Stephane Guerin - Academia.edu (original) (raw)

Papers by Stephane Guerin

The Journal of Chemical Physics, 2019

We present a model for the lowest two potential energy surfaces (PESs) that describe the photoind... more We present a model for the lowest two potential energy surfaces (PESs) that describe the photoinduced ring-opening reaction of benzopyran taken as a model compound to study the photochromic ring-opening reaction of indolinobenzospiropyran and its evolution toward its open-chain analog. The PESs are expressed in terms of three effective rectilinear coordinates. One corresponds to the direction between the equilibrium geometry in the electronic ground state, referred to as the Franck-Condon geometry, and the minimum of conical intersection (CI), while the other two span the two-dimensional branching space at the CI. The model correctly reproduces the topography of the PESs. The ab initio calculations are performed with the extended multiconfiguration quasidegenerate perturbation theory at second order method. We demonstrate that accounting for electron dynamic correlation drastically changes the global energy landscape since some zwitterionic states become strongly stabilized. Quantum...

Physical chemistry chemical physics : PCCP, Jan 18, 2015

We investigate the radiationless decay of photoexcited pyrazine to its ground electronic state us... more We investigate the radiationless decay of photoexcited pyrazine to its ground electronic state using multireference electronic structure and quantum dynamics calculations. We construct a quadratic vibronic coupling Hamiltonian, including the four lowest electronic states and ten vibrational modes, by fitting to more than 5000 ab initio points. We then use this model to simulate the non-adiabatic excited state dynamics of the molecule using the multi-configuration time-dependent Hartree method. On the basis of these calculations, we propose a new mechanism for this decay process involving a conical intersection between the Au(nπ*) state and the ground state. After excitation to the B2u(ππ*) state, the molecule decays to both the B3u(nπ*) and Au(nπ*) states on an ultrashort timescale of approximately 20 fs. The radiationless decay to the ground state then occurs from the Au(nπ*) state on a much longer timescale.

Physical Chemistry Chemical Physics, 2015

Femtosecond studies of electronic relaxation in aniline reveal evidence for a 3-state conical int... more Femtosecond studies of electronic relaxation in aniline reveal evidence for a 3-state conical intersection and show that tunnelling is unimportant.

The Journal of chemical physics, Jan 7, 2014

We present a full quantum-mechanical study of the laser control of the radiationless decay betwee... more We present a full quantum-mechanical study of the laser control of the radiationless decay between the B3u(nπ(*)) and B2u(ππ(*)) states of pyrazine using the dynamic Stark effect. In contrast to our previous study [Sala et al., J. Chem. Phys. 140, 194309 (2014)], where a four-dimensional model was used, all the 24 degrees of freedom are now included in order to test the robustness of the strategy of control. Using a vibronic coupling Hamiltonian model in a diabatic representation, the multi-layer version of the multi-configuration time-dependent Hartree method is exploited to propagate the corresponding wave packets. We still observe a trapping of the wavepacket on the B2u(ππ(*)) potential energy surface due to the Stark effect for a longer time than the "non-resonant field-free" B2u(ππ(*)) lifetime.

Physical chemistry chemical physics : PCCP, Jan 14, 2014

The excited state dynamics of pyrazine has attracted considerable attention in the last three dec... more The excited state dynamics of pyrazine has attracted considerable attention in the last three decades. It has long been recognized that after UV excitation, the dynamics of the molecule is impacted by strong non-adiabatic effects due to the existence of a conical intersection between the B2u(ππ*) and B3u(nπ*) electronic states. However, a recent study based on trajectory surface hopping dynamics simulations suggested the participation of the Au(nπ*) and B2g(nπ*) low-lying dark electronic states in the ultrafast radiationless decay of the molecule after excitation to the B2u(ππ*) state. The purpose of this work was to pursue the investigation of the role of the Au(nπ*) and B2g(nπ*) states in the photophysics of pyrazine. A linear vibronic coupling model hamiltonian including the four lowest excited electronic states and the sixteen most relevant vibrational degrees of freedom was constructed using high level XMCQDPT2 electronic structure calculations. Wavepacket propagations using th...

