Knowledge Bank

A review is given of the theory of two-photon absorption with emphasis on selection rules, orders-of-magnitude of cross-sections and the concept of a ``two-photon absorption spectrum. The absorption spectrum that interstellar hydrogen molecules would produce through the simultaneous absorption of photons corresponding to intense stellar emission lines and stellar continuum radiation is examined. The theory prediets a large number of interstellar absorption lines, which for the most part would be in the visible region between 4400 {\AA} and 6700 {\AA}, i.e., precisely in the region where numerous unexplained absorption lines have been observed. It is shown that the assignment of virtually all of the observed lines to two-photon transitions of hydrogen is totally consistent with selection rules and with the known energy levels of $H2$. The theory, moreover, is capable of accounting for the unusual breadths of the absorption lines, including that of the broad 4430 {\AA} feature. Finally, continuous absorption is predicted throughout the visible region with increasing strength towards shorter wavelengths. Although two-photon absorption is normally thought of as an extremely weak process, it is shown that a measurable interstellar extinction can reasonably be expected in the present case. This is due mainly to the following three factors: (1) Recent evidence which has led to large upward revisions in the abundance of interstellar $H2$, (2) The presence of anomalously intense emission lines in the same regions of interstellar space as the $H2$ clouds, and (3) The unusually large two-photon cross-sections for $H2$ and the radiation components which would be involved in the interstellar extinction. The enhancement factors in the cross-sections are due to near-resonances of the intense stellar lines and the $H2$ energy level system.