Infrared rotational spectra are excited by electromagnetic radiation in the far-infrared region of the spectrum, 10 - 200 cm−1 and are always observed in absorption. Thus, only increases in the rotational quantum number are observed. Normally, for a molecule to absorb in the far infrared it must have a permanent dipole moment (this is alos true for the closely related microwave spectrum). Molecules with permanent dipole moments do not show any spin statistics in their rotational spectrum because they do not have any symmetry at right angles to the main axis of symmetry of the molecule (if there is one). Suggested "experiments" are
(i) The B slider controls the value of B. You will be able to observe that the spacing of the lines is approximately 2B. You should write out the formula for the lines to see why this is.
(ii) With the value of B fixed vary the temperature and note how the maximum moves further from the band origin as the temperature increases. The reason for the maximum is that as J increases the increasing degeneracy (2J + 1) favours a higher population but the decreasing Boltzmann factor favours a lower one. The different rate at which these factors change generates the maximum. The actual population of the levels and the number of transitions occurring between levels is shown on the right hand side of the diagram.
(iii) Note the effect of varying the distortion constant D. This covers a realistic range in this diagram but only has a relatively small effect, and then only on the highest frequency lines. Why are only the highest frequency lines affected?