The interactions of light with matter Ignoring fluorescence, the interactions of light with matter can be expressed thus: I o = I reflected + I scattered + I absorbed + I transmitted transparent material translucent material

The Beer-Bouger-Lambert Law and  max The Beer’s law: At a given the proportion of light absorbed by a transparent medium is independent of the intensity of the incident light and is proportional to the number of absorbing molecules through which the light passes. I 0 is the intensity of incident light I is the intensity of transmitted light k is the absorption coefficient l is the path length through the sample  is molar extinction coefficient or molar absorptivity c is the concentration in mol/L So k ≈  c

Absorption Intensity and Oscillator Strengths The shape of an electronic absorption is due primarily to the vibrational sublevels of the electronic states (Franck-Condon Principle). The intensity of an absorption band is often and conveniently described in terms of the maximum of molar absorptivity  max but, unfortunately, this value is not directly related to any quantity obtained directly from theory. A more related value is the dipole strength D and represents the electronic transition probability. Δ is the bandwidth at  max /2 In an electronic transition, an electron is promoted from one molecular orbital to a higher-lying molecular orbital while the molecule goes from its ground state (S=0) to its excited states (S=1, 2,3,…). This migration of charge creates a momentary dipole, called the electric transition dipole moment (  ), which has both direction and intensity (vector). ן  ן = √D

Electronic, Vibrational, and Rotational States of Molecules S = 0

The Franck-Condon Principle

Classification of Electronic Transitions in Molecules