Editing Fluorescence (section)

=== Mirror image rule ===
[[File:Stokes shift- Rh6G.png|thumb|The fluorescent dye, [[rhodamine 6G]], is commonly used in applications such as [[highlighter pen]]s, [[dye laser]]s, and automotive leak detection. The absorption profile is a mirror of the emission profile.]]
For many fluorophores the absorption spectrum is a mirror image of the emission spectrum.<ref name=Lakowicz-1999>
{{cite book
 |author=Lakowicz, Joseph R.
 |year=1999
 |title=Principles of Fluorescence Spectroscopy
 |publisher=Kluwer Academic / Plenum Publishers
 |isbn=978-0-387-31278-1
}}
</ref>{{rp|style=ama|pp= 6–8}}
This is known as the mirror image rule and is related to the [[Franck–Condon principle]] which states that electronic transitions are vertical, that is energy changes without distance changing as can be represented with a vertical line in Jablonski diagram. This means the nucleus does not move and the vibration levels of the excited state resemble the vibration levels of the ground state.