The Journal of chemical physics, Jan 21, 2014

The laser control of the radiationless decay between the B(3u)(nπ*) and B(2u)(ππ*) states of pyra... more The laser control of the radiationless decay between the B(3u)(nπ*) and B(2u)(ππ*) states of pyrazine using the dynamic Stark effect has been investigated. A vibronic coupling model Hamiltonian in diabatic representation, including potential energy, transition dipole, and static polarizability surfaces as a function of the four most important vibrational modes of the molecule has been parametrized using multi-reference electronic structure calculations. The interaction of the molecule with a strong non-resonant laser pulse has been analyzed in terms of dressed potential energy surfaces. Because of the large polarizability difference between the vibronically coupled B(3u)(nπ*) and B(2u)(ππ*) states, the Stark effect induced by the non-resonant laser pulse shifts the conical intersection away from the Franck-Condon region. We have shown, by solving the time-dependent Schrödinger equation for the molecule interacting with a relatively weak pump pulse driving the electronic excitation f...

Physical Review A, 2008

We show that a laser pulse designed as an adiabatic ramp followed by a kick allows one to reach a... more We show that a laser pulse designed as an adiabatic ramp followed by a kick allows one to reach a perfect postpulse molecular alignment, free of saturation. The mechanism is based on an optimized distribution of the energy between a weakly efficient but non saturating adiabatic ramp and an efficient but saturating impulsive field. Unprecedent degrees of alignment are predicted using state-of-the-art pulse shaping techniques and non-destructive field intensities. The scheme can be extended to reach high degrees of orientation of polar molecules using designed half-cycle pulses.

Phys. Chem. Chem. Phys., 2014

New insight into the nonadiabatic relaxation dynamics of aniline following excitation to its firs... more New insight into the nonadiabatic relaxation dynamics of aniline following excitation to its first three singlet excited states, 11ππ*, 11π3s/πσ* and 21ππ*.

The Journal of Chemical Physics, 2012

We apply and explore techniques aiming at enhancing the tunneling by laser fields, originally dev... more We apply and explore techniques aiming at enhancing the tunneling by laser fields, originally developed for a one-dimensional model, to a complete six-dimensional vibrational model of the inversion motion in NHD2. The computational study is performed with the multi-configuration time-dependent Hartree method. Assuming an ideal three-dimensional alignment we obtain a driven tunneling time twenty times smaller than the natural one, in rather good agreement with an oversimplified three-state model. In the case of one-dimensional alignment, a linearly polarized field leads to a poor enhancement of the tunneling probability, after averaging over the rotation about the alignment axis, whereas a circularly polarized field improves the rotationally averaged tunneling probability at the end of the pulse.

Chemical Physics, 2014

The ring-opening photoisomerization of benzopyran, which occurs via a photochemical route involvi... more The ring-opening photoisomerization of benzopyran, which occurs via a photochemical route involving a conical intersection, has been studied with quantum dynamics calculations using the multi-configuration time-dependent Hartree method (MCTDH). We introduce a mechanistic strategy to control the conversion of benzopyran to merocyanine with laser pulses. We use a six-dimensional model developed in a previous work for the potential energy surfaces (PES) based on an extension of the vibronic-coupling Hamiltonian model (diabatization method by ansatz), which depends on the most active degrees of freedom. The main objective of these quantum dynamics simulations is to provide a set of strategies that could help experimentalists to control the photoreactivity vs. photostability ratio (selectivity). In this work we present: (i) a pump-dump technique used to control the photostability, (ii) a two-step strategy to enhance the reactivity of the system: first, a pure vibrational excitation in the electronic ground state that prepares the system and, second, an ultraviolet excitation that brings the system to the first adiabatic electronic state; (iii) finally the effect of a non-resonant pulse (Stark effect) on the dynamics.

The Journal of chemical physics, Jan 28, 2014

We investigate the phenomenon of coherent destruction of tunneling in a six-dimensional model of ... more We investigate the phenomenon of coherent destruction of tunneling in a six-dimensional model of the NHD2 molecule. Two regimes are considered for the frequency of the laser field. A non-resonant regime where the frequency of the laser field is high with respect to the ground vibrational state tunneling splitting but smaller than the transition frequencies between the ground and excited vibrational states; and a quasi-resonant regime where the frequency of the laser field is close to the transition frequency between the ground and first excited vibrational states. In each case, we study the laser driven dynamics in the framework of the Floquet formalism and derive simple analytical formulas that explain the shape of the quasienergy curves associated with the two tunneling components of the ground vibrational state. This analysis allows us to obtain the parameters (frequency and amplitude) of the laser field that lead to the coherent destruction of tunneling. The multi-configuration ...

