Ultrafast energy transfer to liquid water by sub-picosecond high-intensity terahertz pulses: An ab initio molecular dynamics study

Liquid water is the most common environment for chemical and biological processes. Most of them occur as a consequence of thermal random fluctuations of the environment, which every once in a while create the conditions for a chemical reaction to occur. Therefore, bringing a large amount of energy to liquid water in a as short as possible time can open new avenues for the controlled exploration of thermally activated chemical reactions in liquids.

In this work we address the sub-picosecond response of liquid water to an intense and ultrashort THz pulse as those that can be generated at X-FELs and on state-of-the-art table top setups. The main questions that we try to answer are: how does energy get
transferred from the THz pulse to different vibrational modes of liquid water? How does the structure of water evolve as a function of time under the effect of the pulse?

We address these questions through ab-initio molecular dynamics simulations of liquid water interacting with a one-cycle THz pulse of intensity 10^10 W/cm2, 100 cm-1 (~3 THz) photon energy and a pulse duration of about 250 fs.

We find that after a very rapid disruption of the hydrogen bonding structure of the liquid, the water molecules start to violently move against each other. In a time-scale of about 500 fs, the temperature jump from 300 to about 900 K is completed and about 25
THz photons per water molecule have been absorbed. The energy flows from inter- to intra-molecular modes and a quasi-equilibrium state is reached within 1 ps. Dramatic structural changes can be seen in the modification of the O-O and O-H radial distribution
functions. A substantial modification of the time resolved X-ray diffraction (TR-XRD) pattern from double to single peaked during the THz pulse is a consequence of such ultrafast rearrangements (See Figure).

The properties as a matrix for chemical processes of the transiently hot and structureless phase of water at the density of the liquid will be the subject of future explorations.

Angew. Chem. Int. Ed. (2013)