Excitation Of Liquid Water Research Articles

A model for Geant4-DNA to simulate ionization and excitation of liquid water by protons travelling above 100 MeV

Biological damage induced by ionizing radiation plays a major role in many application fields as radiotherapy and microdosimetry. Geant4-DNA Monte Carlo track-structure code has the capability to simulate the passage of radiation through liquid water, containing physical, physicochemical and chemical processes that lead the early DNA damage. For proton projectile, current models reach up to an incident energy of 100 MeV. In order to cover the entire energy regime involved in proton radiotherapy, this work presents a new model that extends proton ionization and excitation of liquid water up to 300 MeV. Calculation of cross section dataset is made for ionization of five ionization shells and five excitation levels of liquid water using the Relativistic Plane Wave Born Approximation (RPWBA). Implementation is validated through the spower and range examples of the official release, obtaining an agreement within 1% with respect to reference data published in ICRU90 report.

Coherent polaron dynamics of electrons solvated in polar liquids.

An electron solvated in a polar liquid is an elementary quantum system with properties governed by electric interactions with a fluctuating molecular environment. In the prevailing single particle picture, the quantum ground and excited states are determined by a self-consistent potential, as defined by the particular local configuration of the solvation shell. This description neglects collective many-body excitations, which arise from the coupling of electronic degrees of freedom and nuclear motions of the environment. While recent experiments have demonstrated collective nonequilbrium electronic-nuclear motion, i.e. polaron excitations in liquid water, their relevance in the broader context of polar liquids has remained unexplored. Here, we study the nonequilibrium dielectric response of the, compared to water, less polar alcohols isopropanol, and ethylene glycol, that also display a different hydrogen bond pattern. We demonstrate that ultrafast relaxation of photogenerated electrons impulsively induces coherent charge oscillations, which persist for some 10ps. They emit electric waves in a frequency range from 0.1 to 2 THz, depending on electron concentration. Oscillation frequencies and line shapes are reproduced by a unified polaron picture for alcohols and water, which is based on a Clausius-Mossotti local field approach for the THz dielectric function. The analysis suggests a longitudinal character of many-body polaron excitations and a weak coupling to transverse excitations, supported by the underdamped character of charge oscillations. Polaron dynamics are governed by the long-range Coulomb interaction between an excess electron and several thousands of polar solvent molecules, while local electron solvation geometries play a minor role.

Limitations (and merits) of PENELOPE as a track-structure code

Purpose: To outline the limitations of PENELOPE (acronym of PENetration and Energy LOss of Positrons and Electrons) as a track-structure code, and to comment on modifications that enable its fruitful use in certain microdosimetry and nanodosimetry applications.Methods: Attention is paid to the way in which inelastic collisions of electrons are modelled and to the ensuing implications for microdosimetry analysis.Results: Inelastic mean free paths and collision stopping powers calculated with PENELOPE and two well-known optical-data models are compared. An ad hoc modification of PENELOPE is summarized where ionization and excitation of liquid water by electron impact is simulated using tables of realistic differential and total cross sections.Conclusions: PENELOPE can be employed advantageously in some track-structure applications provided that the default model for inelastic interactions of electrons is replaced by suitable tables of differential and total cross sections.

Ion formation upon water excitation by IR laser radiation in the range of OH stretching vibrations

The ion formation and emission upon the excitation of liquid water in the range of OH stretching vibrations were observed and studied. The radiation source was a nanosecond parametric light oscillator tunable in the range of 2.7–3.3 μm. The ions were detected by an ion-mobility spectrometer.

Interaction-site-model description of collective excitations in liquid water. II: Comparison with simulation results

Theoretical results for dynamical correlation functions characterizing collective excitations in liquid water reported in a previous paper are compared with molecular dynamics simulation results performed on the same system. We also examine the validity and accuracy of the approximation scheme for memory functions employed in our theory by investigating whether the assumed form for memory functions and resulting expressions for dynamical correlation functions can be used as faithful models to reproduce the “experimental” data determined from the simulation.

Interaction-site-model description of collective excitations in liquid water. I: Theoretical study

Collective excitations in liquid water are investigated using the recently developed theory for dynamics of molecular liquids which is based on the interaction-site model for polyatomic fluids, the projection-operator formalism of Zwanzig and Mori, and the simple approximation scheme for memory functions. It is shown that all the essential features of collective excitations in water, reported previously by neutron-scattering experiments, molecular dynamics simulations and dielectric theories, are well reproduced by the present theory.

Collective excitations in liquid water at low frequency and large wave vector

We study the collective modes of water using molecular dynamics with the ST2 model potential, by calculating the dynamic structure factor S(q,ω). We identify the previously reported ‘‘first’’ sound mode to be due to the wave vector independent oxygen bending mode.

A conjecture concerning collective excitations in liquid water

We show that the distribution of rings of water molecules in a molecular dynamics simulation of liquid water is consistent with the hypothesis that there are collective coordinates that convert 6-rings to n-rings.