Re: Excitation and Emission Spectrum

Farooq W wrote:
> The basic question is the the
> absorption spectrum and the excitation spectrum are one and the same
> thing for a luminescent molecule?
>
Not necessicarily. The wavelength of maximum absorbance is a good
starting point for determining fluorescence excitation, but other
wavelengths may be just as good. As an example again, anthracene's
excitation max is not it's absorbance max.

> Let us see this equation from a reference:
> fluorescent intensity I (_f_) = 2.303( Phi_f)(Io)E bC
>
> where phi_f is flurorescence efficiency, Io Intensity of incident beam,
> E is molar absorptivity, b and C are path length and concentration
> respectively.

Nice, ... but,

>
> Now if we assume that Phi-f is wavelength independent,

Does your reference explicitly say this? I don't have a good one
before me, but this seems counterintuitive to me. Although it could be
true. I'm just an analytical lab monkey, after all, not a freakin'
quantum physical chemist.

> then I(_f_), as
> a
> function of exciting wavelength would give a same shape as that of the
> absortion spectrum...this is okay.

> For this reason, the author writes,
> absorption and excitation spectra are used interchangeably in
> fluorescence spectroscopy; but it must be realized that the methods of
> acquiring *normal* absorption spectrum and excitation spectrum are
> fundamentally different! The para ends here with no further reference
> the *fundamental* difference with no clue whatsoever, perhaps he is
> referring to the different geometry used for measuring abs. and excit.
> spectrum.
>
The statement is confusing, you're right. Does the author mean the
case I cite above, or the geometry, or something else. You may have to
take a trip to the local university and talk face to face with a
physical chemist to get to the bottom of this. You may simply been
stymied by some bad terminology or jargon, or some fundamental of
florescence has been glossed over in your reference.

Best of luck.

.

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