Spectra of isotopic ice mixtures

Abstract

Studies of vibrational spectra of ice I and amorphous ice in the stretching mode frequency range were extended to include (a) the observed effect of a full range of isotopic dilution on the infrared spectra and (b) computational modeling of the observed influence of each dilution step on the properties of vibrationally excited states and on infrared and Raman spectra. The quantum-mechanical computational scheme included effects of frequency lowering due to hydrogen bonding, and of intra- and intermolecular coupling between bonds. The H2O/D2O mixtures can be viewed as a collection of clusters of one X2O isotopomer embedded in a matrix of the other isotopomer. The properties of the vibrationally excited states and of the spectra are determined by complex interplay between the size distribution of the embedded clusters, and the inter- and intramolecular coupling. Vibrational excitations are delocalized over large portions of the embedded clusters. In the limit of a pure crystalline isotopomer, the excitations are delocalized over the entire system and thus proton disorder alone is insufficient to induce localization. The excitations in pure amorphous ice show more pronounced localization effects at the band edges. Throughout the entire composition range, the vibrations of molecules in the low frequency regime retain symmetric stretch character, and the vibrations in the high frequency regime retain antisymmetric stretch character. The perpendicularly polarized Raman spectrum peaks in the region of the latter states. The parallel-polarized Raman spectrum peaks in the low frequency end of the band where the states are globally symmetric, i.e., the contributions of excitations of all bonds to a state are of the same sign. The infrared spectrum extends over the entire band and follows roughly the density of states.