Adiabatic ionization potential of Al2 and vibrational spectrum of the X 2Σ+g state of Al+2

We have measured the adiabatic ionization potential of the aluminum dimer, AIP(Al2)=5.989±0.002 eV (±2σ uncertainty), using threshold photoelectron spectroscopy with resonant two photon ionization (ω1 fixed and ω2 scanned). We obtain the Al+2 , X 2Σ+g vibrational constants ωe =178±8 cm−1 and ωexe =2±2 cm−1 (±2σ uncertainty), the first spectroscopic data for a cationic Al cluster. These results are in excellent agreement with ab initio calculations.