rotational spectroscopy
A spinning top, an ice skater, a tossed pen — anything can twirl, but a molecule can only twirl at certain fixed rates, never anything in between. To speed up from one allowed spin to the next it must absorb a precise dose of energy, and those doses are small enough to be carried by microwaves. Rotational spectroscopy watches a molecule step up its spin one allowed notch at a time.
Rotational spectroscopy, usually done in the microwave region, measures the energy a gas-phase molecule absorbs as it jumps between quantised rotational states. Because the spacing of those rotational levels depends directly on the molecule's moment of inertia — that is, on the masses of its atoms and the distances between them — the spectrum gives remarkably exact bond lengths and angles, among the most precise structural measurements in all of chemistry.
Its great strength is this precision: rotational spectra can pin down a bond length to a tiny fraction of a percent. The limitations are equally clear: the molecule must be in the gas phase so it can rotate freely, and it must be polar — it needs a permanent dipole moment for the rotation to interact with light at all — so perfectly symmetric molecules like nitrogen or methane give no pure rotational spectrum.
The rotational spectrum of carbon monoxide gas is a ladder of evenly spaced microwave lines. Measuring the spacing of that ladder yields the carbon–oxygen bond length to within about a thousandth of an ångström — a measurement so sharp it is used to detect and clock CO in distant interstellar clouds.
Evenly spaced microwave lines that measure bond lengths with great precision.
Pure rotational transitions take the least energy of all the molecular jumps, which is why they sit in the microwave region. The next step up — vibration — needs infrared, and lifting an electron needs visible or ultraviolet light.