Nonadiabatic photodissociation dynamics

The photodissociation dynamics of the triatomic (or pseudo‐triatomic) system in the nonadiabatic multiple electronic states is investigated by employing a time‐dependent quantum wave packet method, while the time propagation of the wave packet is carried out using the split‐operator scheme. As a numerical example, the photodissociation dynamics of CH₃I in three electronic states ¹Q₁(A′), ¹Q₁(A″), and ³Q₀₊ is studied and CH₃I is treated as a pseudotriatomic model. The absorption spectra and product vibrational state distributions are calculated and compared with previous theoretical work. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Nonadiabatic decay dynamics of a benzylidene malononitrile

The photoinduced nonadiabatic decay dynamics of 2-[4-(dimethylamino)benzylidene]malononitrile (DMN) in the gas phase is investigated at the semiempirical OM2/MRCI level using surface hopping simulations. A lifetime of 1.2 ps is predicted for the S(1) state, in accordance with experimental observation. The dominant reaction coordinate is found to be the twisting around the C7═C8 double bond accompanied by pronounced pyramidalization at the C8 atom. Motion along this coordinate leads to the lowest-energy conical intersection (CI(01α)). Several other S(0)/S(1) conical intersections have also been located by full optimization but play no role in the dynamics. The time-resolved fluorescence spectrum of DMN is simulated by computing emission energies and oscillator strengths along the trajectories. It compares well with the experimental spectrum. The use of different active spaces in the OM2/MRCI calculations yields similar results and thus demonstrates their internal consistency.     

Nonadiabatic quantum dynamics based on a hierarchical electron-phonon model: exciton dissociation in semiconducting polymers

A hierarchical electron-phonon coupling model is applied to describe the ultrafast decay of a photogenerated exciton at a donor-acceptor polymer heterojunction, via a vibronic coupling mechanism by which a charge-localized interfacial state is created. Expanding upon an earlier Communication [H. Tamura et al., J. Chem. Phys. 126, 021103 (2007)], we present a quantum dynamical analysis based on a two-state linear vibronic coupling model, which accounts for a two-band phonon bath including high-frequency C[Double Bond]C stretch modes and low-frequency ring torsional modes. Building upon this model, an analysis in terms of a hierarchical chain of effective modes is carried out, whose construction is detailed in the present paper. Truncation of this chain at the order n (i.e., 3n+3 modes) conserves the Hamiltonian moments (cumulants) up to the (2n+3)rd order. The effective-mode analysis highlights (i) the dominance of the high-frequency modes in the coupling to the electronic subsystem and (ii) the key role of the low-frequency modes in the intramolecular vibrational redistribution process that is essential in mediating the decay to the charge-localized state. Due to this dynamical interplay, the effective-mode hierarchy has to be carried beyond the first order in order to obtain a qualitatively correct picture of the nonadiabatic process. A reduced model of the dynamics, including a Markovian closure of the hierarchy, is presented. Dynamical calculations were carried out using the multiconfiguration time-dependent Hartree method.     

Nonadiabatic excited-state dynamics of polar pi-systems and related model compounds of biological relevance

Multireference ab initio dynamics simulations have become available as a tool for the investigation of photochemical processes, mainly for those related to nonadiabatic phenomena taking place in the sub-picosecond time scale. For organic molecules, these phenomena are in many cases deeply dependent on the relaxation of the photoexcited pi-system. We review the latest contributions of our group to this subject and report new results for systems studied previously, grouping them in single pi bonds, chains and aromatic rings. The dynamics of ethylene and substituted ethylenes is discussed mainly in connection to the competition between the two available relaxation paths in the excited states and their relation to the conical intersections in large systems. The trans-cis and the cis-trans dynamics of the pentadieniminium cation is investigated as well. Finally, we discuss the photodynamics of aminopyrimidine starting in the S1 and S2 states and the conclusions, which can be drawn from this for the interpretation of the adenine dynamics.     

Nonadiabatic dynamics in multidimensional complex potential energy surfaces

Despite the continuous development of theoretical methodologies for describing nonadiabatic dynamics of molecular systems, there is a lack of approaches for processes where the norm of the wave function is not conserved, i.e., when an imaginary potential accounts for some irreversible decaying mechanism. Current approaches rely on building potential energy surfaces of reduced dimensionality, which is not optimal for more involving and realistic multidimensional problems. Here, we present a novel methodology for describing the dynamics of complex-valued molecular Hamiltonians, which is a generalisation of the trajectory surface hopping method. As a first application, the complex surface fewest switches surface hopping (CS-FSSH) method was employed to survey the relaxation mechanisms of the shape resonant anions of iodoethene. We have provided the first detailed and dynamical picture of the π*/σ* mechanism of dissociative electron attachment in halogenated unsaturated compounds, which is believed to underlie electron-induced reactions of several molecules of interest. Electron capture into the π* orbital promotes C C stretching and out-of-plane vibrations, followed by charge transfer from the double bond into the σ* orbital at the C–I bond, and, finally, release of the iodine ion, all within only 15 fs. On-the-fly dynamics simulations of a vast class of processes can be envisioned with the CS-FSSH methodology, including autoionisation from transient anions, core-ionised and superexcited states, Auger and interatomic coulombic decay, and time-dependent luminescence.

