Quantum Mechanical Studies of Nonadiabatic Systems

Abstract

Understanding nonadiabatic processes is tantamount to understanding the mechanisms underlying phenomena such as energy transfer in photovoltaic cells and catalysis at metal surfaces. A complete quantum description of such events is unfortunately intractable, but recent simulations show great promise in approximately but accurately modeling nonadiabatic systems. Perhaps the first step towards modeling these systems is the proper description of the relevant electronic states involved. We discuss two specific systems in which these states have been calculated, allowing for accurate dynamical simulations to be carried out. The first system described here is a model for intramolecular singlet fission in bipentacenes. Singlet fission, the process by which photoexcited singlet excitons spontaneously split into two lower energy triplet excitons, has received much attention as a promising avenue towards increasing solar cell efficiency. Recently, the blueprint for controlled synthesis of acene dimers has been utilized to create chromophores exhibiting efficient singlet fission, rivaling that of the best crystalline systems. Examining a specific dimer system (2,2-bipentacene), we show that intramolecular singlet fission proceeds nonadiabtically through an avoided crossing between single and multiexcitonic states. Subsequent dynamic calculations reveal singlet fission occurring on ultrafast timescales in agreement with experiment, supporting the proposed mechanism. Bipentacenes are not only interesting for their singlet fission capabilities, but also for their unusual spectral features in the visible region. Depending on bonding geometries, the spectrum of bipentacenes can be significantly altered (a second absorption peak appears in the visible) from that of the monomer. Despite first being observed in 1948, this spectral feature has not been properly described. We algebraically detail the origin of this spectral perturbation and give simple design principles for oligoacenes backed by intuitive molecular orbital arguments. Finally, we discuss a model for energy transfer between diatomics and metal surfaces. Despite being extensively studied experimentally, an adequate theoretical model, accurate across all experimental regimes, has not emerged. Exploiting the simplicity of a Schmidt decomposition of the single particle states of the Newns Hamiltonian, we show how intuitive local states can be constructed and utilized in dynamic calculations.