Phase equilibrium, multiple occupation

B. Phase Equilibrium and the vdWP Theory for Multiple Occupation at High Pressure... 

This long list of assumptions defines the simplest adsorption model. In later sections we shall discuss more complex cases where each one of the above assumptions will be removed. For instance, multiple occupancy is discussed in section 2.9. The case with different types of sites is discussed in section 2.8.7, indistinguishable adsorbing sites in section 2.9.2, dependence between sites in Chapter 3, and equilibrium with a liquid phase in Chapter 8. 

In the previous section we discussed light and matter at equilibrium in a two-level quantum system. For the remainder of this section we will be interested in light and matter which are not at equilibrium. In particular, laser light is completely different from the thennal radiation described at the end of the previous section. In the first place, only one, or a small number of states of the field are occupied, in contrast with the Planck distribution of occupation numbers in thennal radiation. Second, the field state can have a precise phase-, in thennal radiation this phase is assumed to be random. If multiple field states are occupied in a laser they can have a precise phase relationship, something which is achieved in lasers by a teclmique called mode-locking Multiple frequencies with a precise phase relation give rise to laser pulses in time. Nanosecond experiments... 

---