Photodissociation study on Ca⁺(H₂O)_n, n=1-6: Electron structure and photoinduced dehydrogenation reaction

The photodissociation spectra of size-selected Ca⁺(H₂O)_n (n = 1-6) ions are examined in the wavelength region from 330 to 1440 nm by monitoring the total yield of the fragment ions. The absorption bands exhibit redshifts as large as ∼16 000 cm^-1 with respect to the ²P-²S resonance line of the free Ca⁺ ion and are explained by the shift of this transition as a result of hydration. The converging trend in the spectral shifts at n∼6 is discussed in relation to the filling of the first solvation shell for Ca⁺ undergoing the sdσ hybridization. We also discuss the possible contribution of charge-transfer character in the observed transitions in conjunction with our recent results on the photoelectron spectra of Na^-(NH₃)_n. The mass spectra of the fragment ions show the existence of two dissociation channels: The evaporation of water molecules and the dehydrogenation reaction to produce the hydrated CaOH⁺ ions. The evaporation process is the dominant decay channel for n = 1, while the reaction is the only one for n≥3. The former process is found to take place from the higher vibrational levels in the ground state being populated through the fast internal conversion induced by the presence of the low-lying ²D-type states. As for n = 1, the reaction takes place only in an excited state which has the Ca⁺p orbital aligned on the intermolecular axis. The state-specific reaction for n = 1 is explained in terms of charge-transfer interaction between the Ca⁺ ion and the water molecule. On the other hand, the reactivity for the larger clusters drastically increases with increasing cluster size. These reaction features are discussed in comparison with those for Mg⁺(H₂O)_n reported previously.