UV-VIS Spectroscopy

UV, VIS spectroscopy is based on the excitation of electrons in the outer electron shell (orbital) of an atom or molecule. The energy input of the excitation radiation transfers electrons from the highest occupied orbital - the ground state - to the lowest unoccupied orbital - the excited state. The energy required for this transition is obtained by absorption from the excitation radiation. Theoretically, the absorption of organic molecules can be well described by the so-called HOMO-LUMO concept (HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital), while the absorption of elements (especially the often colored compounds of groups 3-12 in the periodic table) is well described by the ligand field theory. Further absorptions in the UV-VIS occur in compounds e.g. by free electron pairs or multiple bonds. In general, the more extensive an electron system is, the less energy is required for electron excitation and the more the absorption is shifted to longer wavelengths (i.e. into visible light). These 'colouring units' of molecules are traditionally called 'chromophores'. The electron transitions additionally follow certain selection rules, so that not all theoretically possible electronic transitions can actually be observed in every case (there are symmetry forbidden transitions as well as spin forbidden transitions).

For UV-VIS spectroscopy, a wide range of possible combinations of excitation source, optics, measurement geometry and sample chamber as well as detectors are available, which can be specifically selected according to requirements. If, for example, color values are to be determined in a defined way, a calibrated excitation source is required. If observations of the layer thickness by interference evaluation are the goal, then a goniometer may be required to adjust the angle of incidence and loss of the radiation.