As explained above, an atom or a molecule has a number of energy levels which are quantized. The transitions take place only between threes energy levels according to certain rules, called selection rules (as will be deskilled later).The expressions for the different types of energy levels of a molecule will be derived later in this chapter. However, at the moment, it is sufficient to know that the transition between any two energy levels can take place in either of the following two ways.

(i)    The transition may take place form lower energy level to higher energy level by absorbing energy. It is then called absorption spectroscopy and the result obtained as a result of number of such transitions is called “absorption spectrum.”
(ii)    The transition may take place from higher energy level to a lower energy level thereby emitting the excess energy as a photon. It is then called emission spectroscopy and the result obtained as a result of number of such transitions is called “emission spectrum”.

In fact, a number of groups of closely spaced lines are observed. Each such group of closely lines is called a “band”. Thus a number of bands are observed. We can , therefore, conclude that whereas atoms give line spectra, molecules give band spectra.

In either case, the energy of the photon hv emitted or absorbed is given by Bohr’s frequency formula
                              E₂ - E₁  =  hv

or in terms of wavelength,     

or in terms of wave number (in cm^-1 if is in cm or velocity c is in cm s^-1
   


                      (a) Transition involving absorption of photon.
                      (b) Transition involving emission photon.

It may be noted that emission and absorption spectroscopy give the same information about the energy level separations. However, this he practical consideration decides as to which technique should be used. Generally absorption spectroscopy is used as absorption spectra are easier to interpret than emission spectra. Emission spectroscopy is used when the visible or ultraviolet incident radiations are used. In this chapter, we shall, therefore, mainly concentrate on absorption spectroscopy.

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