Despite the continuous development of methods for describing nonadiabatic dynamics, there is a lack of multidimensional approaches for processes where the wave function norm is not conserved. A new surface hopping variant closes this knowledge gap.     

Nonadiabatic dynamics of charge transfer in diatomic anion clusters

We have studied the photodissociation and recombination dynamics of the diatomic anions X(2)(-) and XY(-) designed to mimic I(2)(-) and ICl(-), respectively, by using a one-electron model in size-selected N(2)O clusters. The one-electron model is composed of two nuclei and an extra electron moving in a two-dimensional plane including the two nuclei. The main purpose of this study is to explain the salient features of various dynamical processes of molecular ions in clusters using a simple theoretical model. For heteronuclear diatomic anions, a mass disparity and asymmetric electron affinity between the X and Y atoms lead to different phenomena from the homonuclear case. The XY(-) anion shows efficient recombination for a smaller cluster size due to the effect of collision-mediated energy transfer and an inherent potential wall on excited state at asymptotic region, while the recombination for the X(2)(-) anion is due to rearrangement of solvent configuration and faster nonadiabatic transitions. The results of the present study illustrate the microscopic details of the electronically nonadiabatic processes which control the photodissociation dynamics of molecular ions in clusters.     

Nonadiabatic decay dynamics of 9H-guanine in aqueous solution

The nonadiabatic decay of the biologically relevant guanine tautomer (9H-guanine) in aqueous solution has been investigated by trajectory surface hopping simulations in a quantum mechanical-molecular mechanical (QM-MM) framework. The QM part (9H-guanine) was treated at the semiempirical OM2/MRCI level, while the MM part (water) was described by the TIP3P force field. The optimized geometries for the relevant minima and conical intersections are qualitatively similar for 9H-guanine in the gas phase and in aqueous solution, while there are notable solvent-induced shifts in the computed vertical excitation energies (up to about 0.4 eV). Overall, the results from the static OM2/MRCI-based calculations are in reasonable agreement with the available ab initio and experimental data. The dynamics simulations show ultrafast nonradiative decay for 9H-guanine in water that is even slightly faster than in the gas phase, with time constants of 20 fs and around 0.3 ps for the S(2)→ S(1) and S(1)→ S(0) internal conversions, respectively. They predict a change in the S(1)→ S(0) decay mechanism when going from the gas phase to aqueous solution: the major pathway for 9H-guanine in water involves a conical intersection with an out-of-plane distortion of the carbonyl oxygen atom, which does not play any significant role in the gas phase, where the decay mainly proceeds via two other conical intersections characterized by ring distortions and out-of-plane displacement of the amino group, respectively. Possible reasons for this change in the mechanism are analyzed.     

Nonadiabatic Ehrenfest molecular dynamics within the projector augmented-wave method

We derive equations for nonadiabatic Ehrenfest molecular dynamics within the projector augmented-wave (PAW) formalism. The discretization of the electrons is time-dependent as the augmentation functions depend on the positions of the nuclei. We describe the implementation of the Ehrenfest molecular dynamics equations within the real-space PAW method. We demonstrate the applicability of our method by studying the vibration of NaCl, the torsional rotation of H(2)C=NH(2)(+) in both the adiabatic and the nonadiabatic regimes, and the hydrogen bombardment of C(40)H(16).     

Nonadiabatic deactivation of 9H-adenine: a comprehensive picture based on mixed quantum-classical dynamics

Mixed quantum-classical dynamics simulations at the multireference configuration interaction (MR-CIS) level were performed for 9 H-adenine in order to understand its ultrafast nonradiative decay process. Dynamics simulations were also performed for the model system 6-aminopyrimidine. MR-CIS and complete active space perturbation theory (CASPT2) have been employed to characterize a large variety of qualitatively different conical intersections, the branches of the crossing seam connecting them, and the reaction paths from the Franck-Condon region for 9 H-adenine. The results show a two-step process consisting of ultrashort deactivation from S 3 to S 1 and a longer exponential decay step corresponding to the conversion from S 1 to S 0.     

