Excitation Effects for Fluorescence Spectroscopy

[fsonnichsen]

I have read from time to time (e.g. Lakowicz) that the fluorescence spectrum of a substance is not generally influenced by the wavelength excitation source. On the other had there is a correlation as exemplified in excitation-emission matrix spectra for example.

The obvious case is for fluorescence at wavelengths higher that the excitation. Omitting Raman phenomenon this should not happen-the excitation cannot promote electrons at an energy higher than the stimulus.

For excitation wavelengths greater than the energy levels of an atom, I would not expect much correlation-the energy can overcome the promotion barrier, the electron moves to the higher state, and fluorescence ensues. That said, clearly fluorescence analysis is done using the excitation correlation as a parameter so there must be a strong effect.

Does anyone know of an elaboration on this process? I have a few texts (Ingle) and articles here but little is said. The interactions with vibrational/rotational modes, analyte and solvent etc clearly play into this and I expect this is where the conversation begins.

Thanks
Fritz

 

[DrDu]

The obvious case is for fluorescence at wavelengths higher that the excitation. Omitting Raman phenomenon this should not happen-the excitation cannot promote electrons at an energy higher than the stimulus.

For excitation wavelengths greater than the energy levels of an atom, I would not expect much correlation-the energy can overcome the promotion barrier, the electron moves to the higher state, and fluorescence ensues.

You seem to have got something wrong here: greater wavelength-> lower energy

The effect of excitation wavelength on fluorescence depends mainly on the timescale of IVR to fluorescence. As long as the first one is much shorter than the second, fluorescence spectra are not much influenced by excitation. This certainly has been studied excessively but I don't know of a handy reference. Much has also been learned from femtoscecond pump probe experiments.

 

[fsonnichsen]

Excuse my poor wording-to clarify- we would agree that greater wavelength means lower energy (E=h/Lamda). My point being that higher wavelengths (omitting Raman effects) should not be able to promote electrons past the energy for the wavelength.

Once over that threshold it is not clear that there would be further correlation.

I do not understand your reply, probably because I don't now what IVR is?

Thanks
Fritz

 

[DrDu]

IVR=intramolecular vibrational relaxation

 

[fsonnichsen]

Ha- Thanks! Will the world ever run out of acronyms before it runs out of free energy?

So-I think you are referring to the fact that as higher vibe/rot levels are populated a dependence on the excitation is incurred. I believe the dogma is that fluorescence occurs from transitions of the lowest vibrational state of each level. I would suppose the degree of population of higher vibe/rot levels would distort the broad "peaks" of the spectrum, hence the excitation dependancy.

I have pulled a few texts on this and it is ill-described. It certainly would merit some discussion in a thorough text. The excitation wavelength issue makes ones job difficult when selecting a laser(s) for excitation as it could result in an expensive mistake! I have generally assumed that the lower the wavelength the better the chances of exciting more emissions and this seems to bear out in the laboratory.

If you look at my other post on this topic, an interesting question is raised on the appearance of emissions above the excitation line in energy.

thanks again,
Fritz

 

[DrDu]

The very fact you are referring to is known as Kasha's rule:
http://en.wikipedia.org/wiki/Kasha's_rule

The following article may be useful, eventually Google Scholar will yield more recent articles citing this one:

Freed, Karl F., and Abraham Nitzan. "Intramolecular vibrational energy redistribution and the time evolution of molecular fluorescence." The Journal of Chemical Physics 73 (1980): 4765.

 

[fsonnichsen]

Thanks! - looks like a good article. I'll check it out at work tomorrrow.

cheers
Fritz