The nature of the interaction, as well as the reaction of a metastable rare-gas atom, with a ground-state rare-gas atom is investigated using elastic differential cross sections measured by the crossed-molecular-beam method. In a crossed-molecular-beam experiment, the initial conditions of colliding pairs, such as the relative kinetic energy of collision and the spin state of metastable helium atoms can be defined precisely. Besides, in the measurement of angular distribution of elastically scattered metastable atoms, we are observing the consequences of single collisions. If the quenching processes only occur at relatively small impact parameter such that the depletion of the intensity of scattered metastable atoms due to quenching processes only occurs at angles much larger than classical rainbow angle, it is possible to derive information on both interatomic potentials in the van der Waals region (1) and the probability of quenching processes as a function of impact parameter from the analysis of differential cross sections measured.