Secondary ion emission is analyzed within the framework of linear transport theory. The energy spectra and yield of the ionized particles are evaluated by assuming the ionization probability to be given by a time-dependent force exerted on the emerging atoms by the solid surface. A calculation of these energy spectra shows that, at least where clean metal surfaces are concerned, the ionization probability must be applied to the emerging atoms before their penetration of the surface potential barrier. This ensures a shift in the peak and a broadening of the spectrum, compared with the corresponding results for neutral atoms, which is consistent with the experimental data. Estimations of the absolute yields are also given for positive ion emission.