Evaluation and analysis of vibrationally resolved electronic spectra with ab initio semiclassical dynamics

Linear and nonlinear spectroscopy techniques are widely used to study numerous important chemical and physical processes. However, the interpretation of these experimental spectra often becomes very complicated because a particular spectrum constitutes a mere footprint of a host of various, possibly intertwined, effects. In this spirit, calculations in the time-dependent picture provide a useful tool for decoding such spectra. Nevertheless, the ultimate challenge is to devise a theoretical framework that could yield sufficient efficiency as well as accuracy to describe the molecular system of interest in a satisfactory way. The strategy is relatively straightforward for low dimensional systems that are directly tractable, e.g., with exact quantum dynamics performed on an equidistant grid. Despite the formidable overall exponential scaling, these calculations can be significantly accelerated by using higher-order split-operator propagation schemes. In general however, one is forced to seek an affordable balance between physical accuracy on one hand and computational efficiency on the other by employing, for instance, some of the techniques from the broad family of semiclassical methods based on classical trajectories. To this end, the thawed Gaussian approximation (TGA) is combined with an on-the-fly ab initio scheme (OTF-AI). The resulting OTF-AI-TGA algorithm is efficient enough to treat all vibrational degrees of freedom (DOFs) on an equal footing even in case of larger molecules such as pentathiophene (105 DOFs). Moreover, in sharp contrast to popular approaches based on global harmonic approximation, OTF-AI-TGA reproduces almost perfectly the absorption and photoelectron spectra of ammonia, i.e., spectra with strong dependence on large amplitude motions. In addition to the mere reproduction of experimental spectra, a novel systematic approach is introduced to assess the importance and the dynamical couplings of individual vibrational DOFs. This is in turn used to gain a deeper insight into the associated physical and chemical processes by attributing specific spectral features to the underlying molecular motion. Specifically, in the case of oligothiophenes, this approach was used to assign the dynamical interplay between quinoid and aromatic characters of individual rings to particular spectral patterns and, furthermore, to explain the changes in the vibrational line shape with an increasing number of rings. Furthermore, in systems that are too large to be treated with accurate quantum methods, efficient methods such as OTF-AI-TGA are expected to be useful as a preliminary tool for identification of the subspace of the important DOFs. On this subspace, one can then unleash some of the less efficient yet better-suited methods. In summary, OTF-AI-TGA combined with this novel analysis approach is intended to provide the first crucial step in a hierarchical computational protocol for studying large molecules such as dyes.

Geometric Integrators for Nonadiabatic Molecular Quantum Dynamics Induced by the Interaction with the Electromagnetic Field

Aurélien Kevin Fabrice Patoz

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.

EPFL2017

Growth and characterization of specific self-assembled supramolecular architectures at crystal surfaces

