Glow in the dark

Hey Cecil,
At risk of being labeled a pedantic scientist I’d like to point out what you refer to as fluorescence (Baltimore city paper 28 April) is actually Phosphorescence.

Most molecules exist in unexcited states with low vibrational energy known as the ground state (So),
absorption of a photon (light) is usually by molecules in the So energy level resulting in their excitation to
a higher vibrational state (S1).
Once a molecule is in the excited state (S1) it may decay to the ground state (So) radiatively, emitting a photon as fluorescence (emissive decay) or via radiationless decay with the excitation energy being converted to heat. Fluorescence decay occurs very rapidly (average period a fluorophore remains in the excited state being approx. 10-8 sec.)
The molecule may also make a transition to a triplet manifold (T1) state in which electrons become unpaired by a process known as inter system crossing (ISC). Once in this T1 state the molecule may return to So with a flip of electron spin back to being paired by emitting a photon as phosphorescence or by radiative decay. Phosphorescent lifetimes are considerably longer (ms to seconds) as a result of the transitions between energy states being relatively slow.
I hope this throws some light on the subject.

Keith Dilly
Baltimore