### Concept: Conical intersection

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##### Quasi-diabatic representations of adiabatic potential energy surfaces coupled by conical intersections including bond breaking: A more general construction procedure and an analysis of the diabatic representation

- The Journal of chemical physics
- Published about 7 years ago
- Discuss

The analytic representation of adiabatic potential energy surfaces and their nonadiabatic interactions is a key component of accurate, fully quantum mechanical descriptions of nonadiabatic dynamics. In this work, we describe extensions of a promising method for representing the nuclear coordinate dependence of the energies, energy gradients, and derivative couplings of N(state) adiabatic electronic states coupled by conical intersections. The description is based on a vibronic coupling model and can describe multichannel dissociation. An important feature of this approach is that it incorporates information about the geometry dependent interstate derivative couplings into the fitting procedure so that the resulting representation is quantifiably quasi diabatic and quasi diabatic in a least squares sense. The reported extensions improve both the rate of convergence and the converged results and will permit the optimization of nonlinear parameters including those parameters that govern the placement of the functions used to describe multichannel dissociation. Numerical results for a coupled quasi-diabatic state representation of the photodissociation process NH(3)+hv → NH(2)+H illustrate the potential of the improved algorithm. A second focus in this numerical example is the quasi-diabatic character of the representation which is described and analyzed. Special attention is paid to the immediate vicinity of the conical intersection seam.

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##### Pyridinylidene-phenoxide in strong electric fields: controlling orientation, conical intersection, and radiation-less decay.

- The journal of physical chemistry. A
- Published over 7 years ago
- Discuss

Strong electric fields open new routes for the control of radiation-less decay in molecules with conical intersections. Here, we present quantum chemical and quantum dynamical simulations which demonstrate that the radiation-less decay and related photoisomerization of pyridinylidene-phenoxide can be effectively manipulated with strong electric fields by shifting the conical intersection. Moreover, we show the effects of the electric field on the orientation of the molecules and on the photoexcitation and discuss the conditions for which the field induced coupling between rotational and vibronic states can be neglected.

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##### Topological origins of bound states in the continuum for systems with conical intersections

- The journal of physical chemistry letters
- Published about 2 years ago
- Discuss

Bound states in the continuum (BSCs) were reported in a linear vibronic coupling model with a conical intersection (CI) [Cederbaum et al. Phys. Rev. Lett. 90, 013001 (2003)]. It was also found that these states are destroyed within the Born-Oppenheimer approximation (BOA). We investigate whether a nontrivial topological or geometric phase (GP) associated with the CI is responsible for BSCs. To address this question we explore modifications of the original two-dimensional two-state linear vibronic coupling model supporting BSCs. These modifications either add GP effects after the BOA or remove the GP within a two-state problem. Using the stabilization graph technique we shown that the GP is crucial for emergence of BSCs.

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##### An alternative laser driven photodissociation mechanism of pyrrole via πσ*1∕S0 conical intersection

- The Journal of chemical physics
- Published over 2 years ago
- Discuss

A first principles quantum dynamics study of N-H photodissociation of pyrrole on the S0-(1)πσ(*)(A21) coupled electronic states is carried out with the aid of an optimally designed UV-laser pulse. A new photodissociation path, as compared to the conventional barrier crossing on the πσ*1 state, opens up upon electronic transitions under the influence of pump-dump laser pulses, which efficiently populate both the dissociation channels. The interplay of electronic transitions due both to vibronic coupling and the laser pulse is observed in the control mechanism and discussed in detail. The proposed control mechanism seems to be robust, and not discussed in the literature so far, and is expected to trigger future experiments on the πσ*1 photochemistry of molecules of chemical and biological importance. The design of the optimal pulses and their application to enhance the overall dissociation probability is carried out within the framework of optimal control theory. The quantum dynamics of the system in the presence of pulse is treated by solving the time-dependent Schrödinger equation in the semi-classical dipole approximation.

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##### Non-Condon equilibrium Fermi’s golden rule electronic transition rate constants via the linearized semiclassical method

- The Journal of chemical physics
- Published over 3 years ago
- Discuss

In this paper, we test the accuracy of the linearized semiclassical (LSC) expression for the equilibrium Fermi’s golden rule rate constant for electronic transitions in the presence of non-Condon effects. We do so by performing a comparison with the exact quantum-mechanical result for a model where the donor and acceptor potential energy surfaces are parabolic and identical except for shifts in the equilibrium energy and geometry, and the coupling between them is linear in the nuclear coordinates. Since non-Condon effects may or may not give rise to conical intersections, both possibilities are examined by considering: (1) A modified Garg-Onuchic-Ambegaokar model for charge transfer in the condensed phase, where the donor-acceptor coupling is linear in the primary mode coordinate, and for which non-Condon effects do not give rise to a conical intersection; (2) the linear vibronic coupling model for electronic transitions in gas phase molecules, where non-Condon effects give rise to conical intersections. We also present a comprehensive comparison between the linearized semiclassical expression and a progression of more approximate expressions. The comparison is performed over a wide range of frictions and temperatures for model (1) and over a wide range of temperatures for model (2). The linearized semiclassical method is found to reproduce the exact quantum-mechanical result remarkably well for both models over the entire range of parameters under consideration. In contrast, more approximate expressions are observed to deviate considerably from the exact result in some regions of parameter space.

