ABSTRACTThe focus of my graduate research has been to study how size, composition, and structure, influence the optical-electronic properties of nanoscale systems. Towards this goal, I have utilized ultrafast time-resolved spectroscopy to study a series of monolayer protected clusters (MPCs) and plasmonic nanoparticles in order to elucidate carrier relaxation gold nano-systems in the hope of providing insight for improvement. As a first research accomplishment, I determined the transition size (~1.7 nm) between non-metallic and metallic electron behavior for gold nanoclusters. Having determined this characteristic transitional point, I divided subsequent research into three thrusts. The first was to expand the understanding of composition and structure domain dependence of carrier dynamics in ~1.7 nm size regime using ultrafast transient extinction spectroscopy. The second was to explore the ultrafast carrier dynamics in larger metallic nano-systems that are used widely in photo-driven applications. Primarily, my focus was to understand electron-electron scattering processes which relax in less than 500 fs. A fundamental understanding of this electron-electron scattering process is essential for understanding the quantum efficiency of utilizing the hot electrons. The last was to develop spatially resolved ultrafast spectroscopies in order to push our ability in studying structurally complicated systems such as layer materials which contain interesting optical-electronic properties but also have inherent heterogeneity problems that hinder the correlation of specific properties to the structure information. Explicitly, I developed spatially resolved two-dimensional electronic spectroscopy to fulfill this purpose.After the investigation of structure-dependent carrier relaxation dynamics at this transition point, the influence of structural modifications was characterized at the transition point. Specifically, the influence of Ag alloying on the relaxation pathways of the Au144(SR)60 cluster were studied. It was observed that the efficiency of electron-phonon coupling increased as a function of increasing silver alloying. These structure domain-dependent carrier dynamics studies were achieved by employing a state-selective pump-probe technique. Different vibration-assisted carrier relaxation channels were identified. In chapter 5, I demonstrate that excited carriers in Au144 cluster relax through three observable vibration-assisted channels, 2 THz, 1.44 THz, and 0.67 THz depends on where those carriers were located domain-dependent after excitation. These findings provided insight into carrier relaxation in the 144-atom gold cluster, and potential pathway in the modification of the carrier relaxation through structure engineer in MPCs.After the identification and characterization of the transitional point between metallic and nonmetallic nanoscale gold, two-dimensional electronic spectroscopy (2DES) was developed and utilized to study carrier relaxation in purely metallic systems. Here, plasmonic gold nanorods (NRs) were chosen as a model system of study. Leveraging the ultrafast time resolution and the ability to retrieve the homogeneous linewidth of the sample, I was able to determine the electron-electron scattering time constant to be around 150 fs for the NRs we studied. The process observed in Chapter 6 represents the build-up process of Fermi-Dirac distribution from athermal electron gas.Having observed the sensitive correlation between structural and electronic properties of nanoscale systems, I worked to develop a method designed to better directly probe structural influences. In chapter 7, I described the work of developing a spatially resolved two-dimensional electronic spectroscopy (sr-2DES), which facilitated our correlation of linear extinction and nonlinear sr-2DES signals. As a prototype experiment, thin films of aggregated CdSe nanocrystals were studied to demonstrate the combined spectral, temporal, and imaging capabilities of this method. The structural influence, i.e., the conjugation of the nanocrystal, was observed to result in a redshift of steady absorption and accelerated carrier relaxation dynamics.