Photochemistry and spectroscopy of small hydrated magnesium clusters Mg⁺(H₂O)_n, n = 1-5

Hydrated singly charged magnesium ions Mg⁺(H₂O)_n, n ≤ 5, in the gas phase are ideal model systems to study photochemical hydrogen evolution since atomic hydrogen is formed over a wide range of wavelengths, with a strong cluster size dependence. Mass selected clusters are stored in the cell of an Fourier transform ion cyclotron resonance mass spectrometer at a temperature of 130 K for several seconds, which allows thermal equilibration via blackbody radiation. Tunable laser light is used for photodissociation. Strong transitions to D_1-3 states (correlating with the 3s-3p_x,y,z transitions of Mg⁺) are observed for all cluster sizes, as well as a second absorption band at 4-5 eV for n = 3-5. Due to the lifted degeneracy of the 3p_x,y,z energy levels of Mg⁺, the absorptions are broad and red shifted with increasing coordination number of the Mg⁺ center, from 4.5 eV for n = 1 to 1.8 eV for n = 5. In all cases, H atom formation is the dominant photochemical reaction channel. Quantum chemical calculations using the full range of methods for excited state calculations reproduce the experimental spectra and explain all observed features. In particular, they show that H atom formation occurs in excited states, where the potential energy surface becomes repulsive along the O⋯H coordinate at relatively small distances. The loss of H₂O, although thermochemically favorable, is a minor channel because, at least for the clusters n = 1-3, the conical intersection through which the system could relax to the electronic ground state is too high in energy. In some absorption bands, sequential absorption of multiple photons is required for photodissociation. For n = 1, these multiphoton spectra can be modeled on the basis of quantum chemical calculations.