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Video Source: www.youtube.com/watch?v=lv7W21nWut4
We mentioned that a change of ∆J = ± 2, ± 3, etc was forbidden – in other words, the transition probability for all these changes is zero. Precisely similar calculations show that the probability of all changes with ∆J = ±1 is almost the same, are equally likely to occur. This does not mean however, that all spectral lines will be equally intense. Although the intrinsic probability that a single molecule in the J = 0 state, say, will move from to J = 1 is the same as that of a single molecule moving from J = 1 to J = 2. In a normal gas sample, there will be different numbers of molecules in each level to begin with, and therefore different total numbers of molecules will carry out transitions between the various levels. In fact, since the intrinsic probabilities are identical, the line intensities will be directly proportional to the initial numbers of molecules in each level. The first factor governing the population of the levels is the Boltzmann distribution. Here we know that the rotational energy in the lowest level is zero, since J = 0, so if we have N o molecules in this state, the number in any higher state is given by • #populationofrotationallevels • #spectralintensity • #spectroscopy
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