The energetically lowest five states of a helium atom are: He(1 1 S), He(2 3 S), He(2 1 S), He(2 3 P ), and He(2 1 P ). Long-range interaction coefficients C 3 , C 6 , C 8 , C 9 , and C 10 for all S − S and S − P pairs of these states are calculated precisely using correlated wave functions in Hylleraas coordinates. Finite nuclear isotope mass effects are included.

Long-range interactions between a He(2 3 S) atom and a He(2 3 P ) atom for like isotopes AbstractFor the interactions between a He(2 3 S) atom and a He(2 3 P ) atom for like isotopes, we report perturbation theoretic calculations using accurate variational wave functions in Hylleraas coordinates of the coefficients determining the potential energies at large internuclear separations. We evaluate the coefficient C 3 of the first order resonant dipole-dipole energy and the van der Waals coefficients C 6 , C 8 , and C 10 for the second order energies arising from the mutual perturbations of instantaneous electric dipole, quadrupole, and octupole interactions. We also evaluate the coefficient C 9 of the leading contribution to the third order energy. We establish definitive values including treatment of the finite nuclear mass for the 3 He(2 3 S)-3 He(2 3 P ) and 4 He(2 3 S)-4 He(2 3 P ) interactions.

We report on the anisotropic mass-dependent dispersion coefficients C 6 ͑M =0, ±1͒ for He͑1 1 S͒-He͑2 3 P͒ interaction using two different techniques. The first is a highly accurate Hylleraas basis-set diagonalization, while the second method is a semiempirical assessment based on tabulated oscillator strengths and available photoionization cross sections. The results from the two techniques agree to within 1.5%.