The blue-violet absorption band in the Rb-2 molecule at 430 nm is studied by ultrafast pump-probe spectroscopy and quantum dynamical calculations. We find that two electronic states are excited simultaneously and thereby the dynamics essentially proceeds in two independent channels. One channel exhibits vibrational motion in the D'(3)(1)Sigma(u)(+) shelf state while the other involves the D(3)(1)Pi(u) state which is heavily perturbed by spin-orbit interactions. Moreover, we directly observe the build-up of the wavefunction in the (4)(3)Sigma(u)(divided by) state which together with (1)(3)Delta(u) are proposed to be the major perturbers. (C) 2002 Elsevier Science B.V. All rights reserved.