In situ formation and characterisation of singly ionised atomic europium in rare gas matrices--luminescence spectroscopy and MP2 calculations.

Abstract

Irradiation of atomic europium isolated in the solid rare gases, with low intensity laser excitation of the y(8)P←a(8)S resonance transition at ca. 465 nm, is found to produce singly charged europium cations (Eu(+)) in large amounts in xenon and in smaller amounts in argon. Confirmation of the formation of matrix-isolated Eu(+) is obtained from characteristic absorption bands in the UV and in the visible spectral regions. The luminescence produced with excitation of the cation bands is presented in greatest detail for Eu/Xe and assigned. Excitation of the 4f(7)((8)S7/2)6p3/2 absorption bands of Eu(+) between 390 and 410 nm produces emission which is quite distinct from that resulting from excitation of the 4f(7)((8)S7/2)6p1/2 absorption (430 to 450 nm) features. The latter consists of narrow, resolved emission bands with Stokes shifts ten times smaller than the former. The observed spectral differences are discussed in relation to the different spatial symmetries of the p3/2 and p1/2 orbitals in these j-j coupled (7/2, 3/2)J and the (7/2, 1/2)J levels. Møller-Plesset calculations are conducted to obtain the molecular parameters of the neutral Eu-RG and cationic Eu(+)-RG diatomics (RG = Ar, Kr, Xe). From the short bond lengths and the strong binding energies obtained for the Eu(+)-RG species, these values suggest the isolation of the ion in small, possibly interstitial sites especially in xenon. In contrast, but consistent with previous work [O. Byrne and J. G. McCaffrey, J. Chem. Phys. 134, 124501 (2011)], the interaction potentials calculated herein for the Eu-RG diatomics suggest that the neutral Eu atom occupies tetra-vacancy (tv) and hexa-vacancy (hv) sites in the solid rare gas hosts. Possible reasons for the facile production of Eu(+) in the solid rare gases are discussed. The mechanism proposed is that atomic europium is also acting as an electron acceptor, providing a temporary trap for the ionised electron in the matrices.