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Developing a detailed understanding for the role of core electron excitationin liquid water under proton irradiation has become important due to thegrowing use of proton beams in radiation oncology. Using a first-principles,non-equilibrium simulation approach based on real-time time-dependent densityfunctional theory, we determine the electronic stopping power, thevelocity-dependent energy transfer rate from irradiating ions to electrons. Theelectronic stopping power curve agrees quantitatively over the entire velocityrange for which experimental data is available. Also notably, we observesignificant differences between our first-principles results and commonly-usedperturbation theoretic models. Excitations of the water molecules' oxygen coreelectrons are a crucial factor in determining the electronic stopping powercurve beyond its maximum. The core electron contribution is responsible for asmuch as one-third of the stopping power at the high proton velocity of 8.0 a.u.(1.6 MeV). These K-shell core electron excitations not only provide anadditional channel for the energy transfer but they also significantlyinfluence the valence electron excitation. In the excitation process, generatedholes remain highly localized within a few angstroms around the irradiatingproton path whereas electrons are excited away from the path. In spite of itsgreat contribution to the electronic stopping power, the K-shell electrons playa rather minor role in terms of the excitation density: Only one percent of thehole population comprises K-shell holes, even at the high proton velocity of8.0 a.u.. The excitation behavior revealed here is distinctly different fromthat of X/gamma-ray photon radiation, which is the most commonly used type ofionizing radiation in radiation oncology.

K-shell Core Electron Excitations in the Electronic Stopping of Protons in Water