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##### Combined theoretical and experimental study of the valence, Rydberg and ionic states of fluorobenzene

- The Journal of chemical physics
- Published over 3 years ago
- Discuss

New photoelectron spectra (PES) and ultra violet (UV) and vacuum UV (VUV) absorption spectra of fluorobenzene recorded at higher resolution than previously, have been combined with mass-resolved (2 + 1) and (3 + 1) resonance enhanced multiphoton ionization (REMPI) spectra; this has led to the identification of numerous Rydberg states. The PES have been compared with earlier mass-analyzed threshold ionization and photoinduced Rydberg ionization (PIRI) spectra to give an overall picture of the ionic state sequence. The analysis of these spectra using both equations of motion with coupled cluster singles and doubles (EOM-CCSD) configuration interaction and time dependent density functional theory (TDDFT) calculations have been combined with vibrational analysis of both the hot and cold bands of the spectra, in considerable detail. The results extend several earlier studies on the vibronic coupling leading to conical intersections between the X(2)B1 and A(2)A2 states, and a further trio (B, C, and D) of states. The conical intersection of the X and A states has been explicitly identified, and its structure and energetics evaluated. The energy sequence of the last group is only acceptable to the present study if given as B(2)B2

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##### Simulation of femtosecond two-dimensional electronic spectra of conical intersections

- The Journal of chemical physics
- Published over 4 years ago
- Discuss

We have simulated femtosecond two-dimensional (2D) electronic spectra for an excited-state conical intersection using the wave-function version of the equation-of-motion phase-matching approach. We show that 2D spectra at fixed values of the waiting time provide information on the structure of the vibronic eigenstates of the conical intersection, while the evolution of the spectra with the waiting time reveals predominantly ground-state wave-packet dynamics. The results show that 2D spectra of conical intersection systems differ significantly from those obtained for chromophores with well separated excited-state potential-energy surfaces. The spectral signatures which can be attributed to conical intersections are discussed.

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##### Photoelectron Spectrum and Dynamics of the Uracil Cation

- The journal of physical chemistry. A
- Published about 5 years ago
- Discuss

The photoelectron spectrum of uracil and the molecular dynamics of its radical cation are investigated using the multiconfigurational time-dependent Hartree (MCTDH) method. For this aim, the vibronic coupling model Hamiltonian is used including up to ten important a' modes. Moreover, to account for coupling through conical intersections between states of different symmetry in the system, coupling constants of two a'‘ modes are taken into account. The parameters used in the model are obtained by fitting to ab inito data obtained with extensive EOM-IP-CCSD calculations. The first four cationic states were investigated, which are either of A’‘ (hole in a π orbital) or A’ hole in a nO orbital) symmetry. The results of the wavepacket propagations were used to calculate the corresponding photoelectron spectrum and compare to the experimental spectrum. The MCTDH simulations reproduce the experimental spectrum well. The dynamics starting from the D2 and D3 ionic states show a fast relaxation to the cationic ground state often involving direct D2 -D0 or D3 -D1 transitions.

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##### A Conformer-Resolved Quantum Dynamics Study of the Photodissociation of 3-Pyrroline

- The journal of physical chemistry. A
- Published over 5 years ago
- Discuss

A model Hamiltonian based on the vibronic coupling model is developed to describe the excited state dynamics of 3-pyrroline. Using the method of improved relaxation in conjunction with the MCTDH wavepacket propagation algorithm, vibrational eigenstates corresponding to both the axial and equatorial conformers of 3-pyrroline are calculated and subsequently used in a conformer-resolved study of the photodissociation of 3-pyrroline following excitation to its S_1 (3s/pi sigma^*) and S_2(3p_x) states. In analogy with ammonia, the excited state dynamics of both conformers of 3-pyrroline are found to be dominated by the (quasi-) planarisation of the molecule in its electronically excited states and predominantly diabatic behaviour of dissociation mediated by a conical intersection between the S_1 and S_0 states.

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##### A perturbative formalism for electronic transitions through conical intersections in a fully quadratic vibronic model

- The Journal of chemical physics
- Published over 5 years ago
- Discuss

We consider a fully quadratic vibronic model Hamiltonian for studying photoinduced electronic transitions through conical intersections. Using a second order perturbative approximation for diabatic couplings, we derive an analytical expression for the time evolution of electronic populations at a given temperature. This formalism extends upon a previously developed perturbative technique for a linear vibronic coupling Hamiltonian. The advantage of the quadratic model Hamiltonian is that it allows one to use separate quadratic representations for potential energy surfaces of different electronic states and a more flexible representation of interstate couplings. We explore features introduced by the quadratic Hamiltonian in a series of 2D models, and then apply our formalism to the 2,6-bis(methylene) adamantyl cation and its dimethyl derivative. The Hamiltonian parameters for the molecular systems have been obtained from electronic structure calculations followed by a diabatization procedure. The evolution of electronic populations in the molecular systems using the perturbative formalism shows a good agreement with that from variational quantum dynamics.