X-ray Raman Scattering of Water Near the Critical Point: Comparison of an Isotherm and Isochore

TL;DR

This study investigates how X-ray Raman spectra of water change near the critical point, revealing spectral features that transition from liquid-like to vapor-like as density decreases, with comparisons to theoretical spectra.

Contribution

It provides the first detailed comparison of experimental X-ray Raman spectra of supercritical water along an isotherm and isochore near the critical point.

Findings

Spectra become more vapor-like as density decreases.

A local maximum in spectral contrast occurs at the critical point.

Experimental spectra are compared successfully with computed spectra.

Abstract

X-ray Raman spectra of liquid, sub- and super- critical water at the oxygen K-edge were measured, at densities 1.02 - 0.16 gcm^-3. Measurements were made along both an isotherm and an isochore passing near the critical point. As density is reduced there is a general tendency of the spectra to increasingly resemble that of the vapor phase, with, first, a well separated low-energy peak, and, eventually, at densities below the critical density, peaks appearing at higher energies corresponding to molecular transitions. The critical point itself is distinguished by a local maximum in the contrast between some of the spectroscopic features. The results are compared to computed X-ray absorption spectra of supercritical water.