title: Femtosecond coherent anti-Stokes Raman scattering spectroscopy of hydrogen bonded structure in water and aqueous solutions.
abstract: Femtosecond coherent anti-Stokes Raman scattering (fsCARS) spectroscopy, together with perturbation theory based numerical calculation, is employed to study OH stretching (υOH) of pure water and aqueous lithium chloride solutions. Vibrational OH stretching (υOH) modes of aqueous solutions are Raman-excited by a pair of ultrashort, femtosecond laser pulses, and then probed through inelastic scattering of a third, time-delayed laser field. In order to overcome limited spectral resolution of fsCARS, numerical evaluation of the CARS signal through vibrational wave packet propagation was employed in order to confirm the position of distinctive OH stretching mode that is complicated by intramolecular and intermolecular vibrational coupling. Moreover, in order to come to a microscopic description of the observed CARS spectra for aqueous solutions, we have performed molecular dynamics simulations of aqueous lithium chloride solutions with varying concentrations at ambient conditions. To this end we have analyzed the equilibrium distributions of hydrogen bonds in the first solvation shells of the ions as well as in bulk water and also computed the average number of hydrogen bonds per water molecule. According to our experimental and theoretical results on time evolution of Raman OH stretching band of water, it can be inferred that the dissolved ions mainly influence hydrogen bond strength and structure of water molecules in the first hydration shell, the addition of lithium chloride primarily breaks the tetrahedral hydrogen bonding, promotes formation of the donor hydrogen bonding in water, and slightly increases the amount of free OH bonds.
authors.affiliation: Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China.
authors.forename: Huaning
authors.initials: H
authors.lastname: Zhu
authors.affiliation: Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China.
authors.forename: Yang
authors.initials: Y
authors.lastname: Li
authors.affiliation: Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China; Institute of Physics, Bijenička cesta 46, 10000 Zagreb, Croatia.
authors.forename: Silvije
authors.initials: S
authors.lastname: Vdović
authors.affiliation: Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China.
authors.forename: Saran
authors.initials: S
authors.lastname: Long
authors.affiliation: Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China.
authors.forename: Guiying
authors.initials: G
authors.lastname: He
authors.affiliation: Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China. Electronic address: guoqj@iccas.ac.cn.
authors.forename: Qianjin
authors.initials: Q
authors.lastname: Guo
keywords: Coherent anti-Stokes Raman scattering; Hydrogen bonded structure; Molecular dynamics simulations; Perturbation theory based numerical calculation