258 Coherent Phenomena
populations in the J = 2 and J = 3 rotational state of the upper vibrational level.
A population inversion has been achieved for the vibrational levels (higher total
population in υ = 4 than in υ = 0).
4.3.3. Simplifying a N-Level System to a Two-Level
Transition
Taking into account the detailed level structure of a complex molecule,
following the procedure of the previous section is an arduous task. The atomic or
molecular system can be assimilated to a two-level system (the upper and lower
levels) if the intermediate levels are sufficiently off-resonance. To quantify the
conditions under which multiphoton transitions—as opposed to the cascade of
single photon transitions discussed in the previous sections—will be observed,
two ...