Second order contributions to the absorption of massive particles

Recently, in analogy with multiphoton ionization, it has been suggested that multiparticle ionization can also be induced by massive systems. We explore in this paper the possibility that multiparticle absorption processes can also take place for massive particles. To study it we consider, in a perturbative way, a model of absorption which illustrates the analogies with Glauber's scheme for photons and previous analysis on matter-waves coherence. A major advantage of this approach is that the dependence of the absorption rates on the wavefunction of the incident system can be analyzed in an explicit way. The calculations confirm the form of the second order (two-particle) contributions.