Trends in 3p photoabsorption in the Ar I isoelectronic sequence: spectra of Sc IV and Ti V

The Rydberg series of argon-like Sc IV and Ti V have been observed using the dual-laser-produced plasma method. Lines arising from transitions up to n = 8 have been identified in the Ti V spectrum using the average energy of configuration multiconfiguration Hartree - Fock calculations and Rydberg series expansion. The series in Sc IV is seen to be considerably weaker for n>3 than for adjacent ions. The random-phase approximation with exchange calculations confirm this behaviour and show that it originates from a change in sign of the dipole matrix element between n = 4 and n = 5 in this spectrum.