Mark G. Sceats

Raman spectroscopic studies of the OH stretching region of low density amorphous solid water and of polycrystalline ice Ih

Extensive experimental studies of the OH stretching region of the Raman spectra of polycrystalline ice Ih and low density amorphous solid water (H2O (as)) are reported. Analysis of these spectra, using insights from theoretical calculations, leads to the following conclusions: (i) The overall span and distribution of features in the spectra result from strong intermolecular coupling of OH oscillators. This coupling is so strong that the solids cannot be considered to be composed of weakly perturbed water molecules, and the identification of features of their spectra with molecular vibrations is not useful. (ii) At the low end of the OH stretching region there is a totally symmetric mode—that is the amplitudes of motion on all oscillators are in phase, but these amplitudes vary erratically from site to site in the solid. The temperature dependence of the shift in frequency of this mode is likely all accounted for by thermal expansion, the temperature dependence of its FWHM is accurately described as due to...

The OH stretching region infrared spectra of low density amorphous solid water and polycrystalline ice Ih

The transmission spectra in the intramolecular stretching region have been measured for thin films of H2O and D2O polycrystalline ice Ih and amorphous solid water. The data have been reduced to obtain the complex refractive index in this region via an iterative procedure designed to account for reflection and interference losses. Details of this procedure are described. The values which result from this analysis are converted to give the imaginary part of the dielectric permittivity as a function of wavelength for each sample. In this form the spectra are examined and compared to one another, with regard to recent experimental and theoretical studies. We conclude that it is difficult to obtain detailed structural information for the amorphous solid from an analysis of the OH (OD) stretching vibrational spectrum alone.

The water–water pair potential near the hydrogen bonded equilibrium configuration

An analysis is presented of the contributions to the potential energy of interaction between water molecules near the hydrogen bonded equilibrium configuration. From this analysis we synthesize an effective potential that includes the influences of long‐ranged and nonadditive interactions. The influence of the hydrogen bond bending on the anharmonic properties of ice I and on the spectral properties of amorphous solid water, the denser ice polymorphs, and liquid water are used to test the consistency of the effective potential. We also use that potential to estimate the quasistatic distribution of hydrogen bond angles in liquid water, which distribution is an important characteristic of the random network model of water.

A zeroth order random network model of liquid water

A random network model is used to generate the oxygen–oxygen pair correlation function of liquid water and amorphous solid water. The model is basically an extension of the bent hydrogen bond model proposed by Pople in 1951; it differs from the earlier work in that instead of being a parametric fit to the data, it predicts the dispersion in OO distances for each shell of neighbors, and the dispersion in OOO angles, from independent spectroscopic data. Our random network model is based on the hypothesis that there exists a separation of time scales for various classes of molecular motion in liquid water. In particular we assert that it is useful to regard libration and hindered translation as occurring in a quasistatic network of bonded molecules and diffusion and related relaxation processes as occurring much more slowly. The predicted OO pair correlation function is in good but not perfect agreement with experiment over a wide temperature range. As a further test the model is used to predict the dielectr...

A theoretical study of the OH stretching region of the vibrational spectrum of ice Ih

We describe a theoretical interpretation of the OH stretching region of the vibrational spectrum of ice Ih derived from a study of extended models. The principle conclusions are the following: (i) The breadth and distribution of vibrational modes, and of intensity in the Raman and infrared spectra, are dominated by the influence of strong intermolecular coupling. The intermolecular coupling is comparable in strength to the intramolecular coupling. Long range couplings between molecules not directly hydrogen bonded mainly serve to make the features of the spectrum more diffuse than they would otherwise be, but have no great influence on the spectral distribution. (ii) The intramolecular OH stretch‐OH stretch coupling in ice Ih is of opposite sign to that in the gas phase. (iii) The vibrational modes of ice Ih are found to be complex mixtures of molecular motions, so identification of regions of the Raman or infrared spectra with particular isolated molecule modes is not useful. Rather good agreement is obt...

Pulse-sequenced coherent anti-Stokes Raman scattering spectroscopy: a method for suppression of the nonresonant background

A new technique for suppressing the nonresonant contribution to the coherent anti-Stokes Raman scattering (CARS) signal is presented. This involves two-laser three-beam CARS generation using picosecond pulses in which one of the pump beams is delayed with respect to the two others. A reduction of 2 orders of magnitude is obtained for the 656-cm−1 vibrational line of CS2 in a 10% mixture of CS2 in toluene. This reduction is limited by the ratio of the energy in our pulse laser to that in its trailing background, which is about 100:1. Use of alternative laser systems would reduce the background by about 1012:1.

On the role of Fermi resonance in the spectrum of water in its condensed phases

In this paper we present a theoretical analysis of the intramolecular stretching spectrum of D2O in H2O and H2O in D2O for both ice I and amorphous solid water H2O(as). The model includes the dependence of the harmonic stretching (krr) and stretch–stretch interaction (krr′) force constants on hydrogen bonding as well as Fermi resonance between the stretching fundamentals and the bending overtone. Provided that the width of the bending mode is broadened by intermolecular coupling, then the model reproduces the anomalously large widths and splittings in the stretching spectrum recently reported by Devlin et al. for D2O in H2O ice I and H2O(as). The value of the Fermi resonance interaction force constant k122 required to fit these spectra is accounted for by diagonal harmonic terms in the valence force field representation of the intramolecular potential.

Cooperative vibronic spectra involving rare earth ions and water molecules in hydrated salts and dilute aqueous solutions

Several cooperative vibronic bands have been observed in the excitation and emission spectra of polycrystalline hydrates and dilute aqueous solutions of TbCl3. A radiative transition involving the simultaneous electronic transition of a Tb3+ ion and vibrational transition of a nearby water molecule gives rise to these bands. Theoretical calculations, based on an electrostatic coupling model, explain the intensities of the cooperative vibronic transitions. Selection rules are derived and quantitative estimates of intensities are made.

The 2500–4000 cm−1 Raman and infrared spectra of low density amorphous solid water and of polycrystalline ice I

Abstract New measurements of the 2500–4000 cm −1 region of the vibrational spectra of amorphous solid water, H 2 O (as), and of polycrystalline ice I are reported. These data force the withdrawal of the interpretation of the Raman spectrum of H 2 O (as) put forward by Venkatesh et al. A new interpretation, based on similarities between the spectra of the amorphous and polycrystalline solids, is proposed.

The intramolecular potential of water molecules engaged in hydrogen bonding from analysis of the overtone spectrum of ice I

An analysis of the fundamental and overtone spectra of ice Ih is presented. It is shown that the major features of the spectra of H2O, D2O, and H2O/D2O mixtures can be modeled by a theory which asserts that there is no change in the anharmonic diagonal stretching terms in the intramolecular potential upon hydrogen bonding. The changes in the anharmonic constant arise from the change in the OH stretching amplitude. Both the harmonic and anharmonic terms can be expressed in terms of a hydrogen bond parameter η which can be extracted from the observed OH stretching frequency alone. The influence of intermolecular coupling in dimers and pure crystals gives rise to new spectral features which can be assigned using the model.