Extreme ultraviolet (EUV) photons expose photoresists by complex interactions starting with photoionization that create primary electrons (), followed by ionization steps that create secondary electrons (10 to 60 eV). Ultimately, these lower energy electrons interact with specific molecules in the resist that cause the chemical reactions which are responsible for changes in solubility. The mechanisms by which these electrons interact with resist components are key to optimizing the performance of EUV resists. A resist exposure chamber was built to probe the behavior of electrons within photoresists. Resists were exposed under electron beam and then developed; ellipsometry was used to identify the dependence of electron penetration depth and number of reactions on dose and energy. Additionally, our group has updated a robust software that uses a first principles-based Monte Carlo model called low-energy electron scattering in solids (LESiS) to track secondary electron production, penetration depth, and reaction mechanisms within materials-defined environments. LESiS was used to model the thickness loss experiments to validate its performance with respect to simulated electron penetration depths to inform future modeling work.