Theoretical study on low-lying excited states of B₃ molecule

The electronic states of the smallest boron cluster B₃ with excitation energies up to 5 eV are systematically investigated. Geometries and spectroscopic constants for the low-lying electronic states were calculated using the multireference configuration interaction method with Davidson correction (MRCI+Q). The nondegenerate 1 ²B₂ and 2 ²A₁ states are arising from the degenerate ²E′ state in D_3h symmetry, this is also the case for 2 ²B₂ and 3 ²A₁. Furthermore, vertical excitation energies, oscillator strengths, main configurations, and transitions of the excited state of B₃ were determined. Notably, the theoretically predicted wavelengths for the X²A₁ → 2²A₁ and X²A₁ → 2²B₂ electronic transitions (728 nm and 457 nm, respectively) exhibit excellent agreements with experimental absorption bands observed at 736 nm and 458 nm. These theoretical findings provide critical insights into the electronic structure and geometric configuration of the B₃ cluster.