Reinvestigation of the Rb2 (2) 3Πg- a3∑u + band on helium nanodroplets

It is well known that alkali-metal molecules are preferentially observed in the weak van der Waals-bound high spin states by helium droplet isolation spectroscopy. In [F. R. Brühl, J. Chem. Phys. 115, 10275 (2001)] the Rb₂ (2) ³Π_g-a³∑ _u ⁺ band on He droplets was investigated by laser-induced fluorescence and dispersed emission spectroscopy. At that time no information on the magnitude of spin-orbit coupling was available for the (2) ³Π_g state which connects to the atomic 5s+4d asymptote and it was neglected. In this work we reinvestigate the observed spectra. The dispersed emission spectra, which resulted from free molecules, are consistent with state-of-the-art nonrelativistic potential energy surfaces and effective spin-orbit coupling matrix elements obtained from resonance-enhanced multiphoton ionization spectroscopy of cold Rb dimers [J. Lozeille, Eur. Phys. J. D 39, 261 (2006)]. Having validated the theoretical description of the free molecule state, we use the laser-induced fluorescence spectrum to discuss the influence of the He droplet on the excitation band.