Absorption Spectroscopy Lab Report

An incident light beam interacting with a material can be absorbed, scattered or cannot interact at all, passing through it.

When the level of energy of the photon impacting with the material is correspondent to the to the difference between the ground state and the excited state of the molecules forming the impacted material, the absorbed photon causes a promotion to the higher energy excited state of the molecules.

Absorption spectroscopy measures this change in energetic level detecting the loss of energy of the out coming light after the impact with the sample material.

Nevertheless, the light can also be impacting with the material and scatter from it. In this instance, the photon does not need to be at an energy level able to cover the

=c/

Wavelength indirectly proportional to energy.

The condition in which light is utilised in raman scattering and infrared absorption spectroscopies is quite different. In infrared absorption the analyte is exposed to a radiation of a broad range of wavelengths and when the specific incident light frequency matches the vibrational energy level of the analyte, the molecule is stimulated to a higher vibrational state.

It is analysed the loss of energy of the incident radiation due to these absorptions.

On the other hand, Raman scattering adopt one unique incident radiation at a fix wavelength to interact with the sample material. The scattered light produced by the irradiated analyte is then monitored.

Consequently, opposite to the infrared spectroscopy, Raman scattering does not demand an overlapped incident radiation to the energy change between the ground and excited

Since only an electrons cloud distortion occur in scattering, the light is re-radiated with minor frequency variations. This scattering re-radiation is called elastic scattering and is the principal process. At molecular level, this scattering process is named Rayleigh scattering.

When nuclear motion is involved in the scattering process, energy is shifted from the incident light to the sample or from the molecule to the scattered light. In these instances, the process is deemed to be inelastic and the energy of the scattered light is different from the energy of the incident light by one vibrational energy unit. This is called Raman scattering. The Raman scattering is a weak phenomenon, occurring in only one in every 106 – 108 photons which scattered. In the same time, other radiation processes occur, such as fluorescence and sample