Question

what type of energy interaction contributes to infrared spectra? what are the applications of FTIR

Answer 1

The Vibrational energy interaction contribute to the Infrared spectrum because

The number of vibrational modes (different types of vibrations) in a molecule is 3N-5 for linear molecules and 3N-6 for nonlinear molecules, where N is the number of atoms. So the diatomic molecule we just discussed has 3 x 2 - 5 = 1 vibration: the stretching of the bond between the atoms.

these vibrational modes include the 1) stretching 2) Bending 3) Rocking and 4) Twisting of Bonds which are infrared active because there is a change in the molecular dipole moment during this vibration.

b)

Advantages of Fourier Transform spectroscopy[edit]

There are three principal advantages for an FT spectrometer compared to a scanning (dispersive) spectrometer.

1. The multiplex or Fellgett's advantage. This arises from the fact that information from all wavelengths is collected simultaneously. It results in a higher Signal-to-noise ratio for a given scan-time. For a spectrum with m resolution elements this increase is equal to the square root of m. Alternatively it allows a shorter scan-time for a given resolution. In practice multiple scans are often averaged, increasing the signal-to-noise ratio by the square root of the number of scans.
2. The throughput or Jacquinot's advantage. This results from the fact that, in a dispersive instrument, the monochromator has entrance and exit slits which restrict the amount of light that passes through it. The interferometer throughput is determined only by the diameter of the collimated beam coming from the source. Although no slits are needed FTIR spectrometers do require an aperture to restrict the convergence of the collimated beam in the interferometer. This is because convergent rays are modulated at different frequencies as the path difference is varied. Such an aperture is called a Jacquinot stop. For a given resolution and wavelength this circular aperture allows more light through than a slit, resulting in a higher signal-to-noise ratio.
3. The wavelength accuracy or Connes' advantage. The wavelength scale is calibrated by a laser beam of known wavelength that passes through the interferometer. This is much more stable and accurate than in dispersive instruments where the scale depends on the mechanical movement of diffraction gratings. In practice the accuracy is limited by the divergence of the beam in the interferometer which depends on the resolution.

Another minor advantage is less sensitivity to stray light, that is radiation of one wavelength appearing at another wavelength in the spectrum. In dispersive instruments this is the result of imperfections in the diffraction gratings and accidental reflections. In FT instruments there is no direct equivalent as the apparent wavelength is determined by the modulation frequency in the interferometer.