Physical Chemistry Chemical Physics, 2014

There have been a number of recent experimental investigations of the nonadiabatic relaxation dyn... more There have been a number of recent experimental investigations of the nonadiabatic relaxation dynamics of aniline following excitation to the first three singlet excited states, 1(1)ππ*, 1(1)π3s/πσ* and 2(1)ππ*. Motivated by differences between the interpretations of experimental observations, we have employed CASSCF and XMCQDPT2 calculations to explore the potential energy landscape and relaxation pathways of photoexcited aniline. We find a new prefulvene-like MECI connecting the 1(1)ππ* state with the GS in which the carbon-atom carrying the amino group is distorted out-of-plane. This suggests that excitation above the 1(1)π3s/πσ* vertical excitation energy could be followed by electronic relaxation from the 1(1)ππ* state to the ground-electronic state through this MECI. We find a MECI connecting the 1(1)π3s/πσ* and 1(1)ππ* states close to the local minimum on 1(1)π3s/πσ* which suggests that photoexcitation to the 1(1)π3s/πσ* state could be followed by relaxation to the 1(1)ππ* state and to the dissociative component of the 1(1)π3s/πσ* state. We also find evidence for a new pathway from the 2(1)ππ* state to the ground electronic state that is likely to pass through a three-state conical intersection involving the 2(1)ππ*, 1(1)π3s/πσ* and 1(1)ππ* states.

Physical review. E, Statistical, nonlinear, and soft matter physics, 2015

We propose an exactly solvable multisite interaction spin-1/2 Ising-Heisenberg model on a triangu... more We propose an exactly solvable multisite interaction spin-1/2 Ising-Heisenberg model on a triangulated Husimi lattice for the rigorous studies of chaotic entanglement. By making use of the generalized star-triangle transformation, we map the initial model onto an effective Ising one on a Husimi lattice, which we solve then exactly by applying the recursive method. Expressing the entanglement of the Heisenberg spins, that we quantify by means of the concurrence, in terms of the magnetic quantities of the system, we demonstrate its bifurcation and chaotic behavior. Furthermore, we show that the underlying chaos may slightly enhance the amount of the entanglement and present on the phase diagram the transition lines from the uniform to periodic and from the periodic to chaotic regimes.

Physical Review Letters, 2007

We propose an implementation of the quantum search algorithm of a marked item in an unsorted list... more We propose an implementation of the quantum search algorithm of a marked item in an unsorted list of N items by adiabatic passage in a cavity-laser-atom system. We use an ensemble of N identical three-level atoms trapped in a single-mode cavity and driven by two lasers. In each atom, the same level represents a database entry. One of the atoms is marked by having an energy gap between its two ground states. Appropriate time delays between the two laser pulses allow one to populate the marked state starting from an initial entangled state within a decoherence-free adiabatic subspace. The time to achieve such a process is shown to exhibit the Grover speedup √ N .

Journal of Physics A: Mathematical and Theoretical, 2012

Using a complex time method with the formalism of Stokes lines, we establish a generalization of ... more Using a complex time method with the formalism of Stokes lines, we establish a generalization of the Davis-Dykhne-Pechukas formula which gives in the adiabatic limit the transition probability of a lossy two-state system driven by an external frequency-chirped pulse-shaped field. The conditions that allow this generalization are derived. We illustrate the result with the dissipative Allen-Eberly and Rosen-Zener models.

Physical chemistry chemical physics : PCCP, Jan 18, 2015

We investigate the radiationless decay of photoexcited pyrazine to its ground electronic state us... more We investigate the radiationless decay of photoexcited pyrazine to its ground electronic state using multireference electronic structure and quantum dynamics calculations. We construct a quadratic vibronic coupling Hamiltonian, including the four lowest electronic states and ten vibrational modes, by fitting to more than 5000 ab initio points. We then use this model to simulate the non-adiabatic excited state dynamics of the molecule using the multi-configuration time-dependent Hartree method. On the basis of these calculations, we propose a new mechanism for this decay process involving a conical intersection between the Au(nπ*) state and the ground state. After excitation to the B2u(ππ*) state, the molecule decays to both the B3u(nπ*) and Au(nπ*) states on an ultrashort timescale of approximately 20 fs. The radiationless decay to the ground state then occurs from the Au(nπ*) state on a much longer timescale.

