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The ionization rate of interstellar material by cosmic rays has been a majorsource of controversy, with different estimates varying by three orders ofmagnitude. Observational constraints of this rate have all depended onanalyzing the chemistry of various molecules that are produced followingcosmic-ray ionization, and in many cases these analyses contain significantuncertainties. Even in the simplest case (H3+) the derived ionization ratedepends on an (uncertain) estimate of the absorption path length. In this paperwe examine the feasibility of inferring the cosmic-ray ionization rate usingthe 10830 A absorption line of metastable helium. Observations through thediffuse clouds toward HD 183143 are presented, but yield only an upper limit onthe metastable helium column density. A thorough investigation of He+ chemistryreveals that only a small fraction of He+ will recombine into the triplet stateand populate the metastable level. In addition, excitation to the tripletmanifold of helium by secondary electrons must be accounted for as it is thedominant mechanism which produces He* in some environments. Incorporating thesevarious formation and destruction pathways, we derive new equations for thesteady state abundance of metastable helium. Using these equations in concertwith our observations, we find zeta_He < 1.2*10^-15 s^-1, an upper limit about5 times larger than the ionization rate previously inferred for this sight lineusing H3+. While observations of interstellar He* are extremely difficult atpresent...

Interstellar Metastable Helium Absorption as a Probe of the Cosmic-Ray Ionization Rate