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If we observe any textbook they say that molecules form band spectrum. But a H2 molecule is just 2 H atoms and both form line spectra but somehow combination of 2 H atoms formed band spectrum. Because if even 2 H atoms are present they both absorb the same wavelengths and then this implies they should form line spectra. But the question why do they form band spectra still remains?

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Molecular spectrum is not just a superposition of the atomic spectra, it also has vibrational and rotational degrees of freedom corresponding to the oscillations of the inter-atomic bond lengths and the rotation of a molecule as a whole (or rotation of its parts in case of complex molecules). The frequencies of these degrees of freedom are usually much smaller than the optical frequencies of atomic absorption, and they are rarely directly coupled to the light. Instead they are excited together with an electron transition, and result in appearance of the satellite absorption lines, which appear as bands due to additional broadening because of the kinetic motion.

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