Abstract
Cold water oligomers (H2O)n and (D2O)n with n = 2-5 are assigned in spontaneous Raman scattering spectra of seeded rare gas expansions for the first time. Comparison with infrared spectra provides direct experimental insights into the hydrogen bond-mediated excitonic OH oscillator coupling, which is responsible for ultrafast energy transfer between water molecules, usually suppressed by isotopic dilution in femtosecond experiments for the condensed phase. The experimental coupling constants are compared to those in state-of-the-art full-dimensional water potential energy hypersurfaces, leaving room for improvement in the description of the coupled dynamics in water. Evidence for intensified Fermi resonance between OH stretching and OH bending motion beyond water trimers is collected.
Highlights
The vibrational dynamics of water is of fundamental interest.[1]
While the focus of this contribution is on cyclic trimers, tetramers and pentamers with their low microwave visibility[12] but well-characterized low-frequency dynamics,[2,31] we present the first Raman spectra for isolated water dimers, for which some transitions have previously been observed in He nanodroplets.[32]
We obtained monomer rotational temperatures of 30–50 K based on the relative intensities of Stokes and anti-Stokes transitions
Summary
The vibrational dynamics of water is of fundamental interest.[1] It is mediated by a network of intermolecular hydrogen bonds, which can be introduced step by step in the popular cluster approach.[2,3] While matrix isolation[4,5] and chromophore labeling[6,7] have proven useful in this context, the study of unperturbed, isolated water clusters is attractive due to the close contact with theory[8] and the large amplitude motion in these systems. The Raman selection rules provide essential complementary information on concerted motions, only a pioneering coherent anti-Stokes Raman spectroscopy (CARS) investigation of isolated water clusters has so far been published[13] and discussed controversially.[14,15] The nonlinearity of the CARS experiment rendered a firm assignment of cluster sizes difficult. The characteristic coupling between neighboring OH oscillators in water assemblies, which controls energy flow after local excitation, has remained largely in the dark
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