Can somebody provide Absorption spectrum of Hydrogen molecule? I have searched days for absorption spectrum of Hydrogen molecule in 200 nm - 300 nm UV region and All I got is absorption spectrum of atomic hydrogen. I do find little about absorption spectrum of hydrogen molecule in the region 90 nm to 180 nm in some astrophysics research papers.

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    $\begingroup$ Although now an old book Herzberg, 'Spectra of Diatomic Molecules' gives details of the energy levels of numerous excited states (up to $\approx$ 100000 cm$^{-1}$) and references to the original papers. The ground state dissociation energy is $\approx $ 4.476 eV. $\endgroup$
    – porphyrin
    Dec 15, 2017 at 23:10

1 Answer 1


Being a homonuclear diatomic molecule, molecular hydrogen does not posses a pure rotational of vibrational (electric dipole) spectrum. The lowest excited singlet state is the B state (for historical reasons there is no A state in H$_2$) and transitions from this state to the X ground state are known as the Lyman band which starts around 120 nm, thus outside the range you are interested in. In principle, electric quadrupole transitions may be observed but these are several orders of magnitude weaker than electric dipole transitions. In addition, the dissociation energy of molecular hydrogen lies around 36000 cm$^{-1}$, so that only radiation longer than about 277 nm can result in bound-to-bound transitions. However, I expect that the Franck-Condon factors to these highly excited vibrational states are very poor.

So there reason that you cannot find a spectrum is that there is none.


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