Here's the question... A hydrogen atom is in the ground state at time t = 0. At this time an external electric field of magnitude E(t)=E*exp(-t/tau) is applied along the z direction. Find the first-order probability that the atom will be in the 210 (nlm) state at time t >> tau, assuming that the spontaneous transition probability for the 2p -> 1s transition is negligible at that time. What i'm thinking is I need to find the average pertubation (<psi|H'|psi>) and then use time dependent pertubation theory to solve this... but i'm not really sure what wave functions i'd use to do this. But also i'm conflicted if I should really be doing it this way and not using something for absorption/stimulation in the electric dipole approximation... any suggestions?