1. Vibrational spectra. Vibrational level. Harmonic oscillator

Reminder

In an absorption band, radiation undergoes a decrease in intensity. Lambert's law relates absorbance (optical density) to the thickness d of the absorbing medium: lg I/I ₀ = ad

and, in solution, a is proportional to the concentration c :

lg I/I ₀ = ε _λ cd

with c in mol/L, d in cm.

The molar extinction coefficient ε _λ thus defined is characteristic of the sample at each wavelength. It is homogeneous to one surface area per mole, i.e. to the photon capture cross-section of the molecule.