  Developing a detailed understanding for the role of core electron excitation
in liquid water under proton irradiation has become important due to the
growing use of proton beams in radiation oncology. Using a first-principles,
non-equilibrium simulation approach based on real-time time-dependent density
functional theory, we determine the electronic stopping power, the
velocity-dependent energy transfer rate from irradiating ions to electrons. The
electronic stopping power curve agrees quantitatively over the entire velocity
range for which experimental data is available. Also notably, we observe
significant differences between our first-principles results and commonly-used
perturbation theoretic models. Excitations of the water molecules' oxygen core
electrons are a crucial factor in determining the electronic stopping power
curve beyond its maximum. The core electron contribution is responsible for as
much 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 an
additional channel for the energy transfer but they also significantly
influence the valence electron excitation. In the excitation process, generated
holes remain highly localized within a few angstroms around the irradiating
proton path whereas electrons are excited away from the path. In spite of its
great contribution to the electronic stopping power, the K-shell electrons play
a rather minor role in terms of the excitation density: Only one percent of the
hole population comprises K-shell holes, even at the high proton velocity of
8.0 a.u.. The excitation behavior revealed here is distinctly different from
that of X/gamma-ray photon radiation, which is the most commonly used type of
ionizing radiation in radiation oncology.