Phys. Chem. Chem. Phys., 2015

Femtosecond time-resolved photoelectron spectroscopy experiments have been used to compare the el... more Femtosecond time-resolved photoelectron spectroscopy experiments have been used to compare the electronic relaxation dynamics of aniline and d7-aniline following photoexcitation in the range 272-238 nm. Together with the results of recent theoretical investigations of the potential energy landscape [M. Sala, O. M. Kirkby, S. Guérin and H. H. Fielding, Phys. Chem. Chem. Phys., 2014, 16, 3122], these experiments allow us to resolve a number of unanswered questions surrounding the nonradiative relaxation mechanism. We find that tunnelling does not play a role in the electronic relaxation dynamics, which is surprising given that tunnelling plays an important role in the electronic relaxation of isoelectronic phenol and in pyrrole. We confirm the existence of two time constants associated with dynamics on the 1(1)πσ* surface that we attribute to relaxation through a conical intersection between the 1(1)πσ* and 1(1)ππ* states and motion on the 1(1)πσ* surface. We also present what we believe is the first report of an experimental signature of a 3-state conical intersection involving the 2(1)ππ*, 1(1)πσ* and 1(1)ππ* states.

The Journal of chemical physics, Jan 28, 2014

We investigate the phenomenon of coherent destruction of tunneling in a six-dimensional model of ... more We investigate the phenomenon of coherent destruction of tunneling in a six-dimensional model of the NHD2 molecule. Two regimes are considered for the frequency of the laser field. A non-resonant regime where the frequency of the laser field is high with respect to the ground vibrational state tunneling splitting but smaller than the transition frequencies between the ground and excited vibrational states; and a quasi-resonant regime where the frequency of the laser field is close to the transition frequency between the ground and first excited vibrational states. In each case, we study the laser driven dynamics in the framework of the Floquet formalism and derive simple analytical formulas that explain the shape of the quasienergy curves associated with the two tunneling components of the ground vibrational state. This analysis allows us to obtain the parameters (frequency and amplitude) of the laser field that lead to the coherent destruction of tunneling. The multi-configuration ...

The Journal of chemical physics, Jan 7, 2014

We present a full quantum-mechanical study of the laser control of the radiationless decay betwee... more We present a full quantum-mechanical study of the laser control of the radiationless decay between the B3u(nπ(*)) and B2u(ππ(*)) states of pyrazine using the dynamic Stark effect. In contrast to our previous study [Sala et al., J. Chem. Phys. 140, 194309 (2014)], where a four-dimensional model was used, all the 24 degrees of freedom are now included in order to test the robustness of the strategy of control. Using a vibronic coupling Hamiltonian model in a diabatic representation, the multi-layer version of the multi-configuration time-dependent Hartree method is exploited to propagate the corresponding wave packets. We still observe a trapping of the wavepacket on the B2u(ππ(*)) potential energy surface due to the Stark effect for a longer time than the "non-resonant field-free" B2u(ππ(*)) lifetime.

Physical chemistry chemical physics : PCCP, Jan 14, 2014

The excited state dynamics of pyrazine has attracted considerable attention in the last three dec... more The excited state dynamics of pyrazine has attracted considerable attention in the last three decades. It has long been recognized that after UV excitation, the dynamics of the molecule is impacted by strong non-adiabatic effects due to the existence of a conical intersection between the B2u(ππ*) and B3u(nπ*) electronic states. However, a recent study based on trajectory surface hopping dynamics simulations suggested the participation of the Au(nπ*) and B2g(nπ*) low-lying dark electronic states in the ultrafast radiationless decay of the molecule after excitation to the B2u(ππ*) state. The purpose of this work was to pursue the investigation of the role of the Au(nπ*) and B2g(nπ*) states in the photophysics of pyrazine. A linear vibronic coupling model hamiltonian including the four lowest excited electronic states and the sixteen most relevant vibrational degrees of freedom was constructed using high level XMCQDPT2 electronic structure calculations. Wavepacket propagations using th...

