Electronically excited and ionized states of the chlorine molecule

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Potentials curves for the ground and excited states of the chlorine molecules and its positive and negative ions have been calculated by means of the MRD-CI method. The standard AO basis employed consists of 74 functions including two atomic d and one set of s and p bond species, and the results at the corresponding full CI level are estimated for each state via a perturbation correction. Special emphasis is placed upon the treatment of Rydberg-valence mixing in this system, which phenomenon is found to be essential to the understanding of Cl2 electronic absorption spectrum. All singlet states which correlate with the lowest dissociation limit plus many others which go to ionic Cl++Cl− or Rydberg Cl+Cl asymptotes are given explicit consideration. Among the triplet species of Cl2 which dissociate into the ground state atoms only the 3Πu state is not repulsive. The calculated D0 value for the ground state is 2.455 eV compared to the experimental value of 2.475 eV, while the vertical ionization energy and electron affinity are found to be 11.48 and 2.38 eV respectively, also in very good agreement with the corresponding measured data of 11.50 and 2.51 ± 0.1 eV. In addition to Cl2 laser line is confirmed to result from a 3Πg → 3Πu emission, whereby the calculated downward vertical transition energy of 4.86 eV fits in quite well with the known location of this line at 4.805 eV. The first two dipole-allowed transitions from the ground state of chlorine involve 1Σu+ and 1Πu states which are calculated to be nearly isoenergetic, and these results also match very well with the location of the first absorption band in this spectrum. Finally quite similarly as in O2 it is found that an avoided crossing between Rydberg and valences states produces a relatively steep potential well for an upper state (2 1Σu+), whose location concides with that of a second absorption band recently observed in synchrotron radiation studies.