Collision-Induced Dipole Transitions Associated with High-Lying States. II. Experimental and Theoretical Studies of Calcium in Rare Gases

More than 20 collision-induced dirole (CID) transitions (4s²−4sns with 7≤n≤9, −4snd with 5≤n≤12, −4snf with 4≤n≤8, etc.) have been observed in the absorption spectrum of calcium below 2400 A in the presence of xenon or krypton. Some of them have been also observed in the presence of argon, neon or helium. The measured shifts and the effective oscillator strengths show a close correlation with low-energy-electron-scattering parameters of the rare-gas atoms. Present theoretical calculation of the effective oscillator strengths for 4s2−4snd (8≤n≤12) reproduces the dependence of the measured oscillator strengths on the scattering lengths of the rare-gas atoms and on the effective principal quantum numbers of the upper D levels, but the calculation gives larger values by a factor of about 4 than the experiment.