Terahertz laser helped remove the spectrum of the water trimer in the low frequency region

Scientists have been able to register the absorption spectrum of clusters of three water molecules in the infrared frequency range from 70 to 620 inverse centimeters. Using the method of isolation of molecules in helium nanodrops and free electron laser authors were also able to detect frequency transitions that were not previously registered. The results of a study published in the journal Angewandte Chemie International Edition.

It is no secret that water is a hugely important substance in biology and chemistry. Despite decades of studies of the interactions between water molecules, their accurate description at the molecular level still remains a challenge. To develop a universal model of water that would have predicted its properties in any form and different conditions, researchers have resorted to empirical methods of spectroscopy and theoretical calculations to construct potential energy surfaces (PPE) water clusters.

If we define the energy of intermolecular interactions in water in a condensed state as the difference between the total electronic energy of system and energies of individual molecules, it turns out that the terms of clusters of two and three molecules make up about 98 percent of the total energy. Therefore, to develop a universal model it is important to examine intermolecular interactions of small clusters.

One way spectroscopic studies of aggregated molecules is isolated in nanodrops helium at a temperature of 0.37 Kelvin. This technique is already used to study water molecules, but only in the energy region mod stretching of intermolecular bonds O-H and bending intermolecular N-O-N. Scientists under the leadership of Martina Havenith (Havenith Martina) from the Ruhr University in Bochum were able by this method to obtain spectra of the water trimer in the lower frequency region (from 70 to 620 inverse centimeters), to get information about the energies of the translational and librational modes in the plane of the cluster and outside it.

In the chamber with the droplets of helium added water, the probability of capture molecules which drop of helium was consistent with the Poisson distribution. Fluctuations in the clusters was initiated with the help of free electron lasers. The laser radiation beam was located antiparallel and collinear relative to the helium beam nanodrops. Infrared radiation is absorbed only drops with water molecules. As a result of absorption of a part of helium atoms evaporated from the droplet, and then by electron impact drops were ionized and directed into the mass spectrometer, which recorded the flow of ions in time. The evaporation of helium atoms resulted in a decrease in ion current. Therefore, the absorption of radiation was measured not directly but as a reduction of the ionic current of the charged fragment of the solute in the mass spectrum.

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