Knowledge Bank

It was announced at an earlier meeting$\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$ that many of the diffuse lines in interstellar visible absorption spectra could be attributed to absorption from high excited states of ortho $H2$ to states which lie beyond the predissociation limit. This model gives a highly satisfactory explanation of the complex mixture of diffuse and very diffuse lines which had not previously been identified. However, the assignment of transitions depends on extrapolating known discrete levels to the region of quasi-continuous predissociative energy levels. It is extremely difficult to directly confirm this set of assignments by comparison with reported laboratory measurements due to the relative weakness of the inverse predissociative transitions in emission. A much more critical test of the theory would appear to be the identification of several very sharp lines which are intermingled with some of the diffuse lines near the red end of the spectrum. It is reported here that at least three sharp lines can be accounted for by the same model which explains the diffuse lines. The predicted and observed line centers agree to within $0.02cm-1$ in each case. The lower state for each absorption line is a vibrationally and rotationally excited level of the $B1\Sigma u+$ electronic state. A mechanism which results in selective excitation by interstellar quantized (atomic hydrogen) Lyman energy is described briefly. The excitation involves near-resonance collision between ground state $H2$ and electronically excited H atoms, as well as excitation by intense fluxes of Lyman $\alpha$, $\beta$ and $\gamma$ photons.