It is widely recognized nowadays that conical intersections ofmolecular potential-energy surfaces play a key mechanistic role in thespectroscopy of polyatomic molecules, photochemistry and chemicalkinetics. This invaluable book presents a systematic exposition of thecurrent state of knowledge about conical intersections, which has beenelaborated in research papers scattered throughout the chemicalphysics literature.

The concept of adiabatic electronic potential-energy surfaces, defined by the Born?Oppenheimer approximation, is fundamental to our thinking about chemical processes. Recent computational as well as experimental studies have produced ample evidence that the so-called conical intersections of electronic energy surfaces, predicted by von Neumann and Wigner in 1929, are the rule rather than the exception in polyatomic molecules. It is nowadays increasingly recognized that conical intersections play a key mechanistic role in chemical reaction dynamics. This volume provides an up-to-date overview of the multi-faceted research on the role of conical intersections in photochemistry and photobiology, including basic theoretical concepts, novel computational strategies as well as innovative experiments. The contents and discussions will be of value to advanced students and researchers in photochemistry, molecular spectroscopy and related areas.

The central subject of this thesis is the theoretical description of ultrafast dynamical processes in molecular systems of chemical interest and their control by laser pulses. This work encompasses different cutting-edge methods in quantum chemistry, quantum dynamics and for the rigorous description of the interaction of light and matter at the molecular level. It provides a general quantum mechanical framework for the description of chemical processes guided by laser pulses, in particular near conical intersections, i.e. geometries where the nuclear and electronic motions couple and the molecule undergoes non-adiabatic (or non-Born-Oppenheimer) dynamics. In close collaboration with experimentalists, the author succeeds in making a decisive step to link and to apply quantum physics to chemistry by transferring state of the art techniques and concepts developed in physics to chemistry, such as “light dressed atoms and molecules” and “adiabatic Floquet theory”. He applies these techniques in three prototypic model systems (aniline, pyrazine and NHD2) using high-level electronic structure calculations. Readers will enjoy the comprehensive and accessible introduction to the topic and methodology, as well as the clear structure of the thesis.

It is widely recognized nowadays that conical intersections of molecular potential-energy surfaces play a key mechanistic role in the spectroscopy of polyatomic molecules, photochemistry and chemical kinetics. This invaluable book presents a systematic exposition of the current state of knowledge about conical intersections, which has been elaborated in research papers scattered throughout the chemical physics literature.Section I of the book provides a comprehensive analysis of the electronic-structure aspects of conical intersections. Section II shows the importance of conical intersections in chemical reaction dynamics and gives an overview of the computational techniques employed to describe the dynamics at conical intersections. Finally, Section III deals with the role of conical intersections in the fields of molecular spectroscopy and laser control of chemical reaction dynamics.This book has been selected for coverage in:• CC / Physical, Chemical & Earth Sciences• Chemistry Citation Index(tm)• Index to Scientific Book Contents® (ISBC)

INTRODUCING A POWERFUL APPROACH TO DEVELOPING RELIABLE QUANTUM MECHANICAL TREATMENTS OF A LARGE VARIETY OF PROCESSES IN MOLECULAR SYSTEMS. The Born-Oppenheimer approximation has been fundamental to calculation in molecular spectroscopy and molecular dynamics since the early days of quantum mechanics. This is despite well-established fact that it is often not valid due to conical intersections that give rise to strong nonadiabatic effects caused by singular nonadiabatic coupling terms (NACTs). In Beyond Born-Oppenheimer, Michael Baer, a leading authority on molecular scattering theory and electronic nonadiabatic processes, addresses this deficiency and introduces a rigorous approach--diabatization--for eliminating troublesome NACTs and deriving well-converged equations to treat the interactions within and between molecules. Concentrating on both the practical and theoretical aspects of electronic nonadiabatic transitions in molecules, Professor Baer uses a simple mathematical language to rigorously eliminate the singular NACTs and enable reliable calculations of spectroscopic and dynamical cross sections. He presents models of varying complexity to illustrate the validity of the theory and explores the significance of the study of NACTs and the relationship between molecular physics and other fields in physics, particularly electrodynamics. The first book of its king Beyond Born-Oppenheimer: * Presents a detailed mathematical framework to treat electronic NACTs and their conical intersections * Describes the Born-Oppenheimer treatment, including the concepts of adiabatic and diabatic frameworks * Introduces a field-theoretical approach to calculating NACTs, which offers an alternative to time-consuming ab initio procedures * Discusses various approximations for treating a large system of diabatic Schrödinger equations * Presents numerous exercises with solutions to further clarify the material being discussed Beyond Born-Oppenheimer is required reading for physicists, physical chemists, and all researchers involved in the quantum mechanical study of molecular systems.

Photochemistry is an important part of both chemistry and biology and is of great practical significance for the development of sustainable sources of energy. The mechanisms of photochemistry are far from trivial and far from understood. There are limits to how well theory can describe the processes and how well experiments can resolve them. This book aims to provide an overview of state-of-the-art methods for both theoretical development and experimental techniques, with a focus on ultrafast molecular processes and electronic excitation of organic molecules. These fields are active and progress is being made, carried by the increasing speed of computation and the development of new light sources, most notably X-ray sources at large facilities. Alongside these two layers of theoretical development and experimental techniques is a third layer—model building. In this layer, model building tries to find similarities in seemingly unrelated experimental results and deepen our general knowledge of photoinduced processes. Often, progress is made not by cutting-edge techniques but rather by using well-established techniques with a great variety of molecules—this approach promises less glory but is just as important as the first two layers. Examples mentioned in the text are the Woodward–Hoffman rules and the dynamophore concept. All three layers are crucial to push our knowledge further and, eventually, to use it for developing new and more advanced optical devices.