Ultrafast dynamics of water system under photoexcitation

Abstract

<sec>Experimental techniques and theoretical calculations have made significant breakthroughs recently in realizing the ultrafast time resolution and the understanding of microscopic details on an atomic scale, which has brought new insights into the ultrafast microscopic dynamics of water system and aqueous system. Here we focus on the dynamic processes of ionization, dissociation, as well as plasmonization of water molecules, water clusters, and liquid water under different intensities of light excitation.</sec><sec>The pump-probe technique allows one to extract the information about the orbital-dependent phase shift during photoionization, corresponding to delays on a time scale from tens to hundreds of attoseconds. Delay time in photoionization is found to be proportional to the delocalization of molecular/cluster orbitals. In addition, the information related to the Feshbach resonance is also of interest. By solving the scattering equations, the detailed information about the scattering cross section, the <i>β</i>-parameter, and the involved Rydberg orbitals during valence electron ionization of water is obtained.</sec><sec>For liquid water undergoing irradiation by an optical field, the tunneling electrons are unable to move away and recombine with the parent molecule on a time scale of 100 fs if the external field is not strong enough (e.g. ~100 kV/cm). For terahertz fields as high as 250 kV/cm, electrons will move away from the parent molecule after tunnelling and undergo decoherence on a 10-fs time scale. At the beginning of tunneling the electrons will be more delocalized and will collapse to a certain position on a time scale of ~1 ps, and then slowly diffuse or recombine with holes on a longer time scale. For the strong excitation case, hot electrons may also be formed. When a hot electron is located on a particular water molecule, the O—H bond will be broken.</sec><sec>When an electron ionizes away, a hole will be created. The hole will be located on a water molecule within 10 fs, and will trigger off subsequent processes such as proton transfer and coherent oscillations. In particular, after the hole is localized and before the proton is transferred, there is a brief appearance of the metastable <inline-formula><tex-math id="M1">\begin{document}$ {{{\mathrm{H}}}_{2}{\mathrm{O}}}^{+} $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20240047_M1.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20240047_M1.png"/></alternatives></inline-formula> ion, whose lifetime is experimentally captured and is predicted to be (46±10) fs. The nuclear quantum effect in this process plays a key role.</sec><sec>Using the methods such as real time-time dependent density functional theory (rt-TDDFT), it is found that the water undergoes plasmonization under intense laser pulses corresponding to a field strength amplitude larger than 2.4 V/Å. The effective electron temperature in this period reaches over 20000 K. Strongly excited water in this state exhibits the behaviors of a liquid metal, and extremely strong nonthermal effect and nonadiabatic effect. In the process of plasmonization, a large fraction of chemical bonds in water molecules are broken and reorganize themselves, and many chemical species such as hydrogen molecules may appear, which also implies that laser-induced plasmonization can be used to synthesize new substances.</sec><sec>Although the previous researches have brought about a very rich understanding, we have also found some details that still need to be explored: i) the influence of nuclear quantum effects has not been taken into account in most of theoretical calculations, which may result in the inadequate description and inaccurate prediction; ii) some of the microscopic details observed in simulations do not yet have a direct counterpart in experimental measurements; iii) the current simulation of water plasmonization is for the local behavior under the spatially uniform external field, while in the real situation there are spatial inhomogeneity and energy flow, which urgently need larger-scale excited state dynamics simulations.</sec><sec>With the development of laser technology, the integration of water science and ultrafast technologies will be increasingly strong, so we believe that such a systematic understanding will play a key role in the future. It is expected that new research efforts will continue to contribute to a better understanding and the generation of new technologies in this exciting research field.</sec>