  The optical contrast in a phase change material is concomitant with its
structural transition. We connect these two by first recognizing that Friedel
oscillations couple electrons propagating in opposite directions and supply an
additional Coulomb energy. As the crystal switches phase, this energy acquires
time dependence and the Landau-Zener mechanism operates, steering population
transfer from the valence to the conduction band and vice versa. Spectroscopy
suggests that the oscillator energy dominates the optical properties and a
calculation involving the crystalline field and spin-orbit interaction yields
good estimates for of both structural phases. Further analysis relates the
optical gap with the crystalline-field energy as well as activation energy for
electrical conduction. This last property characterizes the amorphous phase,
thereby furnishing a link between the crystalline field and the activation
energy and ultimately with the resistance drift exponent. Providing optical
means to quantify resistance drift in PCMs could circumvent the need for
fabricating expensive devices and performing time consuming measurements.
