Experimental and theoretical studies of the electronic transitions of BeC

Electronic spectra for BeC have been recorded over the range 30 500-40 000 cm^-1. Laser ablation and jet-cooling techniques were used to obtain rotationally resolved data. The vibronic structure consists of a series of bands with erratic energy spacings. Two-color photoionization threshold measurements were used to show that the majority of these features originated from the ground state zero-point level. The rotational structures were consistent with the bands of ³Π-X³Σ^- transitions. Theoretical calculations indicate that the erratic vibronic structure results from strong interactions between the four lowest energy ³Π states. Adiabatic potential energy curves were obtained from dynamically weighted MRCI calculations. Diabatic potentials and coupling matrix elements were then reconstructed from these results, and used to compute the vibronic energy levels for the four interacting ³Π states. The predictions were sufficiently close to the observed structure to permit partial assignment of the spectra. Bands originating from the low-lying 1⁵Σ^- state were also identified, yielding a ⁵Σ^- to X ³Σ^- energy interval of 2302 ± 80 cm ^-1 and molecular constants for the 1⁵Π state. The ionization energy of BeC was found to be 70 779(40) cm^-1.