Ultraviolet Optical Absorption Spectra of Water Clusters: From Molecular Dimer to Nanoscaled Cage-Like Hexakaidecahedron
Abstract:The lowest maximum absorption band in the ultraviolet (UV) optical absorption spectrum of a single water molecule H2O occurs at 7.4eV. Upon condensation the lowest maximum absorption band of bulk water is blueshifted by 0.8 to 1.3 eV and accompanies a long Urbach tail on its red edge. However, the two interesting spectral features are not well understood despite decades of effort and thus are still the object of ongoing controversy. Since water clusters (H2O)n and their isomers are reminiscent of transient structures that appear in liquid water and in the tetrahedral network of ice, the study of the optical excitations of water clusters may help provide some insights into the understanding of the two spectral features of bulk water. Therefore, in this work we apply the time-dependent density functional theory (TD-DFT) to investigate the optical excitations and electronic spectra of water monomer, dimer, trimer, tetramer, pentamer, hexamer, heptamer, octamer, nonamer, decamer, large prism-like clusters (H2O)n (n = 11–19, 21, 22), pentagonal dodecahedron (H2O)20, tetrakaidecahedron (H2O)24, and hexakaidecahedron (H2O)28. Our calculated vertical singlet excitation energies for water monomer are in good agreement with experiment. The red tail predicted for water dimers strongly supports recent experiments. For all other water clusters, we find that the onsets of their UV absorption spectra and thus the long Urbach tail appear below 7 eV due to the intermolecular interactions, which induce a significant broadening of the energy levels relative to the isolated molecule and lead to a decrease of the transition energies relative to the molecule levels. Contrary to this aggregation-induced broadening effect, we also find in all water clusters the universal occurrence of the lowest maximum absorption band located around 8.7 eV. In particular, the averaged UV spectrum of water hexamers reproduces well the measured absorption spectrum of hexagonal ice. Our present study of the UV absorptions of water clusters has provided for the first time a valuable insight into the still largely mysterious radiation chemistry of bulk water.
Document Type: Research Article
Publication date: May 1, 2007
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