|MadSci Network: Chemistry|
Hi Carolyn, The phenomenon at work in glow-in-the- dark stickers is called "phosphorescence". In general, phosphorescence occurs when light is absorbed by something then released slowly such that it can be observed for a period of time after the initial light source is removed. The light emitted is almost always a pale blue color and is neither bright nor visually interesting to most people. Glow in the dark stickers usually emit a brighter green light (or sometimes red light) because they have a "fluorescent" dye mixed with the phosphorescent material. Fluorescent compounds absorb light then emit it again rapidly so "glowing" is only observed while the initial light source is maintained. What's happening in the stickers is the phosphorescent material absorbs light and emits it slowly as a pale blue light. Then the pale blue light is absorbed by the fluorescent dye and emitted as a green or red light (depending on what dye is used). At least one commonly used phosphorescent material is zinc sulfide mixed with copper. The green fluorescent dye is most likely sodium fluorescein and the red fluorescent dye is a rhodamine. Examples of these appear in the figure below. The initial state of a molecule is the "ground" state. When a photon is absorbed by a molecule, conservation of energy requires that an electronic change occur in the molecule to account for the added energy of the absorbed photon. This higher energy state is called the "first singlet excited state". This excited state is not stable so it rapidly decays to the more stable "first triplet excited state" via a process called "intersystem crossing". Energy is lost to molecular vibrations (can be thought of as a slight temperature increase) during this decay. Finally, this excited state decays back to the ground state by emission of a photon of light at the wavelength corresponding to the energy difference between the excited state and the ground state (wavelength is inversely proportional to energy). The stability of the first triplet excited state is what determines whether a material is fluorescent or phosphorescent because the more stable the excited state is, the slower it will decay back to the ground state. So fluorescence occurs if this excited state is relatively unstable and phosphorescence occurs if it is relatively stable. Have fun! Jeremy.
Try the links in the MadSci Library for more information on Chemistry.