SPECTRA OF ISOTOPIC ICE MIXTURES

Studies of vibrational spectra of ice I and amorphous ice in the stret ching mode frequency range were extended to include (a) the observed e ffect 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 infrare d and Raman spectra. The quantum-mechanical computational scheme inclu ded effects of frequency lowering due to hydrogen bonding, and of intr a- 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 vibrational ly excited states and of the spectra are determined by complex interpl ay between the size distribution of the embedded clusters, and the int er- and intramolecular coupling. Vibrational excitations are delocaliz ed over large portions of the embedded clusters. In the limit of a pur e crystalline isotopomer, the excitations are delocalized over the ent ire system and thus proton disorder alone is insufficient to induce lo calization. The excitations in pure amorphous ice show more pronounced localization effects at the band edges. Throughout the entire composi tion range, the vibrations of molecules in the low frequency regime re tain symmetric stretch character, and the vibrations in the high frequ ency regime retain antisymmetric stretch character. The perpendicularl y polarized Raman spectrum peaks in the region of the latter states. T he parallel-polarized Raman spectrum peaks in the low frequency end of the band where the states are globally symmetric, i.e., the contribut ions 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.