  Understanding how the electronic excitation of DNA changes in response to
different high-energy particles is central to advancing ion beam cancer
therapy. While protons have been the predominant ion of choice in beam cancer
therapy, heavier ions, particularly carbon ions, have drawn significant
attention over the past decade. Carbon ions are expected to transfer larger
amounts of energy accordingly to the linear response theory. However, the
molecular-level details of the electronic excitation under heavier ion
irradiation remain unknown. In this work, we use real-time time-dependent
density functional theory simulations to examine the quantum-mechanical details
of DNA electronic excitations in water under proton, alpha-particle, and carbon
ion irradiation. Our results show that the energy transfer does indeed increase
for the heavier ions, while the excitation remains highly conformal. However,
the increase in the energy transfer rate, measured by electronic stopping
power, does not match the prediction by the linear response model, even when
accounting for the velocity-dependence of the irradiating ions charge. The
simulations also reveal that while the number of holes generated on DNA
increases for heavier ions, the increase is only partially responsible for the
larger stopping power. Larger numbers of highly energetic holes formed from the
heavier ions also contribute significantly to the increased electronic stopping
power.
