This gap, or electron hole, can be thought of in a rather abstract way as a “thing” in itself: a positively charged hole. The idea is that we regard the rest of the magnesium atom as remaining neutral while we have created, through the absorption of the photon, a system consisting of the escaped negative electron and the positive hole it has left behind. This binary system is called an exciton (see figure 4.6) and can be thought of as a tiny battery with positive and negative poles capable of storing energy for later use.
