vertical photoionization in the spirit of the Franck-Condon principle is assumed both for the adiabatic and thermostatic simulations. Both intramolecular proton transfer and fragmentation while only the proton transfer are found in the thermostatic and adiabatic simulations, respectively, for the conformer having the intramolecular hydrogen bond N...H-O. The theoretical predictions are in line with the experimental observations available in the literatures. It is reported for the first time that the thermostatic temperatures strongly affect the fragmentation processes induced by photoionization.     

Ab initio potential energy surfaces and nonadiabatic collision dynamics in H(+)+O(2) system

The adiabatic potential energy surfaces for the lowest five electronic states of (3)A" symmetry for the H(+)+O(2) collision system have been obtained at the multireference configuration interaction level of accuracy using Dunning's correlation consistent polarized valence triple zeta basis set. The radial nonadiabatic coupling terms and the mixing angle between the lowest two electronic states (1 (3)A" and 2 (3)A"), which adiabatically correlate in the asymptotic limit to H((2)S)+O(2) (+)(X (2)Pi(g)) and H(+)+O(2)(X (3)Sigma(g)(-)), respectively, have been computed using ab initio procedures at the same level of accuracy to yield the corresponding quasidiabatic potential energy matrix. The computed strengths of the vibrational coupling matrix elements reflect the trend observed for inelastic vibrational excitations of O(2) in the experiments at collision energy of 9.5 eV. The quantum dynamics has been preformed on the newly obtained coupled quasidiabatic potential energy surfaces under the vibrational close-coupling rotational infinite-order sudden framework at the experimental collision energy of 9.5 eV. The present theoretical results for vibrational elastic/inelastic excitations of O(2) are in overall good agreement with the available experimental data obtained from the proton energy-loss spectra in molecular beam experiments [F. A. Gianturco et al., J. Phys. B 14, 667 (1981)]. The results for the complementary charge transfer processes are also presented at this collision energy.     

The photoisomerization mechanism of azobenzene: a semiclassical simulation of nonadiabatic dynamics

We have simulated the photoisomerization dynamics of azobenzene, taking into account internal conversion and geometrical relaxation processes, by means of a semiclassical surface hopping approach. Both n-->pi* and pi-->pi* excitations and both cis-->trans and trans-->cis conversions have been considered. We show that in all cases the torsion around the N==N double bond is the preferred mechanism. The quantum yields measured are correctly reproduced and the observed differences are explained as a result of the competition between the inertia of the torsional motion and the premature deactivation of the excited state. Recent time-resolved spectroscopic experiments are interpreted in the light of the simulated dynamics.     

Ab initio mass tensor molecular dynamics

Mass tensor molecular dynamics method was first introduced by Bennett [J. Comput. Phys. 19, 267 (1975)] for efficient sampling of phase space through the use of generalized atomic masses. Here, we show how to apply this method to ab initio molecular dynamics simulations with minimal computational overhead. Test calculations on liquid water show a threefold reduction in computational effort without making the fixed geometry approximation. We also present a simple recipe for estimating the optimal atomic masses using only the first derivatives of the potential energy.     

Ab initio dynamics study of poly-para-phenylene vinylene

We present an ab initio dynamics investigation within a density-functional perturbation theory framework of the properties of the conjugated polymer poly-para-phenylene vinylene (PPV) in both the isolated chain and crystalline states. The calculated results show that for an isolated chain, most of the vibrational modes correspond to experimentally observed modes in crystalline PPV. However, additional hitherto unidentified modes have been observed in experiment and our calculations on crystalline material have allowed us to assign these. We also present the results of calculations of the polarizability and permittivity tensors of the material, which are in reasonable agreement with the typical values for conjugated polymers. Dynamical Born effective charges [S. Baroni, S. de Gironcoli, A. Dal Corso, and P. Giannozzi, Rev. Mod. Phys. 73, 515 (2001)] are calculated and compared with atomic charges obtained from Mulliken population analysis [M. D. Segall, C. J. Pickard, R. Shah, and M. C. Payne, Mol. Phys. 89, 571 (1996)] and we conclude that effective charges are more appropriate for use in the study of the dynamics of the system. Notable differences are found in the infrared-absorption spectra obtained for the isolated chain and crystalline states, which can be attributed to the differences in the crystalline packing effects, which clearly play a key role in influencing the lattice dynamics of PPV.     

Ab initio trajectory surface-hopping study on ultrafast deactivation process of thiophene

The ultrafast S(1)((1)ππ*) → S(0) deactivation process of thiophene in the gas phase has been simulated with the complete active space self-consistent field (CASSCF) based fewest switch surface hopping method. It was found that most of the calculated trajectories (～80%) decay to the ground state (S(0)) with an averaged time constant of 65 ± 5 fs. This is in good agreement with the experimental value of about 80 fs. Two conical intersections were determined to be responsible for the ultrafast S(1)((1)ππ*) → S(0) internal conversion process. After thiophene is excited to the S(1)((1)ππ*) state in the Franck-Condon region, it quickly relaxes to the minimum of the S(1)((1)ππ*) state, then overcomes a small barrier near the conical intersection (CI((1)ππ*/(1)πσ*)), and eventually arrives at the minimum of one C-S bond fission (S(1)((1)πσ*)). In the vicinity of this minimum, the conical intersection (CI((1)πσ*/S(0))) funnels the electron population to the ground state (S(0)), completing the ultrafast S(1)((1)ππ*) → S(0) internal conversion process. This decay mechanism matches well with previous experimental and theoretical studies.     

Strong nonadiabatic effects in reactions of photoisomerization

Reactions of photoisomerization proceeding through a “funnel” are discussed (Fig. 1a, θ ? θ*, where θ is the angle of isomerization). Strong nonadiabatic interactions in the region of conical intersection of the multidimensional adiabatic potentials U_s and U_s0 are supposed to be responsible for the ultrafast nonradiative transition S → K₂ S₀ (K₂ ? 10¹⁰-10¹² s^?1). The K₂ dependence on the solvent viscosity (isomerization of t-stilbene in series ethanol--octanol) and polarity (isomerization of cyanine dyes in polar solvents) was determined to be in a good agreement with experiments.     

Ab initio studies of ultrafast x-ray scattering of the photodissociation of iodine

We computationally examine various aspects of the reaction dynamics of the photodissociation and recombination of molecular iodine. We use our recently proposed formalism to calculate time-dependent x-ray scattering signal changes from first principles. Different aspects of the dynamics of this prototypical reaction are studied, such as coherent and noncoherent processes, features of structural relaxation that are periodic in time versus nonperiodic dissociative processes, as well as small electron density changes caused by electronic excitation, all with respect to x-ray scattering. We can demonstrate that wide-angle x-ray scattering offers a possibility to study the changes in electron densities in nonperiodic systems, which render it a suitable technique for the investigation of chemical reactions from a structural dynamics point of view.