More than 20 collision-induced dipole (CID) transitions have been observed in the absorption spectrum of calcium below 2400 A in the presence of xenon or krypton. Some have also been observed in the presence of argon, neon or helium. The measured shifts and the effective oscillator strengths show a close correlation with the s-wave amplitude for the electron scattering by a rare-gas atom. This correlation leads us to the conclusion that the low-energy-electron-scattering process is responsible for the appearance of the CID transitions
