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Concept# Couplage vibronique

Résumé

En chimie théorique, les termes de couplage vibronique (pour des molécules discrètes) ou de couplage électron-phonon (pour des cristaux ou des objets bi- ou tridimensionnels), négligés dans l'approximation de Born-Oppenheimer, sont proportionnels à l'interaction entre les mouvements électroniques et nucléaires des objets chimiques. Le terme « vibronique » provient de la concaténation des termes et « électronique ». Le mot couplage dénote l'idée que dans un objet chimie, les états vibrationnels (ou phonons) et électroniques interagissent et s'influencent réciproquement. Le couplage est parfois qualifié d' effet pseudo-Jahn-Teller, en raison de sa proximité conceptuelle avec l'effet Jahn-Teller, bien connu par ailleurs.
Description
Le couplage vibronique / électron-phonon est important lorsque deux surfaces d'énergie potentielle adiabatiques deviennent proches l'une de l'autre, c'est-à-dire lorsque la différence d'énergie entre elles est de l'ordre de grandeur d'un quantum

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Approximation de Born-Oppenheimer

L’approximation de Born et Oppenheimer permet de simplifier drastiquement l’équation de Schrödinger pour le calcul de la fonction d'onde d'une molécule. Elle consiste

Intersection conique

thumb|Intersection conique idéale entre deux surfaces d'énergie potentielle. Les axes horizontaux représentent les positions nucléaires, l'axe vertical est l'énergie des deux états possibles.
En chimi

Chimie numérique

La chimie numérique ou chimie informatique, parfois aussi chimie computationnelle, est une branche de la chimie et de la physico-chimie qui utilise les lois de la chimie théorique exploitées dans de

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MSE-486: Organic electronic materials

This course will introduce students to the field of organic electronic materials. The goal of this course is to discuss the origin of electronic properties in organic materials, charge transport mechanisms, chemical synthesis, materials processing, and device fabrication.

CH-453: Molecular quantum dynamics

The course covers several exact, approximate, and numerical methods to solve the time-dependent molecular Schrödinger equation, and applications including calculations of molecular electronic spectra. More advanced topics include introduction to the semiclassical methods and Feynman path integral.

CH-351: Molecular dynamics and Monte-Carlo simulations

Introduction to molecular dynamics and Monte-Carlo simulation methods.

Understanding the elementary steps involved in a chemical reaction forms the cornerstone of physical chemistry research. One way to deepen this understanding is by studying chemical and physical processes using linear and nonlinear spectroscopic techniques. However, the outcomes of such experiments can be difficult to decipher due to the interweaving of several effects. Therefore, in order to help experimentalists to disentangle such spectra, the role of theorists is to develop efficient tools that are able to accurately describe molecular systems. The starting point of such tools is solving the time-dependent Schrödinger equation. In this thesis, after implementing geometric integrators, which are based on a combination of the split-operator algorithm and Magnus expansion, for the exact nonadiabatic quantum dynamics of a molecule interacting with a time-dependent electromagnetic field, we derive and implement these geometric integrators for the time-dependent perturbation theory, the Condon, rotating-wave, and ultrashort-pulse approximations, as well as every possible combination thereof. As verified in several model systems, these integrators exactly preserve the geometric invariants, and achieve an arbitrary prescribed order of accuracy in the time step and an exponential convergence in the grid spacing. We also explore in more detail the ultrashort-pulse approximation and derive an analytical expression for the combination with the time-dependent perturbation theory; this expression significantly accelerates numerical calculations. We show that in the limit of the zero pulse width, the d-pulse approximation is recovered. We illustrate the performance of the introduced approximations, using a three-dimensional model of pyrazine, in which it is essential to go beyond the d-pulse limit in order to describe the dynamics correctly. The high-order algorithms are also applied to the photodissociation dynamics of iodomethane (CH3I), following its excitation to the A band. We implement a general split-operator with both discrete-variable and finite-basis representations that can treat one non-Cartesian, such as angular coordinate. To test the effect of various degrees of freedom and of the nonadiabatic dynamics, we apply these algorithms to one-, two-, and three-dimensional models of iodomethane, both in the presence and in the absence of nonadiabatic couplings. A full quantum calculation is, however, limited to problems with low dimensionality (approximately ten degrees of freedom). Beyond this, one must seek an affordable balance between computational efficiency and physical accuracy and can employ, for example, semiclassical methods that are based on classical trajectories. A simple semiclassical approximation that can treat larger systems and requires only local knowledge of the potential is the on-the-fly ab initio thawed Gaussian approximation. We implement a generalization of the method that goes beyond the Franck-Condon approximation and treats Herzberg-Teller active molecules. Our method is used to compute absorption spectra of phenyl radical and of benzene, for which the Herzberg-Teller contribution is essential.

