Abstract-Simulated XUV Photoelectron Spectra of THz-pumped Liquid Water

 Caroline Arnold, Ludger Inhester, Sergio Carbajo, Ralph Welsch, Robin Santra

https://arxiv.org/abs/1901.07792

Highly intense, sub-picosecond terahertz (THz) pulses can be used to induce ultrafast temperature jumps (T-jumps) in liquid water. A supercritical state of gas-like water with liquid density is established, and the accompanying structural changes are expected to give rise to time-dependent chemical shifts. We investigate the possibility of using extreme ultraviolet (XUV) photoelectron spectroscopy as a probe for ultrafast dynamics induced by sub-picosecond THz pulses of varying intensities and frequencies. To this end, we use ab initio methods to calculate photoionization cross sections and photoelectron energies of (H2O)20 clusters embedded in an aqueous environment represented by point charges. The cluster geometries are sampled from ab initio molecular dynamics simulations modeling the THz-water interactions. We find that the peaks in the valence photoelectron spectrum are shifted by up to 0.4 eV after the pump pulse, and that they are broadened with respect to unheated water. The shifts can be connected to structural changes caused by the heating, but due to saturation effects they are not sensitive enough to serve as a thermometer for T-jumped water.

Abstract-Molecular polarizability anisotropy of liquid water revealed by terahertz-induced transient orientation

Peter Zalden, Liwei Song, Xiaojun Wu, Haoyu Huang, Frederike Ahr, Oliver D. Mücke, Joscha Reichert, Michael Thorwart, Pankaj Kr. Mishra, Ralph Welsch, Robin Santra, Franz X. Kärtner, Christian Bressler,

https://www.nature.com/articles/s41467-018-04481-5

Reaction pathways of biochemical processes are influenced by the dissipative electrostatic interaction of the reagents with solvent water molecules. The simulation of these interactions requires a parametrization of the permanent and induced dipole moments. However, the underlying molecular polarizability of water and its dependence on ions are partially unknown. Here, we apply intense terahertz pulses to liquid water, whose oscillations match the timescale of orientational relaxation. Using a combination of terahertz pump / optical probe experiments, molecular dynamics simulations, and a Langevin dynamics model, we demonstrate a transient orientation of their dipole moments, not possible by optical excitation. The resulting birefringence reveals that the polarizability of water is lower along its dipole moment than the average value perpendicular to it. This anisotropy, also observed in heavy water and alcohols, increases with the concentration of sodium iodide dissolved in water. Our results enable a more accurate parametrization and a benchmarking of existing and future water models.

Abstract-On the Prospects of Using High-Intensity THz Pulses to Induce Ultrafast Temperature-Jumps in Liquid Water

Pankaj Kr. Mishra, Vincent Bettaque, Oriol Vendrell, Robin Santra, Ralph Welsch,

https://pubs.acs.org/doi/10.1021/acs.jpca.8b00828

Ultrashort, high-intensity THz pulses, e.g., created 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 × 1011 to 5 × 1012W/cm2 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 towards 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.

Abstract-2D THz-THz-Raman Photon-Echo Spectroscopy of Molecular Vibrations in Liquid Bromoform

Ian A. Finneran, Ralph Welsch, Marco A. Allodi, Thomas Francis Miller, and Geoffrey A. Blake

http://pubs.acs.org/doi/abs/10.1021/acs.jpclett.7b02106

Fundamental properties of molecular liquids are governed by long range interactions that most prominently manifest at terahertz (THz) frequencies. Here we report the detection of nonlinear THz photon-echo (rephasing) signals in liquid bromoform, using THz-THz-Raman spectroscopy. Together, the many observed signatures span frequencies from 0.5-8.5 THz, and result from couplings between thermally populated ladders of vibrational states. The strongest peaks in the spectrum are found to be electric dipole-forbidden and polarizability-allowed and may arise from nonlinearities in the intramolecular dipole moment surface driven by intermolecular interactions.

Abstract-Coherent two-dimensional terahertz-terahertz-Raman spectroscopy

1. Ian A. Finnerana, 
2. Ralph Welscha, 
3. Marco A. Allodia,1, 
4. Thomas F. Miller, IIIa, and 
5. Geoffrey A. Blakea,b,2

1. http://www.pnas.org/content/early/2016/06/02/1605631113.abstract

We present 2D terahertz-terahertz-Raman (2D TTR) spectroscopy, the first technique, to our knowledge, to interrogate a liquid with multiple pulses of terahertz (THz) light. This hybrid approach isolates nonlinear signatures in isotropic media, and is sensitive to the coupling and anharmonicity of thermally activated THz modes that play a central role in liquid-phase chemistry. Specifically, by varying the timing between two intense THz pulses, we control the orientational alignment of molecules in a liquid, and nonlinearly excite vibrational coherences. A comparison of experimental and simulated 2D TTR spectra of bromoform (CHBr3), carbon tetrachloride (CCl4), and dibromodichloromethane (CBr2Cl2) shows previously unobserved off-diagonal anharmonic coupling between thermally populated vibrational modes.