The Journal of Chemical Physics, 2019

We present a model for the lowest two potential energy surfaces (PESs) that describe the photoind... more We present a model for the lowest two potential energy surfaces (PESs) that describe the photoinduced ring-opening reaction of benzopyran taken as a model compound to study the photochromic ring-opening reaction of indolinobenzospiropyran and its evolution toward its open-chain analog. The PESs are expressed in terms of three effective rectilinear coordinates. One corresponds to the direction between the equilibrium geometry in the electronic ground state, referred to as the Franck-Condon geometry, and the minimum of conical intersection (CI), while the other two span the two-dimensional branching space at the CI. The model correctly reproduces the topography of the PESs. The ab initio calculations are performed with the extended multiconfiguration quasidegenerate perturbation theory at second order method. We demonstrate that accounting for electron dynamic correlation drastically changes the global energy landscape since some zwitterionic states become strongly stabilized. Quantum...

Physical chemistry chemical physics : PCCP, Jan 18, 2015

We investigate the radiationless decay of photoexcited pyrazine to its ground electronic state us... more We investigate the radiationless decay of photoexcited pyrazine to its ground electronic state using multireference electronic structure and quantum dynamics calculations. We construct a quadratic vibronic coupling Hamiltonian, including the four lowest electronic states and ten vibrational modes, by fitting to more than 5000 ab initio points. We then use this model to simulate the non-adiabatic excited state dynamics of the molecule using the multi-configuration time-dependent Hartree method. On the basis of these calculations, we propose a new mechanism for this decay process involving a conical intersection between the Au(nπ*) state and the ground state. After excitation to the B2u(ππ*) state, the molecule decays to both the B3u(nπ*) and Au(nπ*) states on an ultrashort timescale of approximately 20 fs. The radiationless decay to the ground state then occurs from the Au(nπ*) state on a much longer timescale.

Physical Chemistry Chemical Physics, 2015

Femtosecond studies of electronic relaxation in aniline reveal evidence for a 3-state conical int... more Femtosecond studies of electronic relaxation in aniline reveal evidence for a 3-state conical intersection and show that tunnelling is unimportant.

The Journal of chemical physics, Jan 7, 2014

We present a full quantum-mechanical study of the laser control of the radiationless decay betwee... more We present a full quantum-mechanical study of the laser control of the radiationless decay between the B3u(nπ(*)) and B2u(ππ(*)) states of pyrazine using the dynamic Stark effect. In contrast to our previous study [Sala et al., J. Chem. Phys. 140, 194309 (2014)], where a four-dimensional model was used, all the 24 degrees of freedom are now included in order to test the robustness of the strategy of control. Using a vibronic coupling Hamiltonian model in a diabatic representation, the multi-layer version of the multi-configuration time-dependent Hartree method is exploited to propagate the corresponding wave packets. We still observe a trapping of the wavepacket on the B2u(ππ(*)) potential energy surface due to the Stark effect for a longer time than the "non-resonant field-free" B2u(ππ(*)) lifetime.

Physical chemistry chemical physics : PCCP, Jan 14, 2014

The excited state dynamics of pyrazine has attracted considerable attention in the last three dec... more The excited state dynamics of pyrazine has attracted considerable attention in the last three decades. It has long been recognized that after UV excitation, the dynamics of the molecule is impacted by strong non-adiabatic effects due to the existence of a conical intersection between the B2u(ππ*) and B3u(nπ*) electronic states. However, a recent study based on trajectory surface hopping dynamics simulations suggested the participation of the Au(nπ*) and B2g(nπ*) low-lying dark electronic states in the ultrafast radiationless decay of the molecule after excitation to the B2u(ππ*) state. The purpose of this work was to pursue the investigation of the role of the Au(nπ*) and B2g(nπ*) states in the photophysics of pyrazine. A linear vibronic coupling model hamiltonian including the four lowest excited electronic states and the sixteen most relevant vibrational degrees of freedom was constructed using high level XMCQDPT2 electronic structure calculations. Wavepacket propagations using th...

