Absorbance Spectroscopy: Overview

Synonyms

Absorbance of light; Transmission of light

Definition

Most spectroscopic phenomena arise from the interaction of a molecule or other chemical species with one photon at a time. Multiphoton processes are less commonly encountered. The molecule may either absorb or scatter the photon; it may also emit a photon. The simplest form of spectroscopy is absorption, which measures how much electromagnetic radiation (e.g., light) of a given frequency is absorbed by a collection of molecules. If a molecule absorbs a photon of frequency ν, it increases its energy by

$$ \Delta E= h\nu = h c/\lambda $$

(1)

where h is Planck’s constant, λ is the wavelength of the light, and c is the speed of light. Only light of the correct frequency to cause a molecule to jump from one energy level to another may be absorbed. When molecules absorb photons of visible or ultraviolet (UV) light, the magnitude of the energy of these photons is (2–12) × 10⁻¹⁹ J/molecule (λ= 170–800 nm) which is the amount...