Absorption spectroscopy and transition polarizations: pictorial description

Definition: Absorption can be pictorially viewed as either the electric field or the magnetic field (or both) of the radiation pushing the molecular electron density from a starting arrangement to a higher energy final one (Atkins and de Paula 2006; Atkins and Friedman 2005; Hollas 2004). The electric field is far more effective than the magnetic field in achieving the push that gets a photon absorbed. The direction of net linear displacement of charge is known as the polarization of the transition. The polarization and intensity of a transition are summarized by its electric dipole transition moment, which is a vectorial property having a well-defined direction (the transition polarization) within each molecule and a well-defined length (which is proportional to the square root of the absorbance). The transition moment may be regarded as an antenna by which the molecule absorbs light. Each transition thus has its own antenna and the maximum probability of absorbing light is obtained when the molecular antenna and the electric field of the light are parallel. Conversely, the absorption is zero when the light polarization and antenna are perpendicular to one another.