Relaxation of (CS₂)₂^- to its global minimum mediated by water molecules: Photoelectron imaging study

The coexistence of several isomers of (CS₂)₂ ^- is examined via photoelectron imaging at 355 and 266 nm. Assisted by theoretical calculations, the bands in the photoelectron spectra are assigned to the CS₂^-·CS₂ ion-molecule complex (C_s symmetry, ²A′ electronic state) and two covalently bound dimer-anion structures: C_2v (²B ₁) and D_2h (²B_3g). The isomer distribution depends sensitively on the ion source conditions, particularly the presence of water in the precursor gas mixture. The intensity variation of the photoelectron bands suggests that the presence of water enhances the formation of the global-minimum C_2v (²B₁) structure, particularly relative to the metastable (local-minimum) ion-molecule complex. This trend is rationalized with two assumptions. The first is that the presence of H₂O at the cluster formation stage facilitates the nonadiabatic transitions necessary for reaching the global-minimum dimer-anion equilibrium when starting from the CS₂^- + CS₂ asymptote. The second is that the initial clusters formed in the presence of water tend to have, on average, more internal energy, which is needed for overcoming the potential energy barriers separating the metastable equilibria from the global-minimum structure. As the covalent bonds are formed, excess solvent molecules are evaporated from the cluster, giving rise to stable (CS ₂)₂^- dimer anions. In the (CS₂) _n^-, n > 3, and (CS₂)^{2 -}(H₂O)_m, m > 0, clusters, the population of the covalent-dimer core structures diminishes drastically due to more favorable solvent interactions with the monomer-anion (i.e., CCS₂^-) core.