The spin-orbit interaction of the 5d electron needs to be taken into account to give the proper energy structure for the 4fN-15d configuration of heavy lanthanide ions occupying a site with ligands forming an octahedron. This paper derives theoretical results for the energy structure by treating the t2 orbitals as quasi p orbitals and then using angular-momentum coupling techniques. An analytic expression for the electric dipole absorption line strengths between 4fN multiplets and 4fN-15d states is given in terms of various angular-momentum quantum numbers and re-coupling coefficients. The result is then applied to interpret the excitation spectrum of Cs2NaYF6:Er3+. The high-spin and low-spin states of Cs2NaYF6:Er3+ are discussed in terms of the wavefunctions obtained by using the developed theoretical model.