The theory of pump-probe diffraction experiments shows that probability density distributions of vibrational wave functions are experimentally observable. In the experiment a laser prepares a molecule in a selected vibrational state, in either the same or a different electronic manifold. The diffraction pattern of the molecule in the excited state is the Fourier transform image of the nuclear probability density distribution, as determined by the vibrational eigenfunction of the molecule. This suggests the possibility to directly observe components of molecular vibrational wave functions. Model calculations illustrate the concept on iodine molecules, and sodium dimers. The relevance to time-resolved pump-probe experiments that prepare vibrational wave packets is discussed.