Generally, wavenumbers are indicated in cm-1. To convert the wavelength in wavenumbers, the
reciprocal of the wavelength in nm is multiplied by 107. Eq. 1 shows that the shorter the wavelength,
the higher the wavenumber and the higher the energy of the photon.
The electromagnetic spectrum is divided into several regions. Every region stands for a specific kind of
molecular or atomic transition and thus for a specific spectroscopic technique. Gamma rays and Xrays
have wavelengths of only a few nm and are very harmful, because they break chemical bonds
and ionize molecules. UV radiation covers the region 190–350 nm and visible radiation (VIS) the
region 350–780 nm. Photon absorption in the UV/VIS range transfers electrons from low-energy to
high energy molecule orbitals and returning to the ground state easily proceeds via fluorescence.
Radiation referred to as near-infrared defines the range between 780 and 2500 nm. Molecules can
absorb infrared light without later reemission by exciting certain vibrational frequencies. The sample
absorbs the frequencies of polychromatic light that corresponds to its molecular vibrational
transitions.