Absorption of Radiant Energy by Molecules

In reality, the molecules are as energetic as the modem teenagers. They invariably rock, roll, twist, jerk, and bend, and if the music is of the right rhythm, choice, and frequency, the electrons within the molecule shall move from the ground state to the excited state . 

Explicitly, the total energy in a molecule is the sum of the energies associated with the translational, rotational, vibrational and electronic motions of the molecule/or electrons/or nuclei in the molecule. These four motion-related-energies are briefly explained below  

Electrons generally found in the conjugated double bonds invariably give rise to spectra in the UV and visible regions of the electromagnetic spectrum. 

It is pertinent to mention here that an excited electron normally returns to the ground state in about 10 9 to 10-8 seconds. Consequently, energy must now be released to compensate for the energy absorbed by the system. In actual practice however, the following three situations arise, namely  

Firstly, if the electron returns directly to the ground state, the net effect would be evolution of heat. 

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The situation becomes even worse when the Boltzmann formula is used to interpret the absorption of radiant energy by molecules. Electromagnetic radiation considered as a fluctuating electric field interacts with electrons in... 

In the unimolecular reactions which are also of the first order, only one molecule takes part in the reaction. The process of activation in unimolecular reactions, if caused by collisions should ordinarily lead to second order reactions. How then the observed rate of reaction could be of first order. If however, the activation is by absorption of the radiant energy, this problem can be avoided. But many unimolecular reactions take place under conditions where there is no absorption of radiant energy. For example... 

Excited state. An energy-rich state of an atom or a molecule, produced by the absorption of radiant energy. 

Absorption spectroscopy records depletion by the sample of radiant energy from a continuous or frequency-tunable source, at resonance frequencies that are characteristic of various energy levels ia atoms or molecules. The basic law of absorption, credited to Bouguer-Lambert-Beer, states that ia terms of the iacident, Jq, and transmitted, light iatensities, the absorbance, M (or transmittance, T), is given by equation 1 ... 

Scattering of photons by gas molecules and particles can also reduce the radiant energy transmitted through a layer. In contrast to absorption, the radiant energy scattered remains in the form of radiation, but its direction is altered from that of the incident radiation. If we return to Figure 3.7 and assume that the medium does not absorb but only scatters radiation, then we can write, similarly to (3.10),... 

Absorption and scattering occur simultaneously because all molecules (and particles) both absorb and scatter. The attenuation of radiant energy in a medium is expressed by the extinction coefficient, which is the sum of absorption and scattering,... 

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