For this study, a new microwave resonance cavity (figure ) was designed and fabricated to measure solution-phase samples based on a design by Fessenden et. al.. The experimental set-up consists of a 5 mm diameter EPR tube placed in the cavity at one electric field maximum of the T E 102 standing wave, and a single slit in the end wall allowing for photoexcitation. A circular iris structure is placed between the feeding waveguide and the cavity in order to couple microwave radiation into the cavity, and define one end of the resonator. For all samples in this work, a 9.20 mm diameter iris was used unless otherwise stated. Maxwell's equations were solved using the computer aided drafting (CAD) model and radio-frequency (RF) simulation package COMSOL Multiphysics (v4.3a). This allowed us to optimize the sample position and iris diameter before fabricating the microwave cavity; this design process has been described in our previous work. 45 After manufacturing the cavity, the reverse process was used to model the manufactured cavity in COMSOL to account for manufacturing defects, and extract quantitative values for the sensitivity of the cavity characteristics to the complex permittivity of the sample (ε and ε ). [described in section COMSOL Simulation of Microwave Cavities of the SI] By iteratively stepping the simulated sample's ε and ε inside the EPR tube, we create a look-up table of simulation results that we then use as a numerical fit function to quantify the dielectric properties of solution-phase samples; this enables us to convert measured microwave power reflectance as a function of frequency about the cavity resonance to ε and ε .