The spectroscopic signatures of proton transfer in the water dimer cation were investigated.
The six lowest electronic states were characterized along the reaction coordinate
using the equation-of-motion coupled-cluster method with single and double substitutions
for ionized systems. The nature of the dimer states was explained in terms of
the monomer states using a qualitative molecular orbital framework. We found that
proton transfer induces significant changes in the electronic spectrum, thus suggesting
effective use time-resolved electronic femtosecond spectroscopy to monitor the dynamics
following ionization. The electronic spectra at vertical and proton-transferred configurations
include both local excitations (features similar to those of the monomers) and
charge-transfer bands. Ab initio calculations were used to test the performance of a
self-interaction correction for density functional theory (DFT). It is found that the corrected
DFT/BLYP method is capable of reproducing the correct ordering of the (H2O)+2
isomers, and thus may be employed in calculations of larger systems.