第一性原理分子动力学研究低能碳射弹对水的辐解
Water Radiolysis by Low-Energy Carbon Projectiles from First-Principles Molecular Dynamics
摘要：
用第一性原理分子动力学研究了低能碳射弹对水的辐解。动能在175 eV和2.8 keV之间的碳射弹穿过液态水。除了平移、旋转和振动激发外，它们还产生水离解。最丰富的产物是H和OH片段。我们发现，辐解产物的最大空间生产，不仅发生在低速度，但也远低于最大的能量沉积，达到一个H每5昂在最低速度研究（1玻尔/fs），解离碰撞更显着，而在低速度所需的能量解离水是恒定的，远小于弹丸的能量。转移到碎片上的能量，特别是高速射弹的能量，有很大一部分是以动能的形式转移的，这些碎片本身成为次级射弹。高速射弹引起定义明确的二元碰撞，这应该服从二元近似。对于低速情况则不是这样，在低速情况下会观察到多个碰撞事件。H次级射弹在高速时倾向于以自由基的形式运动，而在低速时倾向于以阳离子的形式运动。我们观察到新物种的产生，如过氧化氢和甲酸。前者发生在碰撞过程中产生的O自由基攻击O位点的水分子时。后者是当C射弹完全停止并与两个水分子反应时。
英文摘要：
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.
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