Rydberg-type electronic excited state dynamics in small sodium–water clusters

Abstract

Equilibrium geometries and thermodynamic potentials of the neutral and ionised species of the Na ⋯ ( H 2 O ) n ( n = 1 , . . , 8 ) mixed clusters were computed at MN15/def2-TZVPD level of density functional theory (DFT). The vertical and adiabatic ionisation energies and enthalpies were computed and their cluster size dependence was discussed. Laser-induced ionisation involves electronic excitation through Rydberg-type excited states, which have been characterised using the TDDFT method, including the ωB2PLYP double-hybrid exchange-correlation functional. Ab initio molecular dynamics calculations were performed on a time scale of 20 picoseconds. Fluctuations of the charge and the sodium–oxygen atomic distances predict that, the 3 s 1 electron of the sodium atom are transferred from the delocalised Rydberg orbitals to the Rydberg orbitals around the water molecules and the sodium atom becomes positively charged with around 0.6e after the first 10 ps . On the other hand, some of the water molecules can move away up to 5 Å  from the sodium with a significant negative charge on them. It has been shown that non-radiative relaxation cannot be excluded, they can mostly occur for cases n ⩾ 4 . The results confirm that the adiabatic photo-ionisation can occur on the basis of cluster disintegration.