Water radiolysis by low-energy carbon projectiles from first-principles molecular dynamics

RESEARCH ARTICLE Water radiolysis by low-energy carbon projectiles from first-principles molecular dynamics Jorge Kohanoff1,2*, Emilio Artacho1,3,4,5 a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 1 Atomistic Simulation Centre, Queen’s University Belfast, Belfast BT7 1NN, Northern Ireland, United Kingdom, 2 Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, United Kingdom, 3 Theory of Condensed Matter, Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, United Kingdom, 4 CIC Nanogune and DIPC, Tolosa Hiribidea 76, 20018 San Sebastián, Spain, 5 Basque Foundation for Science Ikerbasque, 48013 Bilbao, Spain * Abstract OPEN ACCESS Citation: Kohanoff J, Artacho E (2017) Water radiolysis by low-energy carbon projectiles from first-principles molecular dynamics. PLoS ONE 12 (3): e0171820. doi:10.1371/journal.pone.0171820 Editor: Danilo Roccatano, University of Lincoln, UNITED KINGDOM Received: July 18, 2016 Accepted: January 26, 2017 Published: March 7, 2017 Copyright: © 2017 Kohanoff, Artacho. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Water radiolysis by low-energy carbon projectiles is studied by first-principles molecular dynamics. Carbon projectiles of kinetic energies between 175 eV and 2.8 keV are shot across liquid water. Apart from translational, rotational and vibrational excitation, they produce water dissociation. The most abundant products are H and OH fragments. We find that the maximum spatial production of radiolysis products, not only occurs at low velocities, but also well below the maximum of energy deposition, reaching one H every 5 Å at the lowest speed studied (1 Bohr/fs), dissociative collisions being more significant at low velocity while the amount of energy required to dissociate water is constant and much smaller than the projectile’s energy. A substantial fraction of the energy transferred to fragments, especially for high velocity projectiles, is in the form of kinetic energy, such fragments becoming secondary projectiles themselves. High velocity projectiles give rise to well-defined binary collisions, which should be amenable to binary approximations. This is not the case for lower velocities, where multiple collision events are observed. H secondary projectiles tend to move as radicals at high velocity, as cations when slower. We observe the generation of new species such as hydrogen peroxide and formic acid. The former occurs when an O radical created in the collision process attacks a water molecule at the O site. The latter when the C projectile is completely stopped and reacts with two water molecules. Data Availability Statement: All relevant data are within the paper and its Supporting Information files, except MD trajectories which are accessible at the following page: URL: https://www.repository. cam.ac.uk/handle/1810/262388 DOI: https://doi. org/10.17863/CAM.7651. Introduction Funding: This work was supported by Wellcome Trust: flexible travel award 084069/Z/07/Z, https://wellcome.ac.uk/; Electron-Stopping 333813, European Commission, Marie-Curie CIG, http://ec. europa.eu/research/mariecurieactions/; FIS2012-37549-C05, Spanish Ministry of Water dissociation and the formation of other molecules by the action of radiation is one of the most important radiolytic processes, and has been studied for over a century by many authors. [1] While the main interest in the subject is traditionally related to biological implications, [1, 2] and to nuclear reactor design, [3] it recently came into focus also within the energy context, due to the possibility of generating hydrogen at low cost. [4] We will focus this study on ionic projectiles, and will not consider electromagnetic radiation. The two main natural PLOS ONE | DOI:10.1371/journal.pone.0171820 March 7, 2017 1 / 11 Water radiolysis by carbon projectiles from first-principles Science, http://www.idi.mineco.gob.es/stfls/ MICINN/Ayudas/; Exp. 97/14 (Wet Nanoscopy) from the Programa Red Guipuzcoana de Ciencia, Tecnologia e Innovacion, Diputacion Foral de Gipuzkoa, http://www.gipuzkoa.eus/ subvenciones/; and UKCP consortium EP/ F037325/1, http://www.ukcp.ac.uk/. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist. occurrences of ions are: in space in the form of cosmic rays (mostly protons, α-particles and electrons), and as products of radioactive decay in radionuclides. However, high-energy ions can also be produced in accelerators and used as radio-therapeutic tools (hadron-therapy). In either case, it is of major interest to understand, at the microscopic level, how do protons, αparticles and heavier ions like C+q interact and split water or, in a biological context, produce reactive fragments that induce biological end-point effects such as DNA damage. Most of these particles are very energetic (keV to MeV). When water is exposed to radiation of this nature the main effect is ionization, whereby electrons in the water orbitals are removed. The result is a characteristic distribution of secondary electrons whose kinetic energy peaks at low kinetic energies and then decreases monotonically. [5, 6] Other collision channels such as ion-molecule direct impact have exceedingly small cross sections in this regime, and can be ignored. The ionization regime can be described quite well in terms of binary collisions with individual water molecules (gas phase) where the electronic structure is corrected for the influence of the environment (condensed phase). [7] The information on scattering cross sections can then be used to study radiation tracks via Monte Carlo simulations. [7, 8] As ions travel through the medium ionizing the water, they gradually lose their energy. Initially, the ionization cross section is small, but when their velocity approaches that of the electrons in the water orbitals, a resonance phenomenon takes place and a peak in the absorbed dose is observed (Bragg peak), which for carbon corresponds to a state of charge approximately C3+. [9] Beyond the Bragg peak, the ionization cross section and the velocity of the ions rapidly decrease while the ions capture additional electrons. Below a certain threshold, ionic projectiles do not have enough kinetic energy to ionize water. The electronic excitation channel remains open, but only briefly. Water is an electronic wide gap insulator like LiF, for which the existence of a projectile-velocity threshold for electronic excitation has been shown [10] (and partly understood. [11, 12]) to be between 0.1 and 0.2 atomic units of velocity. For carbon projectiles and using what learned for LiF, the electronic excitation channel should es (...truncated)