Why does signal yield increase with shorter wavelength in Raman spectroscopy?

In summary, the article discusses why longer wavelengths (i.e. lower frequencies) yield higher Raman signal intensities. This is due to the fact that shorter wavelengths are scattered more extensively by the electrons in the Raman excited state, resulting in a higher number of photons being detected. Modern spectrometers are better at detecting photons/second, rather than watts, due to the fact that the frequency dependence of Raman scattering is slightly different.
  • #1
n0_3sc
243
1
I read in a journal article that in raman spectroscopy "it is well
known that signal yield increases with shorter wavelength". Can anyone please explain why? I would've thought longer wavelengths = shorter frequencies = higher susceptibilities?
 
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  • #2
Sorry to bring this post up again, but I found the answer to my question if anyone is interested:
Turns out the Raman process does NOT depend much on wavelength. It is the detection of the CCD's that have the highest quantum efficiencies in the visible.

Between 600nm and 1000nm there is a 60% difference in detection efficiency.
 
  • #3
n0_3sc said:
Sorry to bring this post up again, but I found the answer to my question if anyone is interested:
Turns out the Raman process does NOT depend much on wavelength. It is the detection of the CCD's that have the highest quantum efficiencies in the visible.

Between 600nm and 1000nm there is a 60% difference in detection efficiency.

well the Raman process DO depent much on wavelengt (to the fourth power). And it's well understood.
The fourth power can appear through a lengthy derivation using timedependent pertubation theory, second quantization of E-field and derivation of interaction between charges and E-field. You don't wanner mess with this...
 
  • #4
evidenso said:
well the Raman process DO depent much on wavelengt (to the fourth power). And it's well understood.
The fourth power can appear through a lengthy derivation using timedependent pertubation theory, second quantization of E-field and derivation of interaction between charges and E-field. You don't wanner mess with this...

Rayleigh Scattering depends on wavelength to the fourth power not Raman Scattering.
 
  • #5
n0_3sc said:
Rayleigh Scattering depends on wavelength to the fourth power not Raman Scattering.

Sorry to disapoint ya.
Raman intensity DO varies with the fourth power of the observed frequency for normal Raman scattering, which, in turn, depends on laser frequency.
It can be derived from the classical treatment of scattering from an oscillating induced dipole, with the intensity expressed in watts.

Therefore, in biological Raman scattering one has to find a compromise between reduced fluorescence and reduced Raman signal by choosing bigger wavelength to obtain signals.

In the full quantum picture it appears when you combine final density of states, laser intensity and scattered intensity with the Kramer Heisenberg equation

You should note that modern Raman spectrometers,
which usually measure photons/seconds rather than watts, are governed by a
slightly different frequency dependence.
 
  • #6
I'm now confused :(

Do you have any material I can refer too? In all the theory I've gone through I don't see any fourth power dependence. (haven't done QM though)

Is it:
Raman intensity varies with fourth power wavelength or,
Raman intensity varies with fourth power frequency?

Why are modern spectrometers different?
 
  • #7
n0_3sc said:
I'm now confused :(

Do you have any material I can refer too? In all the theory I've gone through I don't see any fourth power dependence. (haven't done QM though)
This is a basic non QM practical guide
"Raman Spectroscopy in Chemical Analysis wiley 2000"

n0_3sc said:
Is it:
Raman intensity varies with fourth power wavelength or,
Raman intensity varies with fourth power frequency?
Raman intensity varies with fourth power of frequency
and you know that [tex]c=\lambda*frequency[/tex]

n0_3sc said:
Why are modern spectrometers different?
This is explained in "Raman Spectroscopy in Chemical Analysis wiley 2000"
It's because there is a factor [tex]E=hv[/tex] in difference between watts and photon/sec
 
  • #8
hi, can you point me to a reference for the quantized field treatment? googling didn't help much, seems this is really a topic that not many are willing to touch.
thx in advance
 
  • #9
"Atom-photon interactions" by Cohen-Tannodji for instance?
 

1. What is Raman spectroscopy?

Raman spectroscopy is a scientific technique used to analyze the vibrational energy levels of molecules. It involves shining a laser onto a sample and measuring the scattered light to determine the molecular structure and composition of the sample.

2. How does Raman spectroscopy work?

Raman spectroscopy works by measuring the inelastic scattering of light when it interacts with a sample. When a laser is shined onto a sample, some of the photons in the light are absorbed and re-emitted at a different energy level, which is related to the vibrational energy levels of the molecules in the sample.

3. What are the advantages of using Raman spectroscopy?

Raman spectroscopy has several advantages, including its ability to analyze samples non-destructively, its high sensitivity to small changes in molecular structure, and its ability to analyze a wide range of samples, including solids, liquids, and gases.

4. What are the limitations of Raman spectroscopy?

Some limitations of Raman spectroscopy include its inability to analyze samples that are highly fluorescent or strongly absorbing, as well as its limited depth of penetration into the sample. Additionally, Raman spectroscopy may not be suitable for analyzing samples with low concentrations of molecules or samples with complex structures.

5. How is Raman spectroscopy used in scientific research?

Raman spectroscopy is used in a variety of scientific research fields, such as chemistry, biology, and material science. It can be used to identify unknown substances, study molecular structures and interactions, and monitor chemical reactions in real-time. It is also used in quality control and forensic analysis.

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