HYPER-RAMAN SPECTROSCOPY ON WATER AND GYPSUM

The importance of water and water bearing minerals is especially highlighted in recent missions to Mars and to comets. Water is a key ingredient for life as we know it. Therefore, studying properties of water and water bearing minerals at near surface conditions provides important information for the search for life beyond planet Earth in future space missions. It also improves our understanding of the complexity of water in its many icy and amorphous phases.

For the first time, hyper-Raman spectroscopy has been applied to study water and single crystals of gypsum at ambient conditions. In centrosymmetric materials hyper-Raman is sensitive to infrared active vibrations and is a powerful technique to directly measure mode frequencies and LO-TO splitting including the far-infrared range. The effective volume used in the present studies is directly comparable to sample sizes in a diamond anvil cell for pressures up to 10 GPa. This clearly shows the feasibility of hyper-Raman under conditions ranging from near surface to the interiors of planetary bodies.

Spectra taken on water clearly resolve low frequency infrared modes at 61 and 208 cm^-1. These modes are notoriously difficult to resolve using conventional far-infrared spectroscopy [e.g. Hasted et al. 1985]. Additionally, far-infrared spectroscopy can not be applied to small samples typical for high pressure studies in icy phases. The low frequency vibrations strongly contribute to the physical properties at temperatures where most forms of water ice are stable. Furthermore, the lineshape of the extensively studied mid-infrared band around 3500 cm^-1 is strongly affected by low frequency modes. Recent results and a detailed analysis of the vibrational spectrum of water and water-ice will be presented. In gypsum, hyper-Raman spectroscopy shows significantly higher resolution when compared to conventional infrared studies [Long et al., 1993] and can be used to directly identify LO-TO splitting. The improved data can be used to calculate thermodynamic properties and reflectivity spectra used in remote sensing.