THE THz/FIR SPECTRUM OF SMALL WATER CLUSTERS IN HELIUM NANODROPLETS

The microscopic properties of water that are relevant for bulk solvation processes are still not fully understood. Here, we combine mass selective Helium nanodroplet spectroscopy with the powerful Terahertz (THz) and far-infrared (FIR) capabilities of the free electron laser facility FELIX to study the fingerprint of small neutral water clusters in the wavelength range from 90-900wn. _x000d_ Helium nanodroplets are a gentle, superfluid matrix and allow aggregation of pre-cooled moieties at ultra-cold temperatures (0.37 K). The fast cooling rate allows in some cases to stabilize not only the global minimum structure but also local minimum structures. _x000d_ The FELIX facility in Nijmegen provides narrowband ($Deltanu / nu=0.5$) pulsed radation covering the frequency range from 80--3300 wn. We used a repetition rate of 10 Hz and typical pulse energies from 10 mJ at the 90wn and 40 mJ at 900wn. This corresponds to average powers of 100--400 mW far beyond those available using other radiation sources in this frequency range._x000d_ The observed spectrum is exceptionally rich and includes lines that are close to or below our resolution limit. By mass selective detection and by varying the pickup pressure, we were able to identify contributions from dimer, trimer, tetramer and pentamer. The number of resonances indicates stabilization of at least two trimer structures in He nanodroplets. A comparison with theoretical predictions is on the way. We are confident that our experiments will contribute to understand the very special behavior of water in a bottom up approach

IR spectroscopy of pyridine-water structures in helium nanodroplets

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.We present the results of an IR spectroscopic study of pyridine–water heterodimer formation in helium nanodroplets. The experiments were carried out in the frequency range of the pyridine C–H stretch region (3055–3100 cm−1) and upon water deuteration in the D–O stretch region (2740–2800 cm−1). In order to come to an unambiguous assignment we have determined the angle between the permanent dipole and the vibrational transition moment of the aggregates. The experiments have been accompanied by theoretical simulations which yielded two minimum structures with a 16.28 kJ mol−1energy difference. The experimentally observed bands were assigned to two structures with different H-bonds: an N⋯H bond and a bifurcated O⋯H–C bond.DFG, FOR 618, Die Aggregation kleiner Moleküle mit präzisen Methoden verstehen - Experiment und Theorie im Wechselspie

Strong Anisotropy in Liquid Water upon Librational Excitation using Terahertz Laser Fields

Tracking the excitation of water molecules in the homogeneous liquid is challenging due to the ultrafast dissipation of rotational excitation energy through the hydrogen-bonded network. Here we demonstrate strong transient anisotropy of liquid water through librational excitation using single-color pump-probe experiments at 12.3 THz. We deduce a third order response of chi^3 exceeding previously reported values in the optical range by three orders of magnitude. Using a theory that replaces the nonlinear response with a material response property amenable to molecular dynamics simulation, we show that the rotationally damped motion of water molecules in the librational band is resonantly driven at this frequency, which could explain the enhancement of the anisotropy in the liquid by the external Terahertz field. By addition of salt (MgSO4), the hydration water is instead dominated by the local electric field of the ions, resulting in reduction of water molecules that can be dynamically perturbed by THz pulses

Observation of the Low-Frequency Spectrum of the Water Trimer as a Sensitive Test of the Water-Trimer Potential and the Dipole-Moment Surface

© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. Intermolecular interactions in bulk water are dominated by pairwise and non-pairwise cooperative interactions. While accurate descriptions of the pairwise interactions are available and can be tested by precise low-frequency spectra of the water dimer up to 550 cm−1, the same does not hold for the three-body interactions. Here, we report the first comprehensive spectrum of the water trimer in the frequency region from 70 to 620 cm−1 using helium-nanodroplet isolation and free-electron lasers. By comparison to accompanying high-level quantum calculations, the experimentally observed intermolecular bands can be assigned. The transition frequencies of the degenerate translation, the degenerate in-plane and the non-degenerate out-of-plane libration, as well as additional bands of the out-of-plane librational mode are reported for the first time. These provide a benchmark for state-of-the-art water potentials and dipole-moment surfaces, especially with respect to three-body interactions

An Isolated Water Droplet in the Aqueous Solution of a Supramolecular Tetrahedral Cage

Water under nanoconfinement at ambient conditions has exhibited low-dimensional ice formation and liquid-solid phase transitions, but with structural and dynamical signatures which map onto known regions of waters phase diagram. Using THz absorption spectroscopy and ab initio molecular dynamics, we have investigated the ambient water confined in a supramolecular tetrahedral assembly, and determined that a distinct network of 9-10 water molecules is present within the nanocavity of the host. The low-frequency absorption spectrum and theoretical analysis of the water in the $Ga_4$$L_6$$^{-12}$ host demonstrate that the structure and dynamics of the encapsulated droplet is distinct from any known phase of water. A further inference is that the release of the highly unusual encapsulated water droplet creates a strong thermodynamic driver for the high affinity binding of guests in aqueous solution for the $Ga_4$$L_6$$^{-12}$ supramolecular construct

MICROSOLVATION STUDIES IN HELIUM NANODROPLETS

In bulk aqueous solutions the interactions between solute and solvent are still not fully understood. We apply spectroscopy in Helium nanodroplets to investigate solvation processes step by step (bottom up approach). Recently, the Bochum helium nanodroplet spectrometer has been equipped with a quantum cascade laser spanning the frequency ranges from 1000--1400, 1600--1700, and 2500--2600 wn. First results with the extended setup will be presented

MICROSOLVATION STUDIES IN HELIUM NANODROPLETS

In bulk aqueous solutions the interactions between solute and solvent are still not fully understood. We apply spectroscopy in Helium nanodroplets to investigate solvation processes step by step (bottom up approach). Recently, the Bochum helium nanodroplet spectrometer has been equipped with a quantum cascade laser spanning the frequency ranges from 1000--1400, 1600--1700, and 2500--2600 wn. First results with the extended setup will be presented

Bohendi@felix: probing the far-infrared fingerprint of small clusters in helium nanodroplets with a free electron laser

Recently, we have installed a helium nanodroplet machine [1,2] at the free electron beamline FELIX in Nijmegen. The current setup allows to study neutral molecules and molecular complexes in the full spectral range from 500--3000 cm$^{-1}$. First proof of principle experiments using the strong absorber SF$_6$ were used to verify the overall alignment between helium nanodroplet beam and the FELIX radiation source. Applications so far included the study of small water clusters and the investigation of microsolvation of small solutes. These results will be presented and compared to recent theoretical predictions of the Bowman group.[3] \vspace{1 em} [1] K. von Haeften et al., Phys. Rev. B. 73, 054502 (2006) [2] Choi et al., Int. Rev. Phys. Chem. 25, 15 (2006) [3] Samantha et al., Acc. Chem. Res. 47, 2700 (2014

Tangent Planes to Quadratic Surfaces

Four different quadratic functions are used to define surfaces. You can vary the point of tangency.Componente Curricular::Educação Superior::Ciências Exatas e da Terra::Matemátic

Wave Packet Dynamics

quantum Mechanics lawsThis Demonstration shows the system's time development in a static harmonic oscillator potentialComponente Curricular::Educação Superior::Ciências Exatas e da Terra::Físic