Physical Principles of Circular Dichroism

Circular dichroism (CD) is the differential absorption of left and right circularly polarized light. It arises from molecular electron oscillations that are driven by both the light's electric and magnetic fields, where the effects are in phase for one circular polarization and out of phase for the other. We describe these interactions, which have not been presented at an intuitive level before, with classical and quantum treatments. A rotational average of the latter solution leads to the Rosenfeld equation, which underlies most CD research. We illustrate its use with a chiral version of a harmonic oscillator, which we then apply to the far-ultraviolet CD of protein a-helices. This presentation is aimed at the level of upper-division undergraduates and could be usefully incorporated into a physical chemistry course.