The highly tunable optical properties of metal nanoparticles make them an ideal building block in any application that requires control over light, heat, or electrons on the nanoscale. Because of their size, metal nanoparticles both absorb and scatter light efficiently. Consequently, improving their performance in specific applications requires control over the balance between absorption and scattering to promote certain desired aspects of their optical response. Scattering by single metal nanoparticles is straightforward to characterize using dark-field scattering spectroscopy; however, methods to characterize pure absorption over a broad wavelength range have not yet been established. This thesis presents photothermal absorption spectroscopy as a tool to evaluate the absorption spectra of single metal nanostructures. The work presented in this thesis is both the refinement of this technique and also its application to studying the influence of nanoparticle coupling and nanoparticle composition on the non-radiative response. First, an experimental system that incorporated a tunable wavelength white light laser into a photothermal heterodyne imaging detection scheme was established. This approach to absorption spectroscopy was verified through agreement between measured and simulated spectra of Au nanospheres and nanorods. Next, the impact of nanoparticle coupling on the non-radiative response was investigated in Au nanoparticle oligomers. Experimental characterization revealed that the optimal excitation wavelength for promoting hot carrier generation coincided with the position of maximum absorption, a result directly relevant to the performance of these types of nanostructures in sensing and photocatalysis. Finally, the impact of nanoparticle composition and local morphology was examined in Pt-decorated Au nanorods. Correlated scattering, photoluminescence, and absorption spectra of single Pt-decorated Au nanorods exhibited a surprising range of diversity. Pt nanoparticle heterogeneity was found to play a significant role in determining the optical response of the hybrid nanostructures in part because Pt’s contributions were amplified by the nanorod’s enhanced local electric field. Overall, these results illustrate the need for experimental characterization of absorption in order to effectively optimize hot carrier generation, heat production, and photocatalytic activity in complex metal nanostructures.