The Journal of chemical physics, Jan 21, 2014

The laser control of the radiationless decay between the B(3u)(nπ*) and B(2u)(ππ*) states of pyra... more The laser control of the radiationless decay between the B(3u)(nπ*) and B(2u)(ππ*) states of pyrazine using the dynamic Stark effect has been investigated. A vibronic coupling model Hamiltonian in diabatic representation, including potential energy, transition dipole, and static polarizability surfaces as a function of the four most important vibrational modes of the molecule has been parametrized using multi-reference electronic structure calculations. The interaction of the molecule with a strong non-resonant laser pulse has been analyzed in terms of dressed potential energy surfaces. Because of the large polarizability difference between the vibronically coupled B(3u)(nπ*) and B(2u)(ππ*) states, the Stark effect induced by the non-resonant laser pulse shifts the conical intersection away from the Franck-Condon region. We have shown, by solving the time-dependent Schrödinger equation for the molecule interacting with a relatively weak pump pulse driving the electronic excitation f...

Physical Review A, 2008

We show that a laser pulse designed as an adiabatic ramp followed by a kick allows one to reach a... more We show that a laser pulse designed as an adiabatic ramp followed by a kick allows one to reach a perfect postpulse molecular alignment, free of saturation. The mechanism is based on an optimized distribution of the energy between a weakly efficient but non saturating adiabatic ramp and an efficient but saturating impulsive field. Unprecedent degrees of alignment are predicted using state-of-the-art pulse shaping techniques and non-destructive field intensities. The scheme can be extended to reach high degrees of orientation of polar molecules using designed half-cycle pulses.

Phys. Chem. Chem. Phys., 2014

New insight into the nonadiabatic relaxation dynamics of aniline following excitation to its firs... more New insight into the nonadiabatic relaxation dynamics of aniline following excitation to its first three singlet excited states, 11ππ*, 11π3s/πσ* and 21ππ*.

The Journal of Chemical Physics, 2012

We apply and explore techniques aiming at enhancing the tunneling by laser fields, originally dev... more We apply and explore techniques aiming at enhancing the tunneling by laser fields, originally developed for a one-dimensional model, to a complete six-dimensional vibrational model of the inversion motion in NHD2. The computational study is performed with the multi-configuration time-dependent Hartree method. Assuming an ideal three-dimensional alignment we obtain a driven tunneling time twenty times smaller than the natural one, in rather good agreement with an oversimplified three-state model. In the case of one-dimensional alignment, a linearly polarized field leads to a poor enhancement of the tunneling probability, after averaging over the rotation about the alignment axis, whereas a circularly polarized field improves the rotationally averaged tunneling probability at the end of the pulse.

Chemical Physics, 2014

The ring-opening photoisomerization of benzopyran, which occurs via a photochemical route involvi... more The ring-opening photoisomerization of benzopyran, which occurs via a photochemical route involving a conical intersection, has been studied with quantum dynamics calculations using the multi-configuration time-dependent Hartree method (MCTDH). We introduce a mechanistic strategy to control the conversion of benzopyran to merocyanine with laser pulses. We use a six-dimensional model developed in a previous work for the potential energy surfaces (PES) based on an extension of the vibronic-coupling Hamiltonian model (diabatization method by ansatz), which depends on the most active degrees of freedom. The main objective of these quantum dynamics simulations is to provide a set of strategies that could help experimentalists to control the photoreactivity vs. photostability ratio (selectivity). In this work we present: (i) a pump-dump technique used to control the photostability, (ii) a two-step strategy to enhance the reactivity of the system: first, a pure vibrational excitation in the electronic ground state that prepares the system and, second, an ultraviolet excitation that brings the system to the first adiabatic electronic state; (iii) finally the effect of a non-resonant pulse (Stark effect) on the dynamics.

The Journal of chemical physics, Jan 28, 2014

We investigate the phenomenon of coherent destruction of tunneling in a six-dimensional model of ... more We investigate the phenomenon of coherent destruction of tunneling in a six-dimensional model of the NHD2 molecule. Two regimes are considered for the frequency of the laser field. A non-resonant regime where the frequency of the laser field is high with respect to the ground vibrational state tunneling splitting but smaller than the transition frequencies between the ground and excited vibrational states; and a quasi-resonant regime where the frequency of the laser field is close to the transition frequency between the ground and first excited vibrational states. In each case, we study the laser driven dynamics in the framework of the Floquet formalism and derive simple analytical formulas that explain the shape of the quasienergy curves associated with the two tunneling components of the ground vibrational state. This analysis allows us to obtain the parameters (frequency and amplitude) of the laser field that lead to the coherent destruction of tunneling. The multi-configuration ...

