Ultrafast Energy Transfer from Solvent to Solute Induced by Subpicosecond Highly Intense THz Pulses

The ultrafast energy transfer from an intense, subpicosecond THz pulse to bulk water at 300 K and density 1 g/cm³ is simulated by ab initio molecular dynamics with explicit inclusion of the laser pulse. A 200 fs subcycle pulse of intensity 5 × 10¹² W/cm² corresponding to a peak field amplitude of 0.6 V/Å and achievable nowadays using optical rectification techniques results in a temperature jump from 300 K up to ∼1000 K within the first picosecond after the pulse. We discuss in detail the time-dependent structural changes caused by the THz pulse in the water medium and suggest possible ways to measure those changes by pump–probe experimental techniques. The ultrafast energy transfer from the energized water molecules to a solute molecule is studied on a test system, phenol. We find that phenol is, in the gas phase, insensitive to the THz pulse and only gains energy in solution via collisional energy transfer with the water molecules in its environment. The reason for this is found in the mode of interaction of the THz pulse with the aqueous medium. In short, water molecules respond mainly through their permanent dipole moments trying to orient themselves in the strong electric field of the pulse and disrupting their hydrogen-bonding structure. As compared with the water molecule, phenol has a smaller but still substantial permanent dipole moment. The moments of inertia of phenol are, however, too large for it to rotate in the short duration of the THz pulse. Therefore, the direct heating-up mechanism is mostly selective to the solvent molecules, whereas the solute heats up indirectly via collisions with its hot environment in about 1 to 2 ps.