A THEORETICAL DESCRIPTION OF THE ION BEC-SPECTRUM OF BEC( AND THE PHOTOELECTRON)

The seven lowest-lying electronic states of the ion BeC+ are described theoretically using the multi-reference single and double excitations configuration interaction approach. Potential energy curves are prese nted for all the states as well as a description of the electronic str ucture characterizing each of them. A whole set of spectroscopic const ants completes the characterization of each state. In its ground state (...3sigma(2)1pi(2)4sigma, X 4SIGMA-) this molecule is more strongly bound (D(e) = 3.18 eV) than the neutral species (D(e) = 2.39 eV) and c onsequently has a shorter equilibrium distance (2.960 a0) compared to 3.150 a0 of BeC. The first two excited states are a 2PI (R(e) = 3.420 a0) and a 2DELTA (R(e) = 2.973 a0) at energies 0.528 and 1.582 eV. The first excited quartet (4PI) lies at 1.971 eV (R(e) = 3.616 a0). Of th e remaining doublets, the state d 2SIGMA+ is distinct for exhibiting a double minimum, with the inner equilibrium distance equal to 2.965 a0 and the outer one equal to 3.681 a0; a barrier of 0.25 eV at 3.263 a0 separates the two wells. A combination of these results with those pr eviously reported for the neutral species allowed the calculation of v ertical and adiabatic ionization potentials and the prediction of phot oelectron spectra based on the Franck-Condon distributions for transit ions from the ground state of BeC to selected bound states of BeC+.