What Distinguishes an Excitation Spectrum from an Absorption Spectrum?

[scarecrow]

What's the difference between an excitation spectrum and an absorption spectrum for the same molecule? Are the words interchangeable, or are they completely different processes?

The excitation spectrum generally is identical to the absorption spectrum as the fluorescence intensity is proportional to the absorption.

In Sharma, A. and Schulman, S. G. (1999). Introduction to Fluorescence Spectroscopy. Wiley interscience, it says "generally", so when is the excitation spectrum not proportional to the absorption?

I don't have that textbook, I found the quote from http://en.wikipedia.org/wiki/Fluorescence_spectroscopy#_note-1

Thanks.

 

[Allywilson]

Wouldn't trust what I say 100% but I am fairly sure excitation spectrum refers to the spectrum produced by flourescence spectroscopy and absorption spectrum from absoprtion spectroscopy, seems a little bit like useless definitions but the two techniques are different, flourescence spectroscopy usually uses UV light which is absorbed by compounds then re-emitted as lower energy visoble photons and you measure the re-emission whereas the absorption spectrum is obtained by looking at what is missing from the white light spectrum normally of atoms or very simple molecules rather than complex compounds.

The generally i can only guess at without the textbook to refer to, assume it is a specific example by example like there may be compounds which behave strangely for chemical rather than physical reasons.


Sure someone will correct me if I am wrong.

 

[mn4j]

Excitation spectrum is the spectrum of energies emitted (fluorescence) by material after exposure to radiation while the Absorption spectrum is the spectrum of energies absorbed by the material.

Usually you expose the material to a large energy/wavelength range of radiation , measure everything that goes through unabsorbed and by comparing with the incident radiation, you can determine the spectrum of absorbed energies which gives you the absorption spectrum.

To obtain the excitation spectrum, you expose the material with radiation, usually just a narrow range of energies, and measure the fluorescence at > 90 degrees from the direction of incidence with a detector that can simultaneously measure and sort photons of different energies. This result is an excitation spectrum.

 

[ashleylein23]

An emission spectrum represents a fluorescence spectrum, not an excitation spectrum.

 

[soli85]

The difference is the following: the absorbance spectrum is experimentally obtaied from absorbance's measurements, the excitation spectrum is obtained from Fluorescence's measurements.
The processes involved are the same: the electronic transition from the ground state to the first and second excited states.
Because of that the two spectra have generally the same shape, and they are superimposable.
Instead absorption and excitation spectra don't corresponds if there are more species in the ground state, or if the sole present species has different forms in the the ground state (aggregates, complexes, tautomericforms etc).
In this case the compararison of the two spectra can give a lot of informations.
In conclusion the two words are not interchangeable. However the two processes are very close.
to asmeylen: the fluorescence is a two step process: excitation and emission.