Van Vleck theory

Van Vleck theory explains how atoms and molecules absorb and emit light through interactions involving their electrons in external electric fields. It describes how the quantum states of electrons are affected by these fields and how this alters the observed spectral lines. Essentially, it provides a detailed understanding of how atomic energy levels shift and how light is absorbed or emitted, accounting for fine details influenced by electron interactions and quantum mechanics. This theory helps explain phenomena like spectral line splitting and broadening in various physical environments.