Spectroscopic characterization of the ionization energy, $^{3}\Sigma _{u}^{+}$, (2)$^{3}\Pi _{g}$, and (3)$^{3}\Pi _{g}$ states of Be$_{2}$

Low-lying electronic states of beryllium dimer are investigated by laser induced fluorescence (LIF) and resonance enhanced multiphoton ionization (REMPI) spectroscopies. Be$_{2}$ is formed by pulsed laser ablation and free jet expansion into vacuum. Comparing 1+1 REMPI and LIF spectra for the X$^{1}\Sigma _{g}^{+}$ (v=0) -$>$ B$^{1}\Sigma _{u}^{+}$(v) bands we find significant perturbations in the REMPI spectra, which are interpreted as autoionizing resonances in the ionization continuum. Photoionization efficiency (PIE) measurements yield an accurate value for the ionization energy, namely 7.40 eV, which is considerably larger than previous theoretical predictions. New CASSCF/MRCI calculations are presented which accurately reproduce the experimental IP. Rotationally resolved spectra for the (1)$^{3}\Sigma _{u}^{+}->$ (2)$^{3}\Pi _{g}$ and (1)$^{3}\Sigma _{u}^{+}->$ (3)$^{3}\Pi _{g}$ band systems of Be$_{2}$ have also been measured for the first time providing further experimental benchmarks for recent ab initio calculations. PIE measurements are also used to accurately determine the X$^{1}\Sigma _{g}^{+}$ $<->$ (1)$^{3}\Sigma _{u}^{+}$ interval.