ELSEVIER 8 September 1995 Chemical Physics Letters 243 (1995) 94-101 CHEMICAL PHYSICS LETTERS + + Emission spectra of calcium dimer. The A u -X l Xg system M.A. Gondal, M.A. Khan, M.H. Rais Laser Research Section, Energy Resources Division, Research Institute, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia Received 8 February 1995; in final form 13 July 1995 Abstract The AIX~+-X 1 + Xg spectral system of Ca 2 has been studied in two independent experiments. The collision-induced fluorescence due to this band was investigated following the resonant excitation of the 4s4p 3P 1 metastable state of calcium using a Nd : YAG laser pumped dye laser. In another experiment, a low current glow discharge in calcium vapor was used. We have assigned 86 spectral lines to this band system for vibrational quantum numbers (v'= 10-13 and v"= 0) by A Xu state dissociation energy D~ is estimated to be ~ 8693.6 + 1 cm-1. The applying a Dunham type analysis. The 1 + processes involved in the collisional excitation of the upper molecular state are discussed. 1. Introduction The alkaline earth metal dimers form a weakly bound van der Waals ground state and are therefore potential candidates for metal vapor excimer lasers. The weakly bound van der Waals character of the 2 2 X 1~; ground state of Ca is due to the tr_~tr, valence electron configuration with two bonding (~rg) and two antibonding (tr.) electrons. Studies have been carded out in the past to investigate the red and green system arising due to A-X and B-X transi- tions in the vapor phase using the laser-induced fluorescence technique [1-5]. However conclusions differed regarding the ground state as well as the excited states [1-5]. Belfour and Whitlock [1] recorded the first high- resolution absorption spectra of Ca 2 and assigned the green system as A IE~+-X 1~. They estimated the ground state dissociation energy D~' to be 1075 + 150 cm -1, while the upper state dissociation, D', is of the order of 6000 cm-l, assuming the state corre- lates in the long range to 1S +lp calcium atoms. Vidal confirmed these vibrational assignments from the laser-induced fluorescence measurements of the green system using an argon ion laser [2]. He im- proved the molecular constants by extending the term value data and calculated RKR (Rydberg- Klein-Rees) and IPA potential curves for both molecular states. He concluded that the green system upper state dissociated to 1S + 10 and that the ground state dissociation energy is -- 1095 ± 1 cm -1 while the excited state has D"--3980 cm -1 . Subsequent studies have, however, introduced considerable con- fusion into this issue. Earlier, Andrews and co- workers conducted absorption, LIF and laser excita- tion experiments on calcium dimer in rare gas matri- 1 + 1 + ces and assigned the red system as A Eu -X Eg arising from the 1S + 1p atomic limit [6-9]. Wyss [3] reported gas-phase observations of the Ca 2 red sys- tem and gave the value of D~' = 2075 cm-1 close to the matrix value. Since this value is quite large compared with the value of 1075 cm-1 reported by 0009-2614/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0009-2614(95)00812-8