A quantum dynamical wave packet description of photoassociation reactions induced by short laser pulses is developed including both vibrational/translation and rotational degrees of freedom. Various levels of approximation (perturbation approach, rotating wave approximation) are discussed. Simulations of the exciplex formation of mercury are performed and related to recent pump-probe experiments by Marvet and Dantus [Chem. Phys. Lett. 245 (1995) 393]. It is shown that in these experiments photoassociation is only due to bound ← free transitions and does not proceed via bound ← bound transitions from a van-der-Waals precursor. The calculated spectra show both vibrational and rotational coherence structure which can be interpreted in terms of quantum beats between different rovibrational states populated during the photoassociation process. The thermally averaged spectra show good qualitative agreement with the experimental data.