Origin of Infrared and Raman Spectra

The selection rule allows any transitions corresponding to Au = 1 if the molecule is assumed to be aharmonic oscillator (Sec. 1.3), Under ordinary conditions, however, only the fundamentals that originate in the transition from u = 0 to i = 1 in the electronic ground state can be observed. This is because the Maxwell-Boltzmann distribution law requires that the ratio of population at i = 0 and v = 1 states is given by 

Overtones represent some multiples of the fundamental, whereas combination bands arise from the sum or difference of two or more fundamentals. 

The origin of Raman spectra can be explained by an elementary classical theory. Consider a light wave of frequency v with an electric field strength E. Since E fluctuates at frequency v, we can write 

TABLE 1.1. Some Representative Laser Lines for Raman Spectroscopy  

A variety of dye solutions are pumped by strong CW or pulsed-laser sources a wide range (440-800 nm) can be covered continuously by choosing proper organic dyes 

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