Physical Chemistry Chemical Physics, 2014

There have been a number of recent experimental investigations of the nonadiabatic relaxation dyn... more There have been a number of recent experimental investigations of the nonadiabatic relaxation dynamics of aniline following excitation to the first three singlet excited states, 1(1)ππ*, 1(1)π3s/πσ* and 2(1)ππ*. Motivated by differences between the interpretations of experimental observations, we have employed CASSCF and XMCQDPT2 calculations to explore the potential energy landscape and relaxation pathways of photoexcited aniline. We find a new prefulvene-like MECI connecting the 1(1)ππ* state with the GS in which the carbon-atom carrying the amino group is distorted out-of-plane. This suggests that excitation above the 1(1)π3s/πσ* vertical excitation energy could be followed by electronic relaxation from the 1(1)ππ* state to the ground-electronic state through this MECI. We find a MECI connecting the 1(1)π3s/πσ* and 1(1)ππ* states close to the local minimum on 1(1)π3s/πσ* which suggests that photoexcitation to the 1(1)π3s/πσ* state could be followed by relaxation to the 1(1)ππ* state and to the dissociative component of the 1(1)π3s/πσ* state. We also find evidence for a new pathway from the 2(1)ππ* state to the ground electronic state that is likely to pass through a three-state conical intersection involving the 2(1)ππ*, 1(1)π3s/πσ* and 1(1)ππ* states.

Physical review. E, Statistical, nonlinear, and soft matter physics, 2015

We propose an exactly solvable multisite interaction spin-1/2 Ising-Heisenberg model on a triangu... more We propose an exactly solvable multisite interaction spin-1/2 Ising-Heisenberg model on a triangulated Husimi lattice for the rigorous studies of chaotic entanglement. By making use of the generalized star-triangle transformation, we map the initial model onto an effective Ising one on a Husimi lattice, which we solve then exactly by applying the recursive method. Expressing the entanglement of the Heisenberg spins, that we quantify by means of the concurrence, in terms of the magnetic quantities of the system, we demonstrate its bifurcation and chaotic behavior. Furthermore, we show that the underlying chaos may slightly enhance the amount of the entanglement and present on the phase diagram the transition lines from the uniform to periodic and from the periodic to chaotic regimes.

Physical Review Letters, 2007

We propose an implementation of the quantum search algorithm of a marked item in an unsorted list... more We propose an implementation of the quantum search algorithm of a marked item in an unsorted list of N items by adiabatic passage in a cavity-laser-atom system. We use an ensemble of N identical three-level atoms trapped in a single-mode cavity and driven by two lasers. In each atom, the same level represents a database entry. One of the atoms is marked by having an energy gap between its two ground states. Appropriate time delays between the two laser pulses allow one to populate the marked state starting from an initial entangled state within a decoherence-free adiabatic subspace. The time to achieve such a process is shown to exhibit the Grover speedup √ N .

Journal of Physics A: Mathematical and Theoretical, 2012

Using a complex time method with the formalism of Stokes lines, we establish a generalization of ... more Using a complex time method with the formalism of Stokes lines, we establish a generalization of the Davis-Dykhne-Pechukas formula which gives in the adiabatic limit the transition probability of a lossy two-state system driven by an external frequency-chirped pulse-shaped field. The conditions that allow this generalization are derived. We illustrate the result with the dissipative Allen-Eberly and Rosen-Zener models.

Physical chemistry chemical physics : PCCP, Jan 18, 2015

We investigate the radiationless decay of photoexcited pyrazine to its ground electronic state us... more We investigate the radiationless decay of photoexcited pyrazine to its ground electronic state using multireference electronic structure and quantum dynamics calculations. We construct a quadratic vibronic coupling Hamiltonian, including the four lowest electronic states and ten vibrational modes, by fitting to more than 5000 ab initio points. We then use this model to simulate the non-adiabatic excited state dynamics of the molecule using the multi-configuration time-dependent Hartree method. On the basis of these calculations, we propose a new mechanism for this decay process involving a conical intersection between the Au(nπ*) state and the ground state. After excitation to the B2u(ππ*) state, the molecule decays to both the B3u(nπ*) and Au(nπ*) states on an ultrashort timescale of approximately 20 fs. The radiationless decay to the ground state then occurs from the Au(nπ*) state on a much longer timescale.

