Assignment of photoelectron spectra of (TiO2)n with n = 1-3

This paper reports a comprehensive study of (TiO(2))(n) (n=1-3) clusters on their structures, vertical electron affinities (VEA) and adiabatic electron affinities (AEA), and excited states that are further correlated with experimental photoelectron spectra (PES). Local density functional and time-dependent density functional methods (TTDFT) with 6-31G(d) basis set are used. For (TiO(2))(n) with n=1 and 3, the ground-state geometries are found to have C(2v) and C(s) structures, respectively, in both neutral and anionic charge states. For neutral (TiO(2))(2), the most stable structure corresponds to a C(2h) conformation. For anionic (TiO(2))(2)(-), two isomers with C(2h) and C(2v) symmetries are identified to be nearly isoenergetic. This conclusion is supported by reoptimization at CCSD/6-31G(d) level and single-point calculation at CCSD(T)/6-311+G(d) level. Hence, they are competitive candidates for the ground-state structure of (TiO(2))(2)(-). For TiO(2) and (TiO(2))(3), the calculated VEA and AEA values agree well with the experimental data. For (TiO(2))(2), the VEA and AEA obtained with the C(2v) structure agree well with the experimental data while those with the C(2h) structure do not. The subsequent assignment of PES indicates that it is the C(2v) isomer of (TiO(2))(2)(-) that represents the structure from which the photoexcitation in experimental PES takes place. TDDFT is used to determine the excited states of (TiO(2))(n) (n=1-3) and the calculated excitation energies are in good agreement with PES experiment.