IL NUOVO CIMENTO VOL. 17 D, N. 1 Gennaio 1995 Laser-Enhanced Ionization Studies of the Blue Satellite Band in the Ca-At System Associated with the Electric-Quadrupole Transition 4S 21S 0 --> 4s3d 1D2 of Calcium (*). M. A. GONDAL, M. A. KHAN and M. H. RhIS Laser Research Laboratory, Energy Resources Division, Research Institute King Fahd University of Petroleum and Minerals - Dhahran 31261, Saudi Arabia (ricevuto il 15 Aprile 1994; approvato il 28 Novembre 1994) Summary. -- The collision-assisted electric-quadrupole transition 4S21So--)4s3d 1D 2 of calcium at 4575/~ has been observed using the laser-enhanced ionization technique for the first time. A blue satellite band due to the interaction of calcium 4s3d 1D 2 state with argon 1S 0 has also been detected at 4527/~ which is shifted by 48/~ from the isolated atomic calcium line. The line broadening cross-section for the blue satellite has been calculated by measuring its collisional broadening at various argon pressures. Important processes involved in the generation of this blue satellite band are qualitatively discussed. PACS 34.50.Fa - Electronic excitation and ionization of atoms (including beam-foil excitation and ionization). PACS 32.70.Jz - Line shapes, widths, and shifts. PACS 31.70 - Effects of molecular interactions on electronic structure. 1. - Introduction. Collision-assisted transitions have been a subject of great interest to atomic spectroscopists for many years [1-4]. Many atoms exhibit transitions of low oscillator strength which violate selection rules for electric-dipole radiation. These transitions are of two types: those which violate the approximate electric-dipole selection rule for LS coupling AS = 0 and those which violate selection rule AL = _+1. Electric- quadrupole and magnetic-dipole transitions are examples of the second category. Both types of these transitions are labeled as ,,forbiddem,. A substantial oscillator strength could, however, be induced into the forbidden line if the atomic radiator or absorber undergoes collisions with a perturber (e.g., buffer gas atom). In the presence of perturbing atoms, these forbidden lines are often accompanied by satellite bands. A satellite can occur when, for a particular separation of emitter and (*) The authors of this paper have agreed to not receive the proofs for correction.