Summary form only given. It is now well established that laser pulses, whose duration is not much larger than the transit time inside the cavity, undergo a beam-parameter evolution (concerning spot size, radius of curvature, far-field divergence, etc.) during the pulse life, from its beginning to its end. In practice, pulsed lasers, such as XeCl excimer lasers generate pulses making only a few transits inside the resonator. As a consequence, the steady state is seldom reached at the end of the pulse and the beam quality of the pulse is worse than that of a steady-state beam.