The valence electrons can be thought of as free electrons, and their three-dimensional nature within the confinement of a solid can be effectively modeled in terms of gas particles in a box. The electron emission microscopes are the earliest of the electron microscopes. In the thermionic electron emission microscope, the actual emission process is somewhat more complicated, and the imaging qualities of a specimen surface tend to be strongly dependent on the residual gases present, and the degree of surface reaction or adsorption. The work function of a metal at 0K is the minimum energy that must be imparted to the most energetic valence electrons to enable them to move into the unfilled levels of the conduction band and finally to escape from the physical interior of the metal to the surface exterior.