Abstract
Molecular behaviour of liquid water under proton irradiation is of great importance to a number of technological and medical applications. The highly energetic proton generates a time-varying field that is highly localized and heterogeneous at the molecular scale, and massive electronic excitations are produced as a result of the field-matter interaction. Using first-principles quantum dynamics simulations, we reveal details of how electrons are dynamically excited through non-equilibrium energy transfer from highly energetic protons in liquid water on the atto/femto-second time scale. Water molecules along the path of the energetic proton undergo ionization at individual molecular level, and the excitation primarily derives from lone pair electrons on the oxygen atom of water molecules. A reduced charge state on the energetic proton in the condensed phase of water results in the strongly suppressed electronic response when compared to water molecules in the gas phase. These molecular-level findings provide important insights into understanding the water radiolysis process under proton irradiation.
Highlights
The use of ion beams for treating cancer was first conceptualized in 19461
The dynamical response of electrons to the energetic proton is simulated using a real-time time-dependent density functional theory (RT-TDDFT) approach as discussed in the Methods section, and technical details of how this non-equilibrium quantum dynamics simulations can be used to study electronic stopping process is discussed in our recent work[24]
The equilibrium structure of the liquid water molecules at room temperature was prepared by performing a separate first-principles molecular dynamics simulation
Summary
The use of ion beams for treating cancer was first conceptualized in 19461. Today, the use of highly energetic protons for cancer therapy has become increasingly more popular, and more than a few dozen ion beam therapy facilities have been constructed worldwide[2,3]. The use of charged-particle radiation such as fast, energetic protons over more conventional radiation based on photons or electrons is often considered more attractive because of its distinct energy deposition profile[4,5] This so-called Bragg curve characterizes the energy loss of ionizing radiation as it penetrates through matter. The electrons in the liquid water are excited by the fast ion through a non-equilibrium energy transfer process[12] For molecular liquids such as water, molecular details of the excitation process are crucial for developing a better understanding of the radiation-induced cancer therapy that utilizes proton and other ion irradiations[13]. Using non-equilibrium electron dynamics simulations based on our new large-scale real-time time-dependent density functional theory approach[22,23], key features in the electronic excitation are revealed at both mesoscopic and molecular levels for deciphering the water radiolysis mechanism under proton irradiation
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