Isomer-specific spectroscopy of the ( H 2 O ) 8 − cluster anion in the intramolecular bending region by selective photodepletion of the more weakly electron binding species (isomer II)

The vibrational predissociation spectra of the two more strongly electron binding forms of the ( H 2 O ) 8 − anion are obtained in the HOH intramolecular bending region. This is accomplished by deconvoluting the overlapping spectra obtained from a mixed ensemble using a population modulation scheme in which the low electron binding isomer (II) is removed from the ion packet prior to spectroscopic analysis. By choosing the energy of the photodepletion laser to lie between the vertical detachment energies of the two isomers, the contribution from isomer II can be quantitatively eliminated, leaving the population of I largely unaffected. The low binding energies involved in the application of the method to the water cluster anions necessitate that this should be carried out in the midinfrared, thus requiring two tunable ir laser systems for implementation. The isolated spectrum of isomer 1 displays a strong, redshifted feature associated with a double H-bond acceptor (AA) water molecule in direct contact with the excess electron and a large gap before higher energy features appear that are typically associated with (acceptor/donor) AD and ADD binding sites in the network. The more weakly binding isomer II does not display the AA feature and instead contributes broad structure at intermediate redshifts that merges with the region associated with neutral water cluster networks.