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FIGURE 3.3 Schematic diagram of energy levels involved in HC1 vibration-rotation transitions at room temperature (from Herzberg, 1950).

electronic energy change for an ideal harmonic oscillator-rigid rotor, the selection rules At' = +1 and A J = + 1 apply. At room temperature, v" = 0 —> v' = 1 is the only significant vibrational transition in absorption, but, as discussed earlier, a variety of rotational transitions can occur.

Figure 3.3 shows some of these possible transitions for HC1. Those with A J = +1 are known as the R branch and occur at the high-energy side of the hypothetical transition At' = 1, A J = 0 (this is not allowed because of the selection rule, A J = +1). Those with A J = -1 on the low-frequency side of the hypothetical transition form the P branch. Figure 3.4 shows the absorption spectrum of HC1 at room temperature, with the rotational transitions responsible for each line. The relative intensities of the lines reflect the relative populations of the absorbing rotational levels; the peaks are doublets due to the separate absorptions of the two chlorine isotopes, that is, H35C1 and H37C1, which have different reduced masses and hence values of the rotational constant B.

(4) Electronic energy and transitions The electronic states of a diatomic molecule are described by several

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