  We review the Lee-Friedrichs model as a model of atomic resonances in the
hydrogen atom, using the dipole-moment matrix-element functions which have been
exactly computed by Nussenzveig. The Hamiltonian $H$ of the model is positive
and has absolutely continuous spectrum. Although the return probability
amplitude $R_{\Psi}(t)=(\Psi,{t\exp(-iHt)\Psi)$ of the initial state $\Psi$,
taken as the so-called Weisskopf-Wigner (W.W.) state, cannot be computed
exactly, we show that it equals the sum of an exponentially decaying term and a
universal correction $O(\beta^{2}\frac{1}{t})$ for large positive times $t$ and
small coupling constants $\beta$, improving on some results of C. King. The
remaining, non-universal part of the correction is also shown to be of the same
qualitative type. The method consists in approximating the matrix element of
the resolvent operator in the W.W. state by a Lorentzian distribution. No use
is made of complex energies associated to analytic continuations of the
resolvent operator to "unphysical" Riemann sheets. Other new results are
presented, in particular a physical interpretation of the corrections and the
characterization of the so-called sojourn time as the average lifetime of the
decaying state, a standard quantity in (quantum) probability.
