Relaxation of a prepared state

Suppose that following the excitation pulse the system was somehow prepared in the excited state 2), so that at t = 0, the starting time of our observation, = 0 and the only non-vanishing element of fr isa22 = l.Keepinginmindthatcrii- -cr22 = k 

The rates A i 2and 2 i satisfy the detailed balance condition k 2lk2- = exp(j6(ii2 — E )) and both may depend on the temperature T. The limit of zero temperature in which 2 1 = 0 is often relevant in optical spectroscopy, even at room temperature if 2 — iii ksT. In this case, k 2 is the total decay rate of level 2—sum of rates associated with different relaxation channels represented by the continuous state manifolds of the model of Fig. 18.1. However, as discussed below, pure dephasing does not exist in this limit. 

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