What is the term for the heating of an element to a high temperature, resulting in the expansion of electrons in an atom?

The heating of an element to a high temperature that causes the expansion of electrons in an atom is referred to as thermionic emission. In this process, when an atom is heated sufficiently, the added energy allows electrons to overcome the attractive forces of the nucleus, leading to their ejection from the atom. This phenomenon is particularly important in the operation of vacuum tubes and cathode ray tubes, where heated electrons contribute to the flow of electric current.

The other terms relate to different phenomena. Electron excitation refers to the process in which an electron absorbs energy and moves to a higher energy state within its atom, but does not necessarily result in the electron being ejected from the atom. Photon emission involves the release of a photon when an electron transitions from a higher energy state to a lower one, but again, this does not involve the heating process that causes electrons to be expelled. Thermal radiation denotes the energy emitted due to heat in the form of electromagnetic waves, but it does not specifically address the behavior of electrons in an atom as a result of heating.