Solar fuel generation utilizing various nanomaterial light absorbers is a promising strategy to address current issues in energy production. Suitable materials for photochemical fuel production must combine properties of visible light absorption, suitable band edge potentials for catalysis, resistance to photooxidation, and long-lived excited states. Such requirements have revealed the need for materials with complex optical and excited state properties. Understanding these properties is important for improving material design for solar fuel generation. Herein, using femtosecond transient absorption (TA) spectroscopy, we study two complex metal alloy systems with properties relevant to solar fuel generation. The first part of this dissertation discusses optical and excited state properties in (Ga1-xZnx)(N1-xOx) nanoparticles. We discovered that (Ga1-xZnx)(N1-xOx) nanoparticles contain excited-state carriers with large reduced effective masses and revealed a free carrier density-dependent Burstein-Moss spectral shift. In addition, decay kinetics were found to exhibit a short-lived component assigned to trap mediated and Auger recombination and a long-lived component assigned to a broad distribution of trap states and trap-limited recombination. Furthermore, we studied these excited state properties in (Ga1-xZnx)(N1-xOx) nanoparticles with various elemental distributions and established that elemental distribution does not have a significant impact on recombination kinetics. The second part of the dissertation discusses the excited state properties in Ag-TiO2 films which are composed of Ag nanoparticles embedded in a mesoporous TiO2 host. TA spectroscopy was used to probe electron transfer between Ag and TiO2 upon visible and ultraviolet illumination. It was proposed that upon UV illumination, electron transfer from excited TiO2 to Ag nanoparticles occurs, and upon visible illumination, the surface plasmon resonance (SPR) in Ag is excited and a direct electron transfer into TiO2 follows. Revealing behavior of carriers in both of these systems upon illumination leads to insights for future material design and applications.</p
