At visible wavelengths, most transitions that are observed in the laboratory are due to electronic transitions within atoms, where electrons move between different energy levels in atoms. However, at infrared (IR) wavelengths, photons have insufficient energy to interact directly with atoms. Instead, at these lower energies molecular absorption mechanisms come into play, which are the core processes in forming the observed IR spectra of planets. Depending on temperature and molecular structure, molecules possess both rotational and vibrational degrees of freedom and absorption of IR photons can promote transitions between these different rotational-vibrational states, as will now be described.
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