ABINITIO STUDY OF SMALL HE CLUSTER IONS HE-2+, HE-3+, HE-4+, HE-5+, AND LOW-LYING RYDBERG STATES OF HE-4

SCF and CEPA calculations are applied to study the structure of small He cluster ions, Hen+, n=2, 3, 4, 5 and some low-lying Rydberg states of He4. The effect of electron correlation upon the equilibrium structures and binding energies is discussed. He3+ has a linear symmetric equilibrium geometry with a bond length of 2.35 a0 and a binding energy De=0.165 eV with respect to He2++He (experimentally:D0=0.17 eV which corresponds to De ≈0.20 eV). He4+ is a very floppy molecular ion with several energetically very similar geometrical configurations. Our CEPA calculations yield a T-shaped form with a He3+ centre (Re = 2.35 a0) and one inductively bound He atom (4.39 a0 from the central He atom of He3+) as equilibrium structure. Its binding energy with respect to He3++He is 0.031 eV. A linear symmetric configuration consisting of a He2+ centre with a bond length of 2.10 a0 and two inductively bound He atoms (4.20 a0 from the centre of He2+) is only 0.02-0.03 eV higher in energy. We expect that in larger He cluster ions structures with He2+ and He3+ centres and n-2 or n-3 inductively bound He atoms have nearly the same energies. In He4 a low-lying metastable Rydberg state (3Π symmetry for linear He4*,3B1 for the T-shaped form) exists which is slightly stronger bound with respect to He3*+He than the corresponding ion. © 1990 Springer-Verlag.