This book explores novel computational strategies for simulating excess energy dissipation alongside transient structural changes in photoexcited molecules, and accompanying solvent rearrangements. It also demonstrates in detail the synergy between theoretical modelling and ultrafast experiments in unravelling various aspects of the reaction dynamics of solvated photocatalytic metal complexes. Transition metal complexes play an important role as photocatalysts in solar energy conversion, and the rational design of metal-based photocatalytic systems with improved efficiency hinges on the fundamental understanding of the mechanisms behind light-induced chemical reactions in solution. Theory and atomistic modelling hold the key to uncovering these ultrafast processes. Linking atomistic simulations and modern X-ray scattering experiments with femtosecond time resolution, the book highlights previously unexplored dynamical changes in molecules, and discusses the development of theoretical and computational frameworks capable of interpreting the underlying ultrafast phenomena.

Since the first stimulated emission pumping (SEP) experiments more than a decade ago, this technique has proven powerful for studying vibrationally excited molecules. SEP is now widely used by increasing numbers of research groups to investigate fundamental problems in spectroscopy, intramolecular dynamics, intermolecular interactions, and even reactions. SEP provides rotationally pre-selected spectra of vibrationally highly excited molecules undergoing large amplitude motions. A unique feature of SEP is the ability to access systematically a wide variety of extreme excitations localized in various parts of a molecule, and to prepare populations in specific, high vibrational levels. SEP has made it possible to ask and answer specific questions about intramolecular vibrational redistribution and the role of vibrational excitation in chemical reactions.

This book offers historical and state-of-the-art molecular spectroscopy methods and applications in dynamic compression science, aimed at the upcoming generation in physical sciences involved in studies of materials at extremes. It begins with addressing the motivation for probing shock compressed molecular materials with spectroscopy and then reviews historical developments and the basics of the various spectroscopic methods that have been utilized. Introductory chapters are devoted to fundamentals of molecular spectroscopy, overviews of dynamic compression technologies, and diagnostics used to quantify the shock compression state during spectroscopy experiments. Subsequent chapters describe all the molecular spectroscopic methods used in shock compression research to date, including theory, experimental details for application to shocked materials, and difficulties that can be encountered. Each of these chapters also includes a section comparing static compression results. The last chapter offers an outlook for the future, which leads the next-generation readers to tackling persistent problems.

Electronic structure calculations have undergone incredible advancement in the past century. Using modern methods and supercomputing infrastructure we are now able to compute precise electron behavior in a variety of large and complex systems. However, these computations are only as good as their applications. To further these computations we consider two applications of efficient force calculations using first principles density functional theory. We compute the vibrational and Raman spectra for B-doped, P-doped, and B-P codoped Si nanocrystals using real-space pseudopotentials constructed within density functional theory. An experimental peak in the Raman spectra near 650 cm−1 observed in codoped nanocrystals can be best explained by the presence of B-P bonds, which are located near the surface of the nanocrystal. We propose that the spectral details of this peak are related to quantum confinement and the breaking of local symmetry associated with the phonon modes involving dopant bonds. We also illustrate an improved method for calculation of nonlocal contributions to interatomic forces is used to perform molecular dynamics simulations. This method results from the real space density functional theory Hamiltonian utilizing a high order Gaussian integration scheme in real space. The efficacy of this method is demonstrated through molecular dynamics simulations of an O2 molecule and a benzene molecule. Our method improves convergence of dynamic variables including stability and vibrational frequency