Spectroscopy Study Materials

0

 

SPECTROSCOPY

Spectroscopy deals with the transition of a quantum system (an atom or a molecule) between two energy levels upon the absorption of a photon of energy hv. If hv is equal to ΔE, the energy difference between the two energy levels, then hv = ΔE or v = ve/h. In atoms, he energy levels are the electronic energy levels whereas in molecules the energy levels are, broadly speaking, of three kinds Electronic, vibrational rotational. If the three kinds of motions are considered independent, then the total energy of a molecule is given by the Bom-oppenheimer approximation.

Absorption and Emission Spectra:

If the molecule absorbs a photon, the resulting spectrum is called absorption spectrum. If the molecule emits a photon as it undergoes a transition from the excited energy state to the ground state, the resulting spectrum is called emission spectrum.

Atomic and Molecular spectra:

Atomic spectra are very simple since there is only electronic motion in atoms. These are called line spectra because the spectrum consists of well-defined sharp lines. Molecular spectra are complicated because the molecule on the absorption of radiation energy may undergo rotational, or vibrational motion or may simultaneously undergo electronic, vibrational and rotational motions.

Molecular spectra are called band spectra because they consist of several overlapping spectral times giving rise to a band. Spectra are best investigated in the gaseous state at very low pressures because under such conditions the intermolecular interactions are negligible. Using the Bom – oppenheimer approximation we can investigate rotational spectra (in the microwave region), vibrational spectra (in the infrared region) and electronic spectra (in the ultraviolet/visible region). These regions of electromagnetic spectrum have the following approximate ranges:

Microwave region 1-100 cm-1

Infrared region 500-4000 cm-1

Visible region        10,000-25,000 cm -1

UV region 25,000 – 70,000 cm -1

Selection rules in spectra arc very important. They tell which quantum numbers change during a transition and by what amount. Transitions which obey a selection rule are called allowed transitions and those which violate the selection rules arc called forbidden transitions. Allowed transitions are more intense (stronger) than forbidden transitions.

PDF DOWNLOAD

TNEB TANGEDCO AE Basic Engineering Study Materials

TNEB TANGEDCO AE Study Materials