Applications of response theory to excited electronic states : metal oxide clusters and non-linear absorption in squaraine dyes
Abstract
Over the past one hundred years, an incredible range of electronic structure methods have been developed providing great insight into the quantum chemistry involved in chemical processes, including reaction pathways, structure and dynamics. The quantum mechanics of excited states provides a challenge for electronic
structure methods. Initially, this thesis inspects the ability of coupled cluster methods and time dependent density functional theory (TD-DFT) to accurately characterize the excitation energies and properties of titanium oxide clusters. By utilizing
a hierarchy of coupled cluster methods, the convergence of excited state properties in terms of systematic electron correlation has been presented along with an
examination of convergence towards the complete basis set limit. Major deficiencies were seen in second order CC2 for the clusters, attributed to a large singles
amplitudes in the Hartree-Fock orbitals. TD-DFT allowed for the study of larger
clusters, too demanding for CC. Chromium oxide clusters yielded similar results.
Having established the successes and limitations of electronic structure methods to
calculate excited state properties of transition metal oxide clusters, attention was
directed towards organic molecular systems - squaraine dyes. Squaraines, which
have an application in photodynamic therapy, exhibit interesting photochemistry
which was explored using linear and quadratic response functions in TD-DFT. The
one-photon and two-photon absorption spectra were determined in this way, while
the a posteriori Tamm-Dancoff Approximation (ATDA) was also utilized to determine two photon absorption cross sections using a three state model. Vertical
excited states from the first excited state geometry gave rise to the ESA spectra
determined by ATDA TD-DFT. To summarise, the power, scope and limitations
of electronic structure methods has been explored in two very different chemical
systems - transition metal oxide clusters and organic molecular squaraines, with
interesting results.