Electronic states and spin-orbit splitting of lanthanum dimer

Lanthanum dimer (La₂) was studied by mass-analyzed threshold ionization (MATI) spectroscopy and a series of multi-configuration ab initio calculations. The MATI spectrum exhibits three band systems originating from ionization of the neutral ground electronic state, and each system shows vibrational frequencies of the neutral molecule and singly charged cation. The three ionization processes are La₂⁺ (a²Σ _g⁺) ← La₂ (X¹Σ _g⁺), La₂⁺ (b²Π _{3/2, u}) ← La₂ (X¹Σ_g ⁺), and La₂⁺ (b²Π _{1/2, u}) La₂ (X¹_g⁺), with the ionization energies of 39 046, 40 314, and 40 864cm^-1, respectively. The vibrational frequency of the X¹_g ⁺ state is 207cm^-1, and those of the a²Σ _g⁺, b²Π_{3/2, u} and b ²Π_{1/2, u} are 235.7, 242.2, and 240cm^-1. While X¹Σ_g⁺ is the ground state of the neutral molecule, a²Σ_g⁺ and b ²Π_u are calculated to be the excited states of the cation. The spin-orbit splitting in the b²Π_u ion is 550cm^-1. An X⁴Σ_g^- state of La₂⁺ was predicted by theory, but not observed by the experiment. The determination of a singlet ground state of La₂ shows that lanthanum behaves differently from scandium and yttrium.