title: Perspective: Echoes in 2D-Raman-THz spectroscopy.
abstract: Recently, various spectroscopic techniques have been developed, which can measure the 2D response of the inter-molecular degrees of freedom of liquids in the THz regime. By employing hybrid Raman-THz pulse sequences, the inherent experimental problems of 2D-Raman spectroscopy are circumvented completely, culminating in the recent measurement of the 2D-Raman-THz responses of water and aqueous salt solutions. This review article focuses on the possibility to observe echoes in such experiments, which would directly reveal the inhomogeneity of the typically extremely blurred THz bands of liquids, and hence the heterogeneity of local structures that are transiently formed, in particular, in a hydrogen-bonding liquid such as water. The generation mechanisms of echoes in 2D-Raman-THz spectroscopy are explained, which differ from those in "conventional" 2D-IR spectroscopy in a subtle but important manner. Subsequently, the circumstances are discussed, under which echoes are expected, revealing a physical picture of the information content of an echo. That is, the echo decay reflects the lifetime of local structures in the liquid on a length scale that equals the delocalization length of the intermolecular modes. Finally, recent experimental results are reviewed from an echo perspective.
authors.affiliation: Department of Chemistry, University of Zurich, Zurich, Switzerland.
authors.forename: Peter
authors.identifier: 0000000311066032
authors.initials: P
authors.lastname: Hamm
authors.affiliation: Department of Chemistry, University of Zurich, Zurich, Switzerland.
authors.forename: Andrey
authors.initials: A
authors.lastname: Shalit