= Rotational spectroscopy
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<Rotational spectroscopy> measures transitions between <molecular rotation> <quantum states>, usually in low-pressure gaseous <molecules>. The measured <spectral lines> become a molecular fingerprint only after <rotational spectral line assignment>: identify which transitions belong together, determine their <quantum numbers>, fit a rotational <Hamiltonian>, and test additional predictions. Line positions constrain <moments of inertia>, while <rotational state populations> and transition coupling determine which lines can be detected.
The simplest ordinary microwave absorption ladder belongs to a polar <linear molecule> obeying <pure rotational electric-dipole selection rules>. A <homonuclear diatomic molecule> has no permanent <electric dipole moment> in its symmetric internal state, but <rotational Raman spectroscopy> can probe its rotation through anisotropic <electric polarizability>. The coupling mechanism must therefore be specified before interpreting a missing spectrum.
The existing <linear rigid rotor>, <rotational constant>, <isotope shift of a linear rotor>, and <centrifugal distortion> explain the forward model. <Molecular structure from rotational constants> asks the reverse question: which arrangements of <chemical atoms> could produce the fitted constants? The <NIST trace-gas FTMW validation experiment> makes the measurement chain concrete and separates identifying a species from measuring its <concentration>.
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