MadSci Network: Chemistry

Re: How do glow-in-the-dark stickers glow?

Date: Fri May 15 02:15:31 1998
Posted By: Jeremy Starr, Grad Student, Chemistry, California Institute of Technology
Area of science: Chemistry
ID: 895071375.Ch

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!


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