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Experimental and theoretical pump-probe studies are performed for the K 2 molecule. Special molecular spectroscopic properties combined with the dynamics induced by "femtosecond state preparation" facilitate the transition from pump-probe to control spectroscopy. Hereby, the intensity of the laser field serves as a control parameter in the observed multiphoton processes. In the monitored transient ion signal we can distinguish the effect of two processes, i.e., multiphoton ionization (MPI) and resonant impulsive stimulated Raman scattering (RISRS). As a consequence, the temporal evolution of the ion signal reflects the induced dynamics of either the first excited state A 1 Σ u + or the ground state X 1 Σ g + of the potassium dimer.

Femtosecond pump-probe experiments on the potassium dimer are simulated by quantum dynamical calculations and compared with the experimentally found results. At moderate laser intensities three neutral electronic states dominantly participate in the ionization process. The transition mechanism is a pure ( I + 2)-photon pump-probe process. The two-photon ionization process is located at the outer turning point of the wave packet in the state of the K2 molecule. Both the favorable spectroscopic properties and the special molecular dynamics induced by the moderate laser intensities combine to open a Franck-Condon window for ionization via the 2Il-4 state which allows the selective detection of the vibrations in the A I L + state.

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