Electronic Excitation Dynamics in Liquid Water under Proton Irradiation

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.

K -Shell Core-Electron Excitations in Electronic Stopping of Protons in Water from First Principles

“…On the other hand, the excited electron distribution is not so localized along the projectile proton path as shown in Figure 4, and the excited electron distribution decreases only by ~10% even at 5 Å away from the path. This indicates that individual water molecules are indeed ionized along the projectile path in the electronic stopping process, consistent with our earlier finding 17 and also with the established notion of proton radiation as ionizing radiation. The K-shell core electron excitations still contribute greatly to the stopping power even when only a small proportion of the excited electrons are excited from the K-shell core states because the core excitation energy is a few orders of magnitude greater than the valence excitation energy.…”

“…In radiation oncology, an empirical factor such as relative biological effectiveness is used to take into account differences between the proton radiation and X-ray photon radiation for convenience, but many now call for a better mechanistic understanding of the radiation at the molecular level 19 . In this Letter, we discuss the role of K-shell core electron excitations in liquid water under proton irradiation by accurately determining the electronic stopping power and simulating quantum dynamics of electronic excitations from first principles.We apply our recently developed non-equilibrium dynamics simulation approach based on real-time timedependent density functional theory (RT-TDDFT) [17][18][20][21][22][23] to simulate the non-perturbative response of the electronic system to a fast-moving projectile proton. In this approach, the electronic stopping power can be obtained from the rate of electronic energy change at different projectile proton velocities as discussed in our earlier work 21,24 .…”

“…We apply our recently developed non-equilibrium dynamics simulation approach based on real-time timedependent density functional theory (RT-TDDFT) [17][18][20][21][22][23] to simulate the non-perturbative response of the electronic system to a fast-moving projectile proton. In this approach, the electronic stopping power can be obtained from the rate of electronic energy change at different projectile proton velocities as discussed in our earlier work 21,24 .…”

Heavy ion ranges from first-principles electron dynamics

“…Relatively recently, time-dependent density functional theory (TDDFT) has been used to determine the electronic stopping for a number of projectile-target combinations. [24][25][26][27][28][29][30] Unlike the free electron gas-based models, 31 TDDFT explicitly takes into account the effects of inhomogeneity in electron density due to the underlying lattice structure, band structure, 32 band gap, 14 and core-state excitations. [33][34][35] Furthermore, the charge state of the projectile need not be imposed in TDDFT calculations, but rather is an outcome of the method itself.…”

X-ray Absorption Spectral Signature of Quantum Nuclei in Liquid Water

“…For intermediate-range and long-range orderings, the observed red and blue shifts of the main-edge and post-edge are attributed to the so-called competing quantum effects, under which both the weak and well-formed H-bonds are promoted. The theoretical spectra are in nearly quantitative agreement with the available experimental data.The nature of H-bond network in water continues to be at the center of scientific interests [1][2][3][4][5][6][7][8][9][10][11][12]. Recently, high-resolution X-ray absorption spectroscopy (XAS), has emerged to be a powerful experimental technique to probe the water structure at molecular scale [13][14][15][16][17][18][19].…”