In this particular article, Features of molecular spectra and molecular energy level we are going to read about basic information of course structure and fine structure, molecular energy levels etc.

Features of molecular spectra and molecular energy level

Course structure

1) The molecular spectra are band spectra. It means that when a molecular spectrum is observed with a low resolving power instrument. Then The spectrum consists not of single, sharp lines but of more or less broad wavelength regions called bands.

2) The bands usually have at one end a sharp edge, called a band head(or band edge), where the intensity falls suddenly to zero. While on the other side the intensity falls off more or less slowly. If the gradual falling off in intensity takes place toward shorter wavelengths. Then the bands are said to be shaded to the violet.  Or If the gradual falling off in intensity takes place towards longer wavelengths, the bands are said to be shaded (degraded) to the red.

3) some spectra, the band heads are not so clearly developed or not presented at all. In rare cases, for example for alkali hydride, no well-developed bands are observed. But only an enormous number of lines, a so-called many-lines spectrum is observed. Again In case of I₂ and halogen molecules band spectra with an extended continuous region as molecular spectra is observed.

4) The arrangement of bands in a molecular spectrum follows rules entirely different from those for the arrangement of lines in atomic spectra. The molecular spectra consist of a series of bands whose separation changes rather slowly. They are called progression has considerable intensity. In the absorption spectra of CO, I₂, and S₂ only one such progression has considerable intensity. However, more than one progression may be present in a molecular spectrum.

Fine structure-

Fine structure of a band in the molecular spectrum

5) When the instrument of greater resolving power are used, it is found that most of the bands (in emission or absorption) consists of a large number of individual lines. In general, the arrangement of individual lines in a band is completely regular, a series of lines which draw farther and farther apart from one another as the distance from the band head increases. The separation of successive lines increases very nearly linearly.

Molecular energy levels

The empirical results of the molecular spectroscopy show that in many cases the energy values bear a simple relation to one another. Such that the energy of the molecule can be conveniently considered to be made up of several parts, called the electronic energy, the vibration energy, and the rotational energy.

The energy level falls into widely separated groups, which are said to correspond to different electronic states. For a given electronic state the levels are again divided into groups, which follow one another at nearly equal intervals. These are said to correspond to successive states of vibration of the molecule. Superimposed on this is the fine structure due to the different states of rotation of the molecule. The successive rotational energy levels being separated by larger and larger intervals with increasing rotational energy. The energy difference between two electronic states energies is of the order of 5 eV. The difference between vibration energy levels is of the order of 10⁻¹ eV. And the difference between rotational energy levels is of the order of 10⁻³eV.

This simplicity of the structure of the energy levels suggests that it should be possible to devise a method of approximate solution of the Schrodinger equation involving its separation into three equations. One dealing with the motion of the nuclei, and one with the rotational motion of the molecule.


In this particular article, Features of molecular spectra and molecular energy level we have discussed the course structure, fine structure and molecular energy levels in detail.


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