Ultraviolet spectrum constants

Spectroscopic methods are physical study procedures involving energy exchange phenomena between matter and electromagnetic radiation. This may involve the absorption or emission of light, the "light" being in or outside the visible part of the spectrum. These exchanges involve well-defined energy levels in matter, and therefore its quantification. By absorbing a photon with an energy corresponding to the difference between two specific energy levels, the molecule is brought into various excited states. The lowest-energy eigenlevel is called the ground state.

Outside an intense magnetic field, there are three ways for the molecule to store absorbed energy: by decreasing values, electronic energy $E_1$ , vibrational energy $E_2$ , rotational energy $E_3$ :

Electron energy quanta are the largest and are located in the UV-visible range. They correspond to the promotion of an electron from an orbital occupied in the ground state (bonding or non-bonding orbital) to an anti-bonding orbital. They therefore have an order of magnitude corresponding to the energy of a bond. Consequently, they are capable of breaking such a bond. For example, bichlor $C{\text{/}}_2$ is colored greenish-yellow. It has an absorption band in the visible range, and it's no coincidence that it dissociates in light into atomic chlorine (initiating radical reactions). In the case of multiple bonds, the molecule may retain its cohesion, but may undergo a major structural transformation (photochemical reaction). UV absorption spectroscopy, the most useful in chemistry, is limited to the near UV (200 to 400 nm) and the visible (400 to 800 nm). As we shall see, it mainly concerns unsaturated systems and involves reversible electronic transitions, a priori without structural change.

In the frequency range corresponding to the visible and near-ultraviolet spectrum, photon energy is such that transitions of the type $\text{n}$...