Nonadiabatic effects on peptide vibrational dynamics induced by conformational changes

Quantum dynamical simulations of vibrational spectroscopy have been carried out for glycine dipeptide (CH(3)-CO-NH-CH(2)-CO-NH-CH(3)). Conformational structure and dynamics are modeled in terms of the two Ramachandran dihedral angles of the molecular backbone. Potential energy surfaces and harmonic frequencies are obtained from electronic structure calculations at the density functional theory (DFT) [B3LYP/6-31+G(d)] level. The ordering of the energetically most stable isomers (C(7) and C(5)) is reversed upon inclusion of the quantum mechanical zero point vibrational energy. Vibrational spectra of various isomers show distinct differences, mainly in the region of the amide modes, thereby relating conformational structures and vibrational spectra. Conformational dynamics is modeled by propagation of quantum mechanical wave packets. Assuming a directed energy transfer to the torsional degrees of freedom, transitions between the C(7) and C(5) minimum energy structures occur on a sub-picosecond time scale (700...800 fs). Vibrationally nonadiabatic effects are investigated for the case of the coupled, fundamentally excited amide I states. Using a two state-two mode model, the resulting wave packet dynamics is found to be strongly nonadiabatic due to the presence of a seam of the two potential energy surfaces. Initially prepared adiabatic vibrational states decay upon conformational change on a time scale of 200...500 fs with population transfer of more than 50% between the coupled amide I states. Also the vibrational energy transport between localized (excitonic) amide I vibrational states is strongly influenced by torsional dynamics of the molecular backbone where both enhanced and reduced decay rates are found. All these observations should allow the detection of conformational changes by means of time-dependent vibrational spectroscopy.     

Nonadiabatic dynamics of uracil: population split among different decay mechanisms

Nonadiabatic dynamics simulations performed at the state-averaged CASSCF method are reported for uracil. Supporting calculations on stationary points and minima on the crossing seams have been performed at the MR-CISD and CASPT2 levels. The dominant mechanism is characterized by relaxation into the S(2) minimum of ππ* character followed by the relaxation to the S(1) minimum of nπ* character. This mechanism contributes to the slower relaxation with a decay constant larger than 1.5 ps, in good agreement with the long time constants experimentally observed. A minor fraction of trajectories decay to the ground state with a time constant of about 0.7 ps, which should be compared to the experimentally observed short constant. The major part of trajectories decaying with this time constant follows the ππ* channel and hops to the ground state via an ethylenic conical intersection. A contribution of the relaxation proceeding via a ring-opening conical intersection was also observed. The existence of these two latter channels together with a reduced long time constant is responsible for a significantly shorter lifetime of uracil compared to that of thymine.     

Nonadiabatic interactions in wave packet dynamics of the bromoacetyl chloride photodissociation

The competitive photodissociation of bromoacetyl chloride BrCH2COCl in the first 1A" state (S1) by 248 nm photons is investigated by nonadiabatic wave packet simulations. We show that the preferential breaking of the stronger C-Cl bond (alpha to the excited carbonyl) over the weaker C-Br bond (beta) could be explained by a diabatic trapping or nonadiabatic recrossing as previously proposed. Our energy resolved flux analysis agrees fairly well with the experimental branching ratio (C-Cl:C-Br=1.0:0.4). Even if this does not prove the mechanism, this at least prevents to discard it. A reduced dimensionality approach based on constrained Hamiltonian is used. The nonadiabatic dissociation is studied in the two C-O/C-X (X=Br, Cl) subspaces to emphasize the role of the C-O vibration upon [nO-->piCO*] excitation. The internal torsion and wagging dihedral angles are frozen at their Franck-Condon value, according to preliminary dynamical tests. The other inactive coordinates are optimized at the trans and Cs constrained geometry in the first excited state. Corresponding 2D cuts in the potential energy surfaces have been computed at the CASSCF level. The nonadiabatic kinetic couplings are highly peaked along an avoided crossing seam in both cases. A two-state diabatic model with a constant potential coupling is proposed in the two C-O/C-X subspaces. The inclusion of the C-O stretching in the active coordinates improves the value of the branching ratio over our previous 1D computation.     

Nonadiabatic excited-state molecular dynamics: numerical tests of convergence and parameters

Nonadiabatic molecular dynamics simulations, involving multiple Born-Oppenheimer potential energy surfaces, often require a large number of independent trajectories in order to achieve the desired convergence of the results, and simulation relies on different parameters that should be tested and compared. In addition to influencing the speed of the simulation, the chosen parameters combined with the frequently reduced number of trajectories can sometimes lead to unanticipated changes in the accuracy of the simulated dynamics. We have previously developed a nonadiabatic excited state molecular dynamics methodology employing Tully's fewest switches surface hopping algorithm. In this study, we seek to investigate the impact of the number of trajectories and the various parameters on the simulation of the photoinduced dynamics of distyrylbenzene (a small oligomer of polyphenylene vinylene) within our developed framework. Various user-defined parameters are analyzed: classical and quantum integration time steps, the value of the friction coefficient for Langevin dynamics, and the initial seed used for stochastic thermostat and hopping algorithms. Common approximations such as reduced number of nonadiabatic coupling terms and the classical path approximation are also investigated. Our analysis shows that, at least for the considered molecular system, a minimum of ~400 independent trajectories should be calculated in order to achieve statistical averaging necessary for convergence of the calculated relaxation timescales.     

