HYPERFINE INDUCED TRANSITIONS AS DIAGNOSTICS OF ISOTOPIC COMPOSITION AND DENSITIES OF LOW-DENSITY PLASMAS

The J = 0 --> J' = 0 radiative transitions, usually viewed as allowed through two-photon decay, may also be induced by the hyperfine (HPF) i nteraction in atoms or ions having a nonzero nuclear spin. We compute new and review existing decay rates for the nsnp P-3(J)0 --> nS(2 1)S( J'= 0) transitions in ions of the Be (n = 2) and Mg (n = 3) isoelectro nic sequences. The HPF induced decay rates for the J = 0 --> J' = 0 tr ansitions are many orders of magnitude larger than those for the compe ting two-photon processes, and when present are typically 1 or 2 order s of magnitude smaller than the decay rates of the magnetic quadrupole (J = 2 --> J' = 0) transitions for these ions. Several HPF induced tr ansitions are potentially of astrophysical interest in ions of C, N, N a, Mg, Al, Si, K, Cr, Fe, and Ni. We highlight those cases that may be of particular diagnostic value for determining isotopic abundance rat ios and/or electron densities from UV or EUV emission-line data. We pr esent our atomic data in the form of scaling laws so that, given the i sotopic nuclear spin and magnetic moment, a simple expression yields e stimates for HPF induced decay rates. We examine some UV and EUV solar and nebular data in light of these new results and suggest possible a pplications for future study. We could not find evidence for the exist ence of HPF induced lines in the spectra we examined, but we demonstra te that existing data have come close to providing interesting upper l imits. For the planetary nebula SMC N2, we derive an upper limit of 0. 1 for C-13/C-12 from Goddard High-Resolution Spectrograph data obtaine d by Clegg. It is likely that more stringent limits could be obtained using newer data with higher sensitivities in a variety of objects.