Phys. Chem. Chem. Phys., 2015

Femtosecond time-resolved photoelectron spectroscopy experiments have been used to compare the el... more Femtosecond time-resolved photoelectron spectroscopy experiments have been used to compare the electronic relaxation dynamics of aniline and d7-aniline following photoexcitation in the range 272-238 nm. Together with the results of recent theoretical investigations of the potential energy landscape [M. Sala, O. M. Kirkby, S. Guérin and H. H. Fielding, Phys. Chem. Chem. Phys., 2014, 16, 3122], these experiments allow us to resolve a number of unanswered questions surrounding the nonradiative relaxation mechanism. We find that tunnelling does not play a role in the electronic relaxation dynamics, which is surprising given that tunnelling plays an important role in the electronic relaxation of isoelectronic phenol and in pyrrole. We confirm the existence of two time constants associated with dynamics on the 1(1)πσ* surface that we attribute to relaxation through a conical intersection between the 1(1)πσ* and 1(1)ππ* states and motion on the 1(1)πσ* surface. We also present what we believe is the first report of an experimental signature of a 3-state conical intersection involving the 2(1)ππ*, 1(1)πσ* and 1(1)ππ* states.

The Journal of chemical physics, Jan 28, 2014

We investigate the phenomenon of coherent destruction of tunneling in a six-dimensional model of ... more We investigate the phenomenon of coherent destruction of tunneling in a six-dimensional model of the NHD2 molecule. Two regimes are considered for the frequency of the laser field. A non-resonant regime where the frequency of the laser field is high with respect to the ground vibrational state tunneling splitting but smaller than the transition frequencies between the ground and excited vibrational states; and a quasi-resonant regime where the frequency of the laser field is close to the transition frequency between the ground and first excited vibrational states. In each case, we study the laser driven dynamics in the framework of the Floquet formalism and derive simple analytical formulas that explain the shape of the quasienergy curves associated with the two tunneling components of the ground vibrational state. This analysis allows us to obtain the parameters (frequency and amplitude) of the laser field that lead to the coherent destruction of tunneling. The multi-configuration ...

The Journal of chemical physics, Jan 7, 2014

We present a full quantum-mechanical study of the laser control of the radiationless decay betwee... more We present a full quantum-mechanical study of the laser control of the radiationless decay between the B3u(nπ(*)) and B2u(ππ(*)) states of pyrazine using the dynamic Stark effect. In contrast to our previous study [Sala et al., J. Chem. Phys. 140, 194309 (2014)], where a four-dimensional model was used, all the 24 degrees of freedom are now included in order to test the robustness of the strategy of control. Using a vibronic coupling Hamiltonian model in a diabatic representation, the multi-layer version of the multi-configuration time-dependent Hartree method is exploited to propagate the corresponding wave packets. We still observe a trapping of the wavepacket on the B2u(ππ(*)) potential energy surface due to the Stark effect for a longer time than the "non-resonant field-free" B2u(ππ(*)) lifetime.

Physical chemistry chemical physics : PCCP, Jan 14, 2014

The excited state dynamics of pyrazine has attracted considerable attention in the last three dec... more The excited state dynamics of pyrazine has attracted considerable attention in the last three decades. It has long been recognized that after UV excitation, the dynamics of the molecule is impacted by strong non-adiabatic effects due to the existence of a conical intersection between the B2u(ππ*) and B3u(nπ*) electronic states. However, a recent study based on trajectory surface hopping dynamics simulations suggested the participation of the Au(nπ*) and B2g(nπ*) low-lying dark electronic states in the ultrafast radiationless decay of the molecule after excitation to the B2u(ππ*) state. The purpose of this work was to pursue the investigation of the role of the Au(nπ*) and B2g(nπ*) states in the photophysics of pyrazine. A linear vibronic coupling model hamiltonian including the four lowest excited electronic states and the sixteen most relevant vibrational degrees of freedom was constructed using high level XMCQDPT2 electronic structure calculations. Wavepacket propagations using th...