DYNAMICAL SCREENING EFFECTS IN SURFACE-IONIZATION - APPLICATION TO C-60

We study the positive ionization probability P+(E(c)) of a carbon atom sputtered from a C-60 target at energy E(c). As the atom leaves the t arget molecule its atomic level progressively decouples from the targe t levels. This can be described by means of the Anderson Hamiltonian w ith time-dependent parameters. We discuss some attempts published in t he literature for solving this kind of time-dependent problem. In our C-60 study we first use an approximate boson technique. Then we presen t a numerical exact solution of the Schrodinger equation. The dimensio n of the N-electron determinant basis is controlled by verifying that an increase of the basis size does not change the results. The exact a nd approximate (boson approximation) calculations are compared. The as pect of the P+(E(c)) curves is globally the same, i.e., a power-law de pendence. The difference between the two approaches is that the expone nt is constant in the approximation and is (slowly) varying with E(c) in the exact calculation. This power-law dependence agrees with typica l experimental results on similar systems. We show that our P+(E(c)) d ependence is obtained because we take into account the excitations whi ch appear in the whole electronic structure, particularly on the targe t levels, during the decoupling process.