How is the Information Presented?

Infrared and Raman data are presented in a spectrum, examples of which are shown in Figure 1. The infrared spectrum is a plot of the amount of light which has been transmitted (% Transmittance) or absorbed (Absorbance) by the sample, versus energy which is denoted by the units wavenumbers (cm^-1) the reciprocal of wavelength (1/λ). The Raman spectrum is a plot of the intensity of the Raman scattered light versus the energy difference (frequency shift), which is also denoted as cm^-1 and is referred to as Raman Shift.

It is interesting to note how the intensity of the bands in the infrared and Raman spectra of polystyrene (Figure 1) differ considering both techniques are measuring the vibrational frequencies of the functional groups present within the sample e.g., C=C, C–H. This is because the vibrational modes derive from different mechanisms i.e., infrared spectroscopy is a one–photon absorption effect and Raman spectroscopy is a two–photon scattering effect. From the diagram it can be seen that the aliphatic C–H stretching modes from 3000–2800 cm^-1 are very intense in the infrared spectrum and weak in the Raman spectrum. Conversely, the C=C stretching modes of the aromatic rings, ~1000 cm^-1, are very intense in the Raman but not in the infrared spectrum. This is because the C–H bonds possess a significant dipole moment whilst the C=C bonds are highly polarizable. Molecular and structural information is obtained by careful examination of the spectra taking special note of the band position, lineshape and intensity.