Spectroscopy
In the past, IR spectra had to be aquired one wavelength at a time, which took a long time. Today quick spectra is due to the

absence of broad spectrum of wavelenth
None of these
the Fourier Transfer Algorithm allows us to scan all frequencies at once
light is faster today that it used to be

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Spectroscopy
Why are rotational transitions of little use to a spectroscopist?

Because rotational transitions are extremely rare
Because, in liquids and solids, spectral lines corresponding to rotational transitions are broadened as the result of molecular collisions and other interactions
All of these
Because the energy required to induce a rotational transition is so small that it cannot be measured

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