Photoprocesses are ubiquitous in nature, science, and engineering. The understanding as well as the optimization of photochemical and photophysical properties of molecular systems requires computational tools that are able to describe the dynamical evolution of the system in electronically excited states. Ab Initio Molecular Dynamics (AIMD) based on Density Functional Theory (DFT) has become an established tool for elucidating mechanisms of chemical reactions that occur in the electronic ground state. However, to describe photoprocesses by AIMD, an underlying electronic-structure method that is able to treat excited states is necessary. This complicates the description of these processes because in the past this implied the use of computationally expensive wavefunction-based methods, which in addition are not straightforward to use. Time-Dependent Density Functional Theory (TDDFT) provides an in principle exact description of electronically excited states, although in practice, approximations have to be introduced. Compared to wavefunction-based methods, TDDFT is computationally less demanding and is relatively straightforward and easy to use. Recently, TDDFT nuclear gradients have become available and allow to carry out AIMD in excited states. In this thesis a TDDFT-based AIMD method that is able to account for non-adiabatic effects is developed and implemented. The non-adiabatic couplings are computed by means of a Kohn-Sham orbital based reconstruction of the many-electron wavefunction for ground and excited states. The non-adiabatic scheme is based on the fewest switches trajectory surface hopping (SH) method introduced by Tully. The method is applied to describe decay processes, such as fragmentation or isomerization, that occur upon photoexcitation of the molecules protonated formaldimine and oxirane. In the case of protonated formaldimine, the results of the TDDFT-SH method are in good agreement with SH simulations based on the state-averaged complete active space (SA-CASSCF) method, both with respect to the observed reaction mechanisms and the excited state life times. In the case of oxirane, the TDDFT-SH simulations confirm the main experimental results and provide an additional refinement of the postulated reaction mechanism. The accuracy of TDDFT is investigated with respect to different issues that are especially important for the proper description of photoprocesses. These aspects include the accuracy of non-adiabatic coupling (NAC) vectors, the description of S1-S0 conical intersections, and the description of locally excited states in systems where charge transfer (CT) states are present, that are affected by the well-known CT failure of TDDFT. Concerning the NAC vectors, a qualitative agreement with SA-CASSCF is found, although magnitudes are underestimated by TDDFT/PBE. Regarding the description of conical intersections to the ground state, we find as expected that TDDFT in the adiabatic approximation (ALDA) is not able to predict an intersection that is strictly conical. However, we find that TDDFT is able to approximate a conical intersection that has a similar shape as the one predicted by SA-CASSCF. For an electron donor-bridge-acceptor molecule it is shown that the CT failure of TDDFT can also considerably affect properties of non-CT states. The use of TDDFT using conventional exchange-correlation functionals is thus not recommended for the description of such systems. Using the second-order approximate coupled cluster (CC2) method in conjunction with a high quality basis set, an accurate and balanced description of both locally excited and CT states can be made. The use of CC2 with large basis sets for AIMD simulations is however still computationally unaffordable for larger systems. TDDFT is still in its infancy and several attempts to cure some of its defiencies have already been made. These attempts mainly concern improvements of the approximations of the exchange-correlation functionals and associated TDDFT kernels. The TDDFT-SH method that has been developed in this thesis can in principle be applied in combination with any approximation for the exchange correlation functional, provided that nuclear gradients for this approximation are available and the computational cost remains acceptable. In this way, the method developed here is able to directly profit from the ongoing improvements in the active research field of exchange-correlation functionals and kernels.

In this thesis, I present an experimental molecular beam surface scattering study of dynamical processes involved when HCl molecules are scattered from Au(111) and Ag(111) surfaces. I investigated vibrational excitation, translational inelasticity and dissociative adsorption in combination with associative desorption. The experiments were conducted in an ultra-high vacuum (UHV) molecular beam/surface scattering apparatus equipped with a pulsed nanosecond infrared (IR) laser source for vibrational state manipulation and pulsed nanosecond ultraviolet (UV) lasers for quantum-state-resolved detection of molecules via resonance enhanced multi-photon ionization (REMPI) before and after the collisions.
For HCl/Au(111), I found surface temperature dependent vibrational excitation probabilities (VEPs) from vibrational state v
= 0 -> 1 to be in the range of 10^-5 - 10^-3 for incidence energies of Ei = 0.67 - 0.99 eV, which is low compared to other molecule-surface systems. On the other hand, VEPs for the v = 1 -> 2 transition were substantially higher at 10^-3 - 10^-2 for comparable incidence energies. In both cases, excitation probabilities could be divided into electronically adiabatic and nonadiabatic contributions where the latter exponentially depended on the surface temperature Ts. This combination of adiabatic and nonadiabatic vibrational excitation has so far been uniquely observed for HCl scattered from Au(111) and Ag(111) surfaces. Extracting Ei and Ts independent interaction coefficients, I found the nonadiabatic excitation to be stronger than the adiabatic one by a factor of 67 for the v = 0 -> 1 and 24 for the v = 1 -> 2 channel.
In comparison, on Ag(111) only the excitation from the vibrational ground state could be observed. With VEPs in the range of 10^-4 - 10^-3 being slightly higher than on gold, this difference was interpreted as enhanced nonadiabatic interactions on silver. Relatively low barriers to dissociation on both metal surfaces (e. g., compared to NO/Au(111)) might have led to the higher v = 1 -> 2 VEPs on Au(111). Further, the absence of higher excitation channels (v = 2 -> 3 on gold, v = 1 -> 2 on silver) might also be explained by the enhanced dissociation of HCl molecules incident in excited vibrational states.
Dissociation probabilities of HCl on metal surfaces were measured by AUGER electron spectroscopy after controlled molecular beam dosage monitored by a quadrupole mass spectrometer. Even though the dissociation probabilities on Ag(111) were predicted to be higher than on Au(111), I could not determine them within the current technical limitations. Additionally, the actual dissociation probabilities I determined on Au(111) employing AES remained below 0.06 - 0.17, which is lower than those predicted by a series of computational studies by a factor of 2 - 6. On the other hand, translational energy losses calculated in the latest AIMDEF study match the experimental results for v = 1 -> 1 and v = 1 -> 2 quite accurately. In sum, the results of this thesis, which all indicate that HCl exhibits both electronically adiabatic and nonadiabatic interactions with metal surfaces during scatter- ing events, can serve as benchmark data to test and improve theoretical descriptions of gas-surface interactions, especially in those cases were nonadiabaticity plays an important role.