Final answer:
Hot bands are molecular transitions that originate from vibrationally excited states due to thermal energy at higher temperatures, similar to the behavior of electrons in a hot gas. They are part of the molecular energy levels that form closely spaced energy bands in solids, involving the valence and conduction bands.
Step-by-step explanation:
The term hot bands refers to transitions in molecular spectroscopy that originate from vibrationally excited states in the electronic ground state of molecules. These bands are called hot because they are generally observed at elevated temperatures, where a significant population of molecules exists in vibrationally excited states due to thermal energy. In these conditions, molecules have absorbed enough energy to occupy higher vibrational levels even before they undergo electronic transitions. The comparison to a hot gas is apt: just as atoms in a hot gas can have electrons in higher energy states because of increased temperature, in hot bands, the molecules already have vibrations of higher energy levels before absorption or emission, reflecting the temperature of the environment they are in.
In solids, as the number of atoms increases, energy bands are formed from the molecular orbitals, and these bands can be described as being continuously spaced with allowed energy states that are closely packed together, especially in large numbers of atoms. These energy bands, made up of molecular orbitals, include the valence band, which is at a lower energy level, and the conduction band, which is at a higher energy level. The gap between these bands, known as the band gap, varies among different materials and determines their electrical conductivity properties.