Europium and dieuropium in rare gas solids.

The low resolution electronic absorption spectrum of 12 K Ar, Kr or Xe matrix isolated atomic europium is reported. Previous studies restricted to the ultraviolet f - d transitions have been extended to the visible region, where an intense absorption due to the 4f(7)6s6p - 4f(7)6s(2) process occurs between the ground a(8)S(o)7/2 and excited y(8)P 9/2,7/2,5.2 states. Matrix induced perturbations of the spectra are discussed. At higher Eu concentrations in Ar a new absorption maximizing at 579 nm develops. Quantitative analysis of the Eu atom concentration dependence of this absorption within a simple kinetic model for multimer formation, suggests that this band is carried by a europium dimer. Thermal annealing and extended Eu atom concentration studies show that the state of Eu self-aggregation depends sensitively on the conditions of matrix preparation and post-treatment. Reference is made to dieuropium in the context of diatomic-bulk metal electronic properties correlations.