A Fluorescence Mystery
Hypotheses that were tested and shot down-
1. The He-Ne laser has some UV output. - Green dye did not fluoresce at all with red laser. Orange dye still fluoresced when laser passed through plate glass or sunglasses.
2. The laser was not very monochromatic and actually had a broad spectrum for some reason. - When the fluorescent dye was replaced by a white paper, just a single spot was observed through the grating.
3. There was some extraneous white light in the room that caused the fluorescence. When the red laser was turned off, no emission spectrum was observed.
(1) The red photons (which are 1.96 eV) can excite the molecules into a fairly broad energy band, rather than into a discreet energy level of exactly 1.96 eV. Electrons in this broad energy band can transfer some energy to other electrons in the band, thereby conserving energy but resulting in a distribution with some electrons having more than 1.96 eV and some having less. A 600nm photon has about .1ev more energy than a 633nm photon, so if this were the case, the energy band would would have to be about .2 eV wide, equivalent to a spread of photon wavelengths from 600-666 nm.
(2) Some percentage of the electrons in the material are not in the ground state at room temperature. These electrons are excited further by the red photons and then drop down to the ground state, giving a photon with a higher energy than that of the red photon.
(3) The electrons end up in an energy state that is semi-stable with a short lifetime that's long enough to be able to be re-excited by a second red photon before they drop back down. They then drop down to ground in various steps.