Newly observed electronic transitions in rhenium monoxide (ReO).

We report laser-induced fluorescence excitation and single-vibronic-level emission spectroscopic studies on gas-phase rhenium monoxide, complemented by high-level ab initio calculations for the Λ-S and Ω electronic states below 30 000 cm-1. Altogether, 14 rotationally resolved vibronic bands were observed in excitation spectra within the energy range of 13 850-23 650 cm-1, and vibrationally resolved emission spectra revealed two Ω components of the ground X2∆ state and three Ω components of the a4Π state below 6500 cm-1. Rovibronic analysis of the spectra yielded electronic symmetries and key molecular constants, including the rotational constant, vibrational frequency, and spin-orbit splitting. In addition, time-resolved emission spectroscopy further identified two cascading decay processes, [17.5]5/2-[17.4*]3/2-X 5/2 and [20.2]5/2-[20.0]5/2-X 5/2. Notably, ten excited states with Ω = 5/2 were characterized in the energy range of 17 000-25 000 cm-1, providing a benchmark for quantum chemical calculations of the 5d-atom-containing molecules, which typically exhibit strong relativistic spin-orbit interactions and electron-electron correlations.