Prospects of Using High-Intensity THz Pulses To Induce Ultrafast Temperature-Jumps in Liquid Water

Abstract

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.

Simulated XUV photoelectron spectra of THz-pumped liquid water

“…The analysis in this work is based on ab initio molecular dynamics (AIMD) trajectories of THz-pumped water that were the subject of previous work 21 sampled ten initial conditions with a time separation of 300 fs. Since the the PBE functional underestimates diffusion constants and prolongates equilibration times 39 , this resulted in a slightly favored orientation of the water molecules in the initial trajectory snapshots with cos ϑ = 0.08 .…”

“…We investigated the possibility of detecting ultrafast THz-induced T-jumps in liquid water by employing XUV photoelectron spectroscopy. The structural changes induced by sub-picosecond, high-intensity THz pulses of various frequencies were analyzed by calculating RDFs, building on AIMD trajectories that were the subject of previous work 21 . We found that, for a lower intensity, the loss of order is frequency-dependent.…”

“…Due to the large number of pump pulse intensities and frequencies considered in Ref. 21, the simulation protocol was restricted. The PBE functional was chosen based on the good description of electronic polarizability 21,25 , however this resulted in overly structured water 39,41 and subpar equilibration in the generation of initial configurations.…”

“…The maximum energy transfer is reached with pump pulse frequencies between 14 and 17 THz, where rotations of water molecules are excited 21,24 . Ultrafast T-jumps of more than 1000 K within tens of femtoseconds are predicted within currently accessible THz intensity levels 21 . The instantaneous temperature can be calculated directly from the molecular dynamics (MD) trajectories of THz-pumped water and was shown to be independent of the simulation box size 24 .…”

“…In Sec. II, we analyze the structural changes that are induced by sub-picosecond, high-intensity THz pulses based on AIMD trajectories 21 . We then describe the theoretical framework for the calculation of photoionization cross sections from liquid water in Sec.…”