The interstellar 2200 A feature and charge transfer absorption

It is suggested that the interstellar 2200 A feature is due to charge transfer between neutral hydrogen and singly ionized iron, silicon and magnesium. Apart from a much weaker feature, near 1600 A arising from charge transfer between neutral hydrogen and singly ionized aluminium, calcium and sodium, no other charge transfer features should be detectable. The hydrogen and the ions must be together on (or in) the interstellar dust grains. The approximate wavelength of the charge transfer feature for each element is given by the difference in the first ionization potentials of hydrogen and the element, and the approximate strength of each feature by the cosmic abundance of the element. (author)