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Why are fluorescence emission bands comparatively broad?
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Farooq W
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Joined: 25 Mar 2005
Posts: 240

PostPosted: Sat Apr 29, 2006 2:37 pm    Post subject: Why are fluorescence emission bands comparatively broad? Reply with quote

I was asked a question during an experiment for the fluorometric
determination of acetylsalicylic acid. I showed the excitation spectrum
and emission spectrum but someone inquired why that the excitation and
absorption spectrum are very narrow and sharp and the emission spectrum
is quite broad. This is what usually observed. At that time I said I'd
let you know later but could the reason be like this:

Initially most of molecules are at the ground state and when they are
excited they may be promoted to higher electronic energy states and to
the associated vibration energy levels. When fluorescence begins from
the first excited singlet, many molecules may "not" return directly to
the ground state but to the higher vibrational levels of the ground
state leading to broadened emission.

The same logic then should lead to broadened excitation spectrum since
the excited states also have vibrational levels.
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