Flash-photolysis time-resolved dielectric-loss (fp-TRDL) spectroscopy is a perturbative technique developed to measure the time dependent evolution of a molecule's electron density distribution; it is one of only a few methods to measure a solution-phase molecule's excited-state dipole moment in situ without relying on emission, which can bias observations to bright states and limit the broad applicability of the technique, for instance with dark molecular triplet states. In this technique, microwave radiation is used to measure the complex permittivity (ε r = εiε ) of a sample by monitoring the phase and amplitude changes of a transmitted waveform. Performing this measurement inside a resonance cavity increases the effective path length through the sample and thus improves the sensitivity to small changes in ε r . At the X-band frequencies used in these experiments (8.2 -12.4 GHz), changes in ε r from a solution are a function of solute concentration, a solute's dipole moment (µ), and its polarizability volume (V p ). Selectively exciting a photoredox catalyst with a pulsed laser allows measurement of ∆ε r due to the solute molecule's change in dipole moment and polarizability volume -the change in charge density and localization from the ground-state to the excited-state. Ultimately, measuring both ε r and ∆ε r captures the dipole moment and polarizability volume of a solution-phase sample, both in the ground and excited-states.