Hf-based dielectric films are susceptible to formation of charge trapping, performance-limiting point defects, making improved characterization of these defects a high priority of research on high-k dielectrics. Here we report noninvasive optical characterization of defect spectra and charge trapping kinetics in Si/SiO2/Hf1-xSixO2 film stacks using internal multiphoton photoemission and time-dependent electrostatic field-induced second-harmonic (TD-EFISH) generation. Oxygen vacancy defects unique to as-deposited HfO2 films are identified by resonant two-photon ionization of the 3.24 eV transition from their occupied mid-gap ground states to conduction-band-edge excited states and subsequent tunneling of the photoelectrons to the Si substrate. These defects are found to be located within the HfO2 bulk, and to be quenched upon annealing or silication of the high-k dielectric layer. Charge trapping in Hf-silicate samples is found to be dominated by traps at the oxide surface. Trapping rate is nearly independent of excitation wavelength and alloy composition. By contrast, trap lifetimes are found to be as much as two orders of magnitude shorter in Hf-silicate surfaces than at HfO2 surfaces.