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A brief introduction

Raman spectra

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Raman spectra

Raman spectra are unique fingerprints of solid, liquid or gaseous materials. These materials can consist of one or even more components. Raman spectra provide answers easily and quickly to the questions:

  • What is it? (Qualitative analysis)
  • How much of one, several or all components contain the sample? (Quantitative analysis)

What is it? can be answered by comparing a sample spectrum with reference spectra or spectra libraries. Analyzing Raman intensities provide the answer to the question: How much?

Raman and infrared spectra complement each other

The Raman and infrared (IR) spectrum of the same material look partly similar. Although both spectroscopic techniques are based on different physical processes most Raman and infrared bands are observed at the same positions. Whether a band is observed and with which intensity depends on selection rules. The whole vibrational information of a material is provided by the knowledge of its Raman and the infrared spectrum.

Wavenumbers

Typically, a spectrum represents intensities depending on energy. The energy scale is expressed in frequencies, wavelengths, and electron volt. In vibrational spectroscopy the energy scale is preferably expressed in wavenumbers. One advantage of the wavenumbers is that they are directly proportional to energy:

Large wavenumbers - High energy
Small wavenumbers - Low energy

Wavenumbers are easily converted into the more familiar wavelength scale by calculating the reciprocal. In semiconductor applications the energy scale is often expressed in electron volts instead of wavenumbers.

Conversion

   
Wavenumbers - Wavelength  

 

Wavenumbers - Frequency  
Wavenumbers - Electron volt  

: Wavenumbers (cm-1)
l: Wavelength (µm)
n: Frequency (s-1)
c: Velocity of light (2.99792458·108 m/s)
e: Elementary charge (1.60217733·10-19 C)
h: Planck's constant (6.6260755·10-34 J·s)
E: Energy (eV)

 

 

   

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