Probing isomer interconversion in anionic water clusters using an Ar-mediated pump-probe approach: Combining vibrational predissociation and velocity-map photoelectron imaging spectroscopies

We present the first results from an experiment designed to explore barriers for interconversion between isomers of cluster anions using an Ar-cluster mediated pump-probe technique. In this approach, anions are generated with many Ar atoms attached, and one of the isomers present is selectively excited by tuning an infrared laser to one of the isomer's characteristic vibrational resonances. The excited cluster is then cooled by evaporation of Ar atoms, and the isomer distribution in the lighter daughter ions is measured after secondary mass selection by recording their photoelectron spectra using velocity-map imaging. We apply the method to the water hexamer anion, (H(2)O)(6) (-), which is known to occur in two isomeric forms with different electron-binding energies. We find that conversion of the high-binding (type I) form to the low-binding (type II) isomer is not efficiently driven in (H(2)O)(6) (-) with excitation energies in the 0.4 eV range even though it is possible to create both isomers in abundance in the ion source. This observation is discussed in the context of the competition between isomerization and electron autodetachment, which depends on the relative positions of the neutral and ionic potential surfaces along the isomerization pathway. Application of the method to the more complex heptamer ion, however, does reveal that interconversion is available among the highest binding isomer classes (I and I(')).