FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups 
 ProfileProfile   PreferencesPreferences   Log in to check your private messagesLog in to check your private messages   Log inLog in 
Forum index » Science and Technology » Chem » Analytical
Why fluorescence emission bands quite broad as compared to excitation spectrum?
Post new topic   Reply to topic Page 1 of 1 [2 Posts] View previous topic :: View next topic
Author Message
Farooq W
science forum Guru Wannabe


Joined: 25 Mar 2005
Posts: 240

PostPosted: Sat Apr 29, 2006 2:39 pm    Post subject: Why fluorescence emission bands quite broad as compared to excitation spectrum? 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.
Back to top
Fabio Mancini
science forum beginner


Joined: 22 Dec 2005
Posts: 17

PostPosted: Sat May 13, 2006 4:17 pm    Post subject: Re: Why fluorescence emission bands quite broad as compared to excitation spectrum? Reply with quote

I think that very much it depends on how the E0 surface is located with
respect of the first excited state surface E1. It will depend on the
location
of the minima and on the slope of the surface in coreespondance of that
points.
I did not know the general rule you cited, it can be true for a vast
majority
of organic compounds.
However, I do not know the answer.
If one consider that an excitation start from E0 (v=0) and follow the
Frank-Condom
principle, one arrives to a steeply surface (shorter internuclear
distance with
respect to the minimum of the E1 surface).

In one consider the fluorescence emission, the transition from E1 (v=0)
to the surface
at E0 - a vertical transition- will cross the E0 surface in a less
steply zone.

Probably this is the only difference between the two transition:
- from E0 to E1 goes toward a steeply surface
- from E1 to E0 goes toward a less steeply surface

bye
fabios



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.




--
Posted via Mailgate.ORG Server - http://www.Mailgate.ORG
Back to top
Google

Back to top
Display posts from previous:   
Post new topic   Reply to topic Page 1 of 1 [2 Posts] View previous topic :: View next topic
The time now is Sun Jun 25, 2017 8:47 pm | All times are GMT
Forum index » Science and Technology » Chem » Analytical
Jump to:  

Similar Topics
Topic Author Forum Replies Last Post
No new posts spectrum of a symmetric tridiagonal random matrix pf.buonsante@gmail.com Math 0 Wed Jul 19, 2006 9:45 am
No new posts Single photon emission - absorbtion time delay experiments ? Boo Research 5 Tue Jul 04, 2006 4:16 am
No new posts Spectrum of T, T^n=0 danipi Math 3 Sat Jul 01, 2006 8:20 am
No new posts A New Hypothesis on photon emission explains the null res... GSS Particle 42 Fri Jun 16, 2006 6:19 pm
No new posts Chapline's dark star compared to Sarfatti's theory Jack Sarfatti Math 0 Thu Jun 08, 2006 10:52 pm

Copyright © 2004-2005 DeniX Solutions SRL
Other DeniX Solutions sites: Electronics forum |  Medicine forum |  Unix/Linux blog |  Unix/Linux documentation |  Unix/Linux forums  |  send newsletters
 


Powered by phpBB © 2001, 2005 phpBB Group
[ Time: 0.0171s ][ Queries: 16 (0.0031s) ][ GZIP on - Debug on ]