Interstellar extinction for lambda >= 1100 angstrom and H-2(+)

The wavelength-dependent photoabsorption cross section sigma(lambda) for H- 2(+) by a transition from a discrete rotation-vibration (RV) state in the 1 s sigma(g) electronic orbital to a continuum state of internuclear motion i n the 2p sigma(u) electronic orbital is discussed. The predicted extinction depends strongly on the, as yet, unknown occupation numbers of the initial RV states appropriate to various parts of the interstellar medium. A simpl e method for adjusting the occupation numbers is introduced. It is applied to three formulae representing the variety observed in interstellar extinct ion for this or other galaxies. The agreement is quantitative for wavelengt hs lambda greater than or equal to 2700 Angstrom. Agreement in the ubiquito us lambda similar to 2175 Angstrom bump region becomes less accurate as the observed relative height increases and the width decreases. The agreement is poor for lambda less than or equal to 1400 Angstrom. The occupation numb ers found for these cases are acceptable on physical grounds and bear a res emblance to those predicted and observed for the ionization of cold H-2 by energetic radiation. The predicted shape would allow this mechanism to be t he sole carrier for interstellar medium (ISM) extinction for lambda greater than or equal to 2700 Angstrom and important far 2700 greater than or equa l to lambda greater than or equal to 1400 Angstrom. However, the [H-2(+)] r equired by this mechanism is considerably larger than predicted by current ISM models. It is suggested that this mechanism be considered, along with t he established dust mechanism and perhaps others, as a contributor to the I SM extinction.