Mishra Pankaj Kr, Bettaque Vincent, Vendrell Oriol, Santra Robin, Welsch Ralph
Center for Free-Electron Laser Science , DESY , Notkestraße 85 , D-22607 Hamburg , Germany.
The Hamburg Centre for Ultrafast Imaging , University of Hamburg , Luruper Chaussee 149 , D-22761 Hamburg , Germany.
J Phys Chem A. 2018 Jun 14;122(23):5211-5222. doi: 10.1021/acs.jpca.8b00828. Epub 2018 Jun 1.
Ultrashort, high-intensity terahertz (THz) pulses, e.g., generated at free-electron laser facilities, allow for direct investigation as well as the driving of intermolecular modes in liquids like water and thus will deepen our understanding of the hydrogen bonding network. In this work, the temperature-jump (T-jump) of water induced by THz radiation is simulated for ten different THz frequencies in the range from 3 to 30 THz and five different pulse intensities in the range from 1 × 10 to 5 × 10 W/cm employing both ab initio molecular dynamics (AIMD) and force field molecular dynamics (FFMD) approaches. The most efficient T-jump can be achieved with 16 THz pulses. Three distinct T-jump mechanisms can be uncovered. For all cases, the T-jump mechanism proceeds within tens of femtoseconds (fs). For frequencies between 10 and 25 THz, most of the energy is initially transferred to the rotational degrees of freedom. Subsequently, the energy is redistributed to the translational and intramolecular vibrational degrees of freedom within a maximum of 500 fs. For the lowest frequencies considered (7 THz and below), translational and rotational degrees of freedom are heated within tens of fs as the THz pulse also couples to the intermolecular vibrations. Subsequently, the intramolecular vibrational modes are heated within a few hundred fs. At the highest frequencies considered (25 THz and above), vibrational and rotational degrees of freedom are heated within tens of fs, and energy redistribution to the translational degrees of freedom happens within several hundred fs. Both AIMD and FFMD simulations show a similar dependence of the T-jump on the frequency employed. However, the FFMD simulations overestimate the total energy transfer around the main peak and drop off too fast toward frequencies higher and lower than the main peak. These differences can be rationalized by missing elements, such as the polarizability, in the TIP4P/2005f force field employed. The feasibility of performing experiments at the studied frequencies and intensities as well as important issues such as energy efficiency, penetration depth, and focusing are discussed.
超短、高强度太赫兹（THz）脉冲，例如在自由电子激光设施中产生的脉冲，能够直接研究并驱动诸如水等液体中的分子间模式，从而加深我们对氢键网络的理解。在这项工作中，利用从头算分子动力学（AIMD）和力场分子动力学（FFMD）方法，针对3至30太赫兹范围内的十种不同太赫兹频率以及1×10至5×10瓦/平方厘米范围内的五种不同脉冲强度，模拟了太赫兹辐射引起的水的温度跃升（T-jump）。使用16太赫兹脉冲可实现最有效的温度跃升。可以发现三种不同的温度跃升机制。在所有情况下，温度跃升机制在几十飞秒（fs）内发生。对于10至25太赫兹之间的频率，大部分能量最初转移到转动自由度。随后，能量在最多500飞秒内重新分配到平动和分子内振动自由度。对于所考虑的最低频率（7太赫兹及以下），由于太赫兹脉冲也耦合到分子间振动，平动和转动自由度在几十飞秒内被加热。随后，分子内振动模式在几百飞秒内被加热。在所考虑的最高频率（25太赫兹及以上），振动和转动自由度在几十飞秒内被加热，并且能量在几百飞秒内重新分配到平动自由度。AIMD和FFMD模拟均显示温度跃升对所采用频率的类似依赖性。然而，FFMD模拟高估了主峰周围的总能量转移，并且在高于和低于主峰的频率处下降过快。这些差异可以通过所采用的TIP4P/2005f力场中缺少诸如极化率等元素来解释。讨论了在所研究的频率和强度下进行实验的可行性以及诸如能量效率、穿透深度和聚焦等重要问题。