Nonadiabatic transition rates in a random motion model. Near adiabatic limit

The near adiabatic limit for nonadiabatic electronic transitions in condensed media is considered. The motion of a classical subsystem is approximated by a one-dimensional stochastic gaussian process. An analytical expression for the near adiabatic transition rate W is obtained in the limits: λ <a and λ ? a, where λ is the step size of classical stochastic motion and a is the size of the nonadiabaticity region. An approx interpolating expression for the intermediate case λ ≈ a is proposed. It is shown that W decreases rapidly with the increase of adiabatic s Application of the obtained expressions to the electron transfer processes in polar liquids is discussed briefly.     

Nonadiabatic electron dynamics in the exit channel of Na-molecule optical collisions

We study optical collisions of Na atoms with N(2), CO, C(2)H(2), and CO(2) molecules in a crossed-beam experiment. Excited electronic states of the collision complex are selectively populated during the collision. We measure the relative population of the Na(3p) fine-structure levels after the collision and observe in this way the nonadiabatic transitions occuring in the final phase of the collision process. For the NaCO, NaC(2)H(2), and NaCO(2) systems new ab initio potential surfaces were generated. The theoretical analysis of the nonadiabatic electron dynamics on the excited potential surfaces is made within the classical-path formalism. The results are in good qualitative agreement with the experimental data and provide insight into the nonadiabatic mechanisms prevailing during the evolution in the upper 3p manifold. The differences between the different collisional systems are related to the presence and system-specific locations of conical intersections and avoided crossing seams in the excited potential surfaces.     

Nonadiabatic photodissociation dynamics in (HI)2 induced by intracluster collisions

The sequential photodissociation dynamics of (HI)2 is studied by means of a nonadiabatic wave packet treatment starting from the I*-HI complex. The model reproduces the main experimental findings for photolysis with 266 nm radiation. The results confirm that some of the H atoms dissociated from the I*-HI complex deactivate the I* atom through a HI* intracluster collision which induces an I*-->I electronically nonadiabatic transition. As a consequence, these H fragments become very fast by acquiring nearly all the I* excitation energy, equivalent to the I*I spin-orbit splitting. A most interesting result is the high production of bound I2 fragments in highly excited rovibrational states in the photolysis, indicating that the H dissociation is mainly direct.     

Nonadiabatic molecular dynamics of photoexcited Li2(+) Ne(n) clusters

We investigate the relaxation of photoexcited Li(2)(+) chromophores solvated in Ne(n) clusters (n = 2-22) by means of molecular dynamics with surface hopping. The simplicity of the electronic structure of these ideal systems is exploited to design an accurate and computationally efficient model. These systems present two series of conical intersections between the states correlated with the Li+Li(2s) and Li+Li(2p) dissociation limits of the Li(2)(+) molecule. Frank-Condon transition from the ground state to one of the three lowest excited states, hereafter indexed by ascending energy from 1 to 3, quickly drives the system toward the first series of conical intersections, which have a tremendous influence on the issue of the dynamics. The states 1 and 2, which originate in the Frank-Condon area from the degenerated nondissociative 1(2)Π(u) states of the bare Li(2)(+) molecule, relax mainly to Li+Li(2s) with a complete atomization of the clusters in the whole range of size n investigated here. The third state, which originates in the Frank-Condon area from the dissociative 1(2)Σ(u)(+) state of the bare Li(2)(+) molecule, exhibits a richer relaxation dynamics. Contrary to intuition, excitation into state 3 leads to less molecular dissociation, though the amount of energy deposited in the cluster by the excitation process is larger than for excitation into state 1 and 2. This extra amount of energy allows the system to reach the second series of conical intersections so that approximately 20% of the clusters are stabilized in the 2(2)Σ(g)(+) state potential well for cluster sizes n larger than 6.     

Nonadiabatic relaxation dynamics of water anion cluster and its isotope effects by ring‐polymer molecular dynamics simulation

We have applied a recently developed hybrid quantum ring‐polymer molecular dynamics method to the nonadiabatic p → s relaxation dynamics in water anion clusters to understand the isotope effects observed in previous experiments. The average relaxation times for (H₂O)₅₀^? and (D₂O)₅₀^? were calculated at 120 and 207 fs, respectively, and are comparable to the experimental results. Therefore, we conclude that nuclear quantum effects play an essential role in understanding the observed isotope effects for water anion cluster nonadiabatic dynamics. The nonadiabatic relaxation mechanisms are also discussed in detail. © 2014 Wiley Periodicals, Inc.