Compact fluorescent lamps or CFLs are deceptively simple devices. Compared to the principles of operations for an incandescent lamp, understanding how a CFL emits light requires knowledge of the electronic structure of the atoms involved in the emission of light. Operating a CFL is simplistic: once the electric current begins flowing through the CFL the interior of the bulb begins to glow and emit visible light. Delving deeper, the CFL contains a few key components involved in this emission of visible light including the presence of elemental mercury vapor, a noble gas (argon, xenon, neon or krypton) and an inner coating called a phosphor which is the substance actually responsible for producing visible light out of the CFL.
Recalling the electronic configuration of an atom and its orbital subshells, every atom contains some varying number of orbital subshells which are correspondingly filled in increasing energy starting with the orbital subshell of lowest energy. For example, helium contains two electrons both located in the 1s2 orbital making this orbital filled. In comparison, a hydrogen atom only holds a single electron in the 1s2 orbital making this orbital partially filled. This principle of full or partially filled orbitals is vital to understanding the operation of a CFL.
The gases which inhabit the hollow interior of the CFL all hold completely filled orbital subshells. Since the electronic configurations of mercury and the noble gases are at their lowest possible energy level called ground state, these types of atoms strongly resist giving up any electrons due to the stability they have already achieved through filled orbital subshells. However, when energy provided through an electric current is allowed to flow through the CFL, the excess flow of electrons impact the atoms of mercury and noble gases. This collision called an inelastic scattering between electron and atom causes an electron from the outermost subshell of the impacted atom to temporarily “jump” or transition into the next highest energy level. This electron is now in an “excited” state but wishes to return to its former stable self so will emit a photon of energy as the excited electron transitions back down to the lower energy level therefore releasing excess energy in the form of that proton.
These emitted photons from the gaseous atoms, though, hold wavelengths in the ultraviolet spectrum and must first be converted into visible light for any usefulness. Here, the inner coating of the CFL called the phosphor works through a similar mechanism as the previously described excitation and transitions from higher to lower energy states. The phosphor will absorb the ultraviolet photons causing a temporary excitation to the next higher energy level and followed by the emission of a photon of lower energy due to the properties of the phosphor material composed of a blend of metallic metals for example: copper, zinc, sulfides, oxides, nitrides, aluminum, selenides, silicon, or rare earth metals. Depending on this composition, the visible light emitted by the CFL can vary in its wavelength and its corresponding visible color.
From ChemPRIME: 5.15: Electron Configurations