An asymptotic limit to optical pumping in an opaque region

Abstract

An asymptotic limit to the optical pumping of low-lying levels of an ion or molecule in an optically thick medium has been found. The net rate of optical pumping approaches zero when optical depth is sufficiently large, even though individual line intensities generally increase as the optical depth increases. The optical pumping rate nears zero because the lines approach a detailed balance resulting from proton conservation; this detailed balance occurs for any geometry, line profiles or redistribution processes. An intermediate domain of large optical depths (the pumping-dominated limit) in which level populations are identical to those in an optically thin region is also found. Results of this optical pumping study may be applied to the excitation of atomic fine-structure levels in H II regions and quasars.