Photoluminescence of Diamondoids: Experiment and Theory

In this thesis, the photoluminescence of diamondoids has been investigated. Diamondoids are a class of purely sp3-hybridized hydrocarbons with a carbon framework congruent with the bulk diamond lattice. The valences at the diamondoid surface are passivated with hydrogen. Diamondoids are size- and shape- selectable, which makes them ideal model systems to investigate the influence of size and shape on optical and electronic properties in nanoparticles. In this work, the vibrationally resolved photoluminescence of twelve different structures with five different sizes has been investigated. For the investigations, samples of a single structural isomers were brought into the gas phase and excited with laser or synchrotron radiation. The radiation emitted in the subsequent relaxation process was spectroscopically analyzed. The photoluminescence spectra of the diamondoids show vibrational fine structure unique to each isomer. To understand the the photophysics behind the spectra, quantum chemical calculations with density functional theory and time-dependent density functional theory have been performed. It was possible to simulate and reproduce the experimentally observed vibrational structure to a very high degree. With the help of the calculations it could be shown that the composition of the spectra exhibits a size dependence. The spectra of smaller diamondoids are dominated by a relaxation into several overtones of a few ground state normal modes and combinations thereof, whereas the spectra of larger diamondoids are almost exclusively combinations of fundamental transitions of many vibrational ground state modes. Another part of the thesis is dedicated to the investigation of thermo-optical effects. Specifically, the influence of excitation energy and temperature on the spectral congestion of the vibrational structure. These and many other phenomena are discussed in detail.