Marta Canas Ventura

In the framework of this thesis self-assembled supramolecular architectures of several molecular species on crystal metal surfaces are characterized. These investigations combine results obtained by means of scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), X-Ray Photoelectron Spectroscopy (XPS), and Near-Edge X-Ray Absorption Fine Structure Spectroscopy (NEXAFS). The interest in controlling the formation of specific arrangements of supramolecular ensembles at surfaces is motivated to a great extent by the prospect to design molecular devices and nanomaterials with customized properties. The goal is a "spontaneous" creation of targeted, well-ordered molecular architectures, which shall be achieved by taking advantage of the molecule's propensity to self-assemble. Control means, in this context, predictability. This term address the accuracy of the prediction of the structures resulting from molecular self-assembly for systems exhibiting different characteristics. Here, system stands for a particular combination of a single-crystal surface and adsorbed molecular species. In a first part, the focus is given to the impact of a Pd(111) surface exhibiting high chemical activity. Due to surface induced deprotonation two resulting chemical states are found for the adsorbed terephthalic acid species. Deprotonation of the carboxyl moieties is observed to affect the topology of the molecular assemblies even though hydrogen bonds (H-bonds) are responsible for the stabilization of assemblies of both, intact and deprotonated molecular species. Therefore, the selective fabrication of a single specific molecular structure becomes a challenge when the surface is prone to induce chemical reactions. Secondly, a molecular species exhibiting conformational flexibility is studied on a largely inert Au(111) surface. It is observed that the characteristic conformational flexibility is also active on the surface, and the molecule is found to adopt different conformations depending on the locally most favorable bonding motif. In particular, phenyl groups may undergo a rotation such that one-dimensional chains can be stabilized by means of intermolecular H-bonds. This phenomenon is very interesting from an adsorbate-substrate interaction point of view, but it limits the predictability of supramolecular structures formed upon self-assembly. In a third step, a comparative study of the self-assembly of three closely related molecular species highlights the impact of concurrent competing intermolecular interactions. Molecules exhibiting an increasing number of possible interaction channels are found to assemble into an increasing number of distinct supramolecular architectures. The most flexible molecular species allowing for different hydrogen-bonding patterns as well as dipolar coupling self-assembles into various structurally complex architectures. Structural predictability, however, is low since the formation of one or the other assembly appears to depend on small variations in sample preparation conditions and on poorly controllable local surface properties. Finally, a system where the resulting structures fully agree with predictions evidences the possibility to control the formation of targeted specific supramolecular architectures. The formation of well ordered bimolecular ribbons and wires over extended length scales is accomplished by the rational choice of both, complementary building blocks for the selective formation of three-fold H-bonds and a nanostructured surface to guide the molecular self-assembly. Furthermore, the impact of H-bond formation on the electronic structure of self-assembled molecular building blocks is probed by STS measurements. The experimental data reveal shifts in the electronic levels depending on the H-bond configuration and on the local characteristics of the underlying surface. In order to improve the precision of such a study, single molecules, homo- and heteromolecular dimers as well as trimers are successfully isolated and assembled by molecular manipulation with the STM tip. Further STS studies on such artificially assembled structures are suggested.

EPFL2008

Efficient ab initio semiclassical dynamics for linear and nonlinear electronic spectroscopy

Tomislav Begusic

Molecular spectroscopy is an essential experimental technique in both fundamental physical sciences and applied research. For example, electronic spectroscopy, in which light induces a change in the electronic state of a molecule, is a powerful tool for studying light-matter interactions appearing in solar energy conversion and light-emitting devices. To explain experimental findings and compare them with theoretical predictions, scientists often compute properties that can only be inferred indirectly, rather than simulating the observed spectroscopic signals. This is partly due to challenges associated with the simulation of electronic spectra, which requires both an accurate description of the electronic structure and the quantum-dynamical treatment of the motion of atomic nuclei. As the exact methods to solve the underlying quantum-mechanical problem scale exponentially with the number of atoms, this task must be performed approximately for all but the smallest molecules. In this thesis, we develop a set of methods to systematically study the effects of different approximations on the computed spectra. As our starting point, we consider a semiclassical method called the thawed Gaussian approximation, which has resurfaced in recent years as an efficient and robust alternative to simple but crude models or highly accurate but costly quantum dynamics methods. First, we present several practical improvements of the method, such as generalization to non-Condon effects or the efficient single-Hessian version, which enable a broader set of molecules to be studied. Second, we introduce a more fundamental modification of the methods - the inclusion of non-zero temperature effects. The underlying theory, which is inspired by the concept of so-called thermo-field dynamics, is exact and applies to any quantum dynamics method. Remarkably, within the thawed Gaussian approximation, the inclusion of non-zero temperature comes at nearly no additional computational cost. These methods, developed originally for steady-state linear spectra, are then employed to compute state-of-the-art, time-resolved spectra, which are typically measured with ultrashort laser pulses. More precisely, we study how the anharmonicity of the potential energy surfaces and mode-mode (Duschinsky) couplings affect pump-probe and two-dimensional electronic spectra. This approach, based on the thawed Gaussian wavepacket propagation, is the first method that can exactly evaluate the nonlinear electronic spectra of many-dimensional, shifted, distorted, and Duschinsky-rotated harmonic potentials. As in the case of linear absorption and emission spectra, we again use the concept of thermo-field dynamics to include non-zero temperature effects in the simulation of two-dimensional electronic spectra. The resulting wavepacket picture of two-dimensional spectroscopy at arbitrary temperature could strongly impact how we interpret and simulate multidimensional spectra in the future.

EPFL2021