# Water Radiolysis by Low-Energy Carbon Projectiles from First-Principles Molecular Dynamics

## TL;DR

This study uses first-principles molecular dynamics to investigate how low-energy carbon projectiles cause water radiolysis, revealing the production of reactive species and energy transfer mechanisms across different projectile velocities.

## Contribution

It provides detailed insights into water radiolysis by low-energy carbon projectiles, including product formation, energy transfer, and collision dynamics, using first-principles simulations.

## Key findings

- Maximum radiolysis product production occurs at low velocities.
- High velocity projectiles produce well-defined binary collisions.
- New chemical species like hydrogen peroxide and formic acid are generated.

## Abstract

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 Ang 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.