Spectroscopic characterization of Be(2) (+) X (2)Sigma(u) (+) and the ionization energy of Be(2).

Rotationally resolved spectra for Be(2) (+) have been recorded using the pulsed-field ionization zero kinetic energy photoelectron technique. Vibrational levels in the range v(+)=0-6 were observed. The rotational selection rules confirmed that the ground state is (2)Sigma(u) (+), resulting from the removal of an electron from the sigma(u) antibonding orbital of Be(2). The bond energy and equilibrium distance for Be(2) (+) were found to be D(e) (+)=16 438(5) cm(-1) and R(e) (+)=2.211(8) A. The ionization energy for Be(2) [59 824(2) cm(-1)] was also refined by these measurements. Comparisons with high-level theoretical results indicate that the bonding in Be(2) (+) is adequately described by multi reference singles and doubles configuration interaction (MRDCI) calculations that employ moderate to large scale basis sets.