Fill in the following blanks with suitable ones.1. One of the important processes to complete the process of equilibrium between the upper and lower transition levels is the process of .............. 2- The probability of self-emission increases with the increase of .......... ........And......... 4- When the energy difference between two atomic or molecular levels is small, the difference in the number of atoms of these two levels is.... 4- The laser is one of the light sources......... that emit within a wide range of the elastic spectrum.