We formulate a semiclassical model for the collective response of an atom to a strong laser pulse and use it to study the behavior of the valence shell of xenon. We find that, in a monochromatic wave, a fraction of the shell ionizes rapidly and the rest is driven coherently. The energy distributions of the emitted electrons show reasonable agreement with experimental distributions that are generated via single-particle ionization, indicating that the spectra are independent of the ionization dynamics. The remaining part of the shell is driven coherently and expands with time, thus exerting minimal effect on the core. We also analyze the effects of pulse shape, pulse duration, and the ponderomotive potential. In particular, we present results for femtosecond pulses
