Ultra-Violet, Visible and Infra-Red Dichroism

Abstract

When a beam, of polarised monochromatic radiation is transmitted through a polymer sample it may, depending on its wavelength λ, be partially absorbed by specific molecular groups within the material. Absorption processes generally involve some periodic relative displacement of atomic nuclei or electrons which yield a dipole moment change, or so-called transition moment M, within the molecule.^1,2 A transition of the molecule may then occur between two states of energy difference ΔE when the frequency of the radiation, ν = c/λ, is equal to ΔE/h Here c and h are the velocity of the radiation and Planck’s constant respectively. Since a given molecule may exhibit various absorption mechanisms, each associated with a different ΔE, radiation may be absorbed at several different wavelengths or wavelength regions. An experimental plot of transmitted intensity against wavelength of the radiation will then exhibit minima at wavelengths of maximum absorption. As is well-known, such a plot is termed the absorption spectrum of the material Figure 1 shows schematically an absorption spectrum and Illustrates the regions of wavelength corresponding to the ultra-violet (UV; λ ≈ 0.1–0.4 μm), the visible (λ ≈ 0.4–0.8 μm) and the infra-red (IR: λ ≈ 1–50 μm).