Subpicosecond energy transfer from a highly intense THz pulse to water: A computational study based on the TIP4P/2005 rigid-water-molecule model

The dynamics of ultrafast energy transfer to water clusters and to bulk water by a highly intense, subcycle THz pulse of duration $\approx 150$ fs is investigated in the context of force-field molecular dynamics simulations. We focus our attention on the mechanisms by which rotational and translational degrees of freedom of the water monomers gain energy from these subcycle pulses with an electric field amplitude of up to about 0.6 V/Å. It has been recently shown that pulses with these characteristics can be generated in the laboratory [C. Vicario, B. Monoszlai, and C. P. Hauri, Phys. Rev. Lett. 112, 213901 (2014)]. Through their permanent dipole moment, water molecules are acted upon by the electric field and forced off their preferred hydrogen-bond network conformation. This immediately sets them in motion with respect to one another as energy quickly transfers to their relative center of mass displacements. We find that, in the bulk, the operation of these mechanisms is strongly dependent on the initial temperature and density of the system. In low density systems, the equilibration between rotational and translational modes is slow due to the lack of collisions between monomers. As the initial density of the system approaches $1 \text{g}/{\text{cm}}^3$, equilibration between rotational and translational modes after the pulse becomes more efficient. In turn, low temperatures hinder the direct energy transfer from the pulse to rotational motion owing to the resulting stiffness of the hydrogen bond network. For small clusters of just a few water molecules we find that fragmentation due to the interaction with the pulse is faster than equilibration between rotations and translations, meaning that the latter remain colder than the former after the pulse. In contrast, clusters with more than a few tens of water molecules already display energy gain dynamics similar to water in condensed phases owing to inertial confinement of the internal water molecules by the outer shells. In these cases, a complete equilibration becomes possible.

Figure 1

Translational $\left(\mathrm{\Delta }\langle E_T\rangle \right)$ and rotational $\left(\mathrm{\Delta }\langle E_R\rangle \right)$ energy increase per water monomer for the monomer and dimer cases. On the right axis the kinetic temperature $T=\frac{2E}{3k}$ with $k$ the Boltzmann factor is shown. The THz pulse profile appears as a dotted line.

Figure 2

Potential energy curve for HOO angle with different electric field amplitudes. The labels (a) and (b) correspond to the dimer configurations as shown in Fig. 3.

Figure 3

Minimum energy configurations of the water dimer (a) without field and (b) with a field of 0.614 V/Å as presented in Fig. 2.

Figure 4

(a) Ensemble averaged total kinetic energy increase $\left(\mathrm{\Delta }\langle E_K\rangle \right)$, (b) translational energy increase $\left(\mathrm{\Delta }\langle E_T\rangle \right)$, and (c) rotational energy increase $\left(\mathrm{\Delta }\langle E_R\rangle \right)$ for the water clusters of sizes 4, 8, 32, and 64 and for bulk water at 200 K. Red circles are showing the points ${\mathrm{t}}_1$ and ${\mathrm{t}}_2$ for 4 water cluster only. ${\mathrm{t}}_1$ is the point when significant change happens in $\mathrm{\Delta }\langle E_K\rangle$ and ${\mathrm{t}}_2$ is the point when $\mathrm{\Delta }\langle E_K\rangle$ is maximum.

Figure 5

Water density difference $\left(\mathrm{\Delta }\rho \right)$ with respect to the density at $-250$ fs (before the THz pulse) at times 250, 750, and 1250 fs in the presence of the THz pulse. $r$ is the distance to the center of the spherical cluster.

Figure 6

Temperature (T)–density $\left(\rho \right)$ phase diagram of pure water [34]. C.P. is the critical point. Red dots represent the thermodynamical states of water in the phase diagram that have been taken as initial conditions to explore the effect of THz pulse.

Figure 7

Snapshots of simulation boxes with density $0.3 \text{g}/{\text{cm}}^3$ and different temperatures ($T=200$ K, 400 K, and 600 K) before the pulse, $t=-250$ fs [(a), (c), (e)], and after the pulse, $t=1250$ fs [(b), (d), (f)].

Figure 8

Total kinetic energy $\left(\mathrm{\Delta }\langle E_K\rangle \right)$, translational energy $\left(\mathrm{\Delta }\langle E_T\rangle \right)$, and rotational energy $\left(\mathrm{\Delta }\langle E_R\rangle \right)$ per water molecule at different temperatures for densities $0.296 \text{g}/{\text{cm}}^3$ [(a), (c), (e)] and $0.019 \text{g}/{\text{cm}}^3$ [(b), (d), (f)].

Figure 9

Snapshots of water systems at $T=300$ K and different densities (0.3, 0.064, and $0.02 \text{g}/{\text{cm}}^3$) before the pulse, $t=-250$ fs [(a), (c), (e)], and after the pulse, $t=1250$ fs [(b), (d), (f)].

Figure 10

Total kinetic energy $\left(\mathrm{\Delta }\langle E_K\rangle \right)$, translational energy $\left(\mathrm{\Delta }\langle E_T\rangle \right)$, and rotational energy $\left(\mathrm{\Delta }\langle E_R\rangle \right)$ per water molecule at different densities (in $\text{g}/{\text{cm}}^3$) for initial temperatures 300 K [(a), (c), (e)] and 500 K [(b), (d), (f)] of the system.