Rami Arieli: "The Laser Adventure" Section 6.4 page 3
Simplified Energy Level Diagram of Dye Laser
The structure of energy levels of organic dye molecules in a solvent is very complex. The explanation below is based on a simplified energy level diagram described in figure 6.34.
Figure 6.34: Simplified Energy Level Diagram of Dye Laser.

The width of each energy band is of the order of tenths of electron volts [eV]. At room temperature, the thermal energy of vibration is of the order of 1/40 [eV]. Thus only the bottom of the each energy level is filled (marked darker in figure 6.34).