Excitation and Emission Spectrum

I am having a minor confusion regarding the definition of excitation
and emission spectrum which I quote below (from Ingle and
Crouch-Spectrochemical Analysis):

The excitation spectrum is a plot of the luminescence vs. excitation
wavelength with a constant emission wavelength. It is used to determine
the best excitation wavelength for analysis and is related to the
absorption spectrum of the analyte.

[One point mentioned in Strobel is that the excitation spectrum is
independent of the monitored wavelength]

A plot of the luminescence signal vs. emission wavelength with a
constant excitation wavelength is denoted an emission spectrum.

Now what confuses me in the "excitation spectrum" is that a
spectrofluorimeter has two monochromators one for choosing the required
wavelength from the source and one for selecting the emission
wavelength. Now in observing an excitation spectrum one is changing the
incident wavelength of light continuously, say from 400- 500 nm, while,
let us assume the emission occurs in the range 450 to 550 nm; suppose
the monitored wavelength is fixed say at 500 nm, the second
monochromator will block emitted light except that of 500 nm.

I. Now suppose I shine 400, 420, 440 and 460 nm light on the sample,the
molecule still emit in the range 450 to 550 nm and only the overall
intensity of each wavelength will be different. If this is right, this
resolves the problem, i.e. no matter what the wavelength of incident
radiation is, the emission would still occur in the range 450 to 550 nm
and we can, in principle, select any wavelength ranging from 450 to 550
nm for making an excitation spectrum and only the intensity of emitted
light will be different, this is the only way an excitation spectrum
would be independent of monitored wavelength.


II. What we say is that the excitation spectrum is _similiar_ to an
absorption spectrum in its structure; but I feel is how it is
practically possible to measure _pure_ absorption spectrum of a
luminescent molecule. For example, we allow the the molecule to absorb
a range of wavelengths say from 400-500 nm, but the luminophore also
emits in the same range, perhaps there is no way to stop the molecule
from emission under ordinary conditions (neglect quenching etc.) So how
do we practically measure a absorption spectrum of a luminescent
molecule?

.

Relevant Pages

  • Re: Any excimer knowledgeable people here?
    ... > energy is released (either by stimulated emission or spontaneous emission) ... > when this molecule comes apart. ... Absorption is what you get when something absorbs a photon. ... light transmitted through a system as a function of wavelength. ...
    (sci.optics)
  • Re: Debunking Physics and Discovering The Process of the Universe
    ... >> Light is formed into a wavelength through its interaction with light. ... >> spread out that unit of emission is. ... The single slit experiment demonstrates that "once the slit width ... approaches the dimension of the wavelength the waves spread out as if ...
    (sci.physics.particle)
  • Sal the crackpot
    ... "Emission theory handles MM very nicely. ... and the wavelength doesn't shift if the emission velocity is ... The wavelength doesn't shift if the emission velocity is fixed at the ...
    (sci.physics.relativity)
  • Re: Debunking Physics and Discovering The Process of the Universe
    ... > Light is formed into a wavelength through its interaction with light. ... > spread out that unit of emission is. ... What do you suppose happens to light when it passes through thin slits? ...
    (sci.physics.particle)