Identification of two distinct electron binding motifs in the anionic water clusters:: A vibrational spectroscopic study of the (H2O)6- isomers

Photoelectron spectroscopy of the water cluster anions, (H2O)n(-), has revealed that several isomeric forms are present for most sizes, and here, we use vibrational spectroscopy to address the structure of the (H2O)6(-)isomer that more weakly binds the extra electron. To overcome the severe line broadening that occurs in the OH stretching region of this isomer caused by fast electron autodetachment, we concentrate on the lowenergy bending modes of the perdeutero isotopomer. Sharp spectroscopic signatures are recovered for two isomers using argon predissociation spectroscopy, and the resulting bands are heavily overlapped. To extract their independent contributions to the observed spectra, we exploit the substantial dependence of their relative populations on the number of attached argon atoms in the (D2O)(6)-center dot Ar-m clusters, determined by photoelectron spectroscopy. The vibrational spectra of each isomer can then be isolated by spectral subtraction, which is implemented with a covariance mapping approach. The resulting band patterns establish that the more weakly binding isomer does not display the characteristic electron-binding motif common to the more strongly bound isomer class. Whereas the strongly binding isomer features a single water molecule pointing toward the excess electron cloud with both of its hydrogen atoms, the spectrum of the more weakly binding isomer suggests a structure where the electron is bound by a number of dangling OH groups corresponding to water molecules in acceptor-donor binding sites.