in addition to the electronic excitation states which are known from atoms, molecules exhibit rotational and vibrational modes whose energy levels are quantized. the smallest energy differences exist between different rotational states: pure rotational spectra are in the far infrared region (about 30 - 150 μm wavelength) of the electromagnetic spectrum. vibrational spectra are in the near infrared (about 1 - 5 μm) and spectra resulting from electronic transitions are mostly in the visible and ultraviolet regions. from measuring rotational and vibrational spectra properties of molecules like the distance between the nuclei can be specifically calculated.