Theory
Under standard temperatures more than 99% of molecular species are found to occupy the ground electronic Furthermore, these molecules can only absorb discrete quanta of elecradiation when an incident photon has an energy coincident with the energy difference between two allowed energy states. The net result of this photon-induced promotion of a few molecules to a higher-energy state. Of course, between each of the electronic states there are vibrational levels, and within each vi- brational level there are still more rotational levels. This entire set of levels electronic-vibrational- makes up the electronic-vibrational