Comprehensive investigation of vibrational relaxation of non-hydrogen-bonded water molecules in liquids

Abstract

The vibrational dynamics of isolated water molecules dissolved in the nonpolar organic liquids 1,2-dichloroethane (C(2)H(4)Cl(2)) and d-chloroform (CDCl(3)) have been studied using an IR pump-probe experiment with approximately 2 ps time resolution. Analyzing transient, time, and spectrally resolved data in both the OH bending and the OH stretching region, the anharmonic constants of the bending overtone (v=2) and the bend-stretch combination modes were obtained. Based on this knowledge, the relaxation pathways of single water molecules were disentangled comprehensively, proving that the vibrational energy of H(2)O molecules is relaxing following the scheme OH stretch-->OH bend overtone-->OH bend-->ground state. A lifetime of 4.8+/-0.4 ps is determined for the OH bending mode of H(2)O in 1,2-dichloroethane. For H(2)O in CDCl(3) a numerical analysis based on rate equations suggests a bending overtone lifetime of tau(020)=13+/-5 ps. The work also shows that full 2-dimensional (pump-probe) spectral resolution with access to all vibrational modes of a molecule is required for the comprehensive analysis of vibrational energy relaxation in liquids.

Anharmonic bend–stretch coupling in neat liquid water

“…Our results are however in stark contrast with recent experiments by Seifert et al [13] on water dissolved in non-polar liquids. For such non-hydrogen bonded water molecules, an initial pump-pulse induced bending excitation leads to two pronounced bleaching signals in the OH-stretching region corresponding to the symmetric and antisymmetric OH-stretching resonances.…”

“…To the best of our knowledge, the derivation of an absorption cross-section from a combination tone transition from femtosecond time-resolved mid-IR pump-probe measurements on hydrogen-bonded water is without precedent. We note that beautiful equivalent experiments were recently carried out by Graener and coworkers on free water dissolved in non-hydrogenbonded environments such as liquid CDCl 3 and C 2 H 4 Cl 2 [13]. Previous complementary studies on the vibrational relaxation dynamics of liquid water have used the differential cross-sections such as those appearing in Eq.…”

Ultrafast anisotropy dynamics of water molecules dissolved in acetonitrile

“…It was recently shown 58 that the coupling is weakly solvent dependent. Therefore, we use the value derived from the linear spectrum of H 2 O in acetonitrile.…”

Direct Vibrational Energy Transfer in Monomeric Water Probed with Ultrafast Two Dimensional Infrared Spectroscopy

“…The short population lifetimes in neat water make the vibrational energy relaxation and dissipation pathway experimentally difficult to access. While the relaxation time constants usually can range from 2 ps to 100 ps for the water molecules in the confined environments, such as water molecules dissolved in the organic solvents [24][25][26][27][28][29][30]. The monomeric water thus provides a model system to study the detailed vibrational relaxation pathway.…”

“…The monomeric water thus provides a model system to study the detailed vibrational relaxation pathway. Seifert and coworkers investigated the vibrational relaxation of monomeric water in different solvents using the infrared pump probe method [24][25][26]. The vibrational relaxation pathway of the OH stretching in monomeric water was proposed where the overtone mode of OH bending is involved as an intermediate step [25].…”