Experimental and theoretical study of the electronic spectrum of BeAl

The electronic structure of BeAl was investigated by laser induced fluorescence and resonance enhanced multiphoton ionization spectroscopy. BeAl was formed by pulsed laser ablation of a Be/Al alloy in the presence of helium carrier gas, followed by a free jet expansion into vacuum. In agreement with recent ab initio studies, the molecule was found to have a ²Π _1/2 ground state. Transitions to two low lying electronic states, (2)²Π_1/2(v′) ← X ²Π _1/2 (v″ = 0) and (1)²Δ(v′) ← X ²Π_1/2 (v″ = 0,1), were observed and rotationally analyzed. An additional band system, identified as (4) ²Σ⁺(v′) ← X ²Π _1/2, was found in the 28 000-30 100 cm^-1 energy range. This transition exhibited an unusual pattern of vibrational levels resulting from an avoided crossing with the (5)²Σ⁺ electronic state. New multi-reference configuration interaction calculations were carried out to facilitate the interpretation of the UV bands. An ionization energy of 48 124(80) cm^-1 was determined for BeAl from photoionization efficiency (PIE) measurements. Fine structure in the PIE curve was attributed to resonances with Rydberg series correlating with vibrationally excited states of the BeAl⁺ ion. Analysis of this structure yielded a vibrational frequency of 240(20) cm^-1 for the cation.