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SCIENTIFIC ABSTRACT Galactic evolution models lead us to believe that the interstellar gas occupying the central few hundred parsecs of the Galaxy has probably undergone a relatively large amount of nuclear processing in stars. This is supported by observations of highly unusual isotopic abundance ratios of molecular lines there, indicative of an enhanced abundance of nuclear burning products via both primary and secondary processes. However, the metallicity of the galactic center region is only crudely determined at present, and there is essentially no unambiguous information on how the metallicity varies with radius in the inner few hundred parsecs. A few measures have been made of mid-IR lines of Ar and Ne, and of the far-IR fine structure lines of [N III] and [O III], and these suggest a metallicity enhancement of about a factor of two, but the treatment of the abundances of unobserved ionization states is quite uncertain. Stars near the galactic center provide some relevant clues - the peak metallicity of K giants in Baade's window is about twice the solar value, though there is a broad range. In sum, the metallicity of the galactic center is poorly known, and yet we cannot understand this environment well without knowing it. The metallicity is needed to 1) constrain models of galactic evolution, 2) model the heating, cooling, and chemistry of the interstellar medium near the galactic center, 3) use molecular lines to derive cloud masses, and 4) understand how metallicity may affect the IMF in the star formation process. In order to determine accurate abundance ratios and to establish the galactic center metallicity and its dependence on galactocentric radius with some reliability, we propose to use the SWS and LWS on ISO for a total of 2.9 hours to observe the accessible ionization states of H (alpha lines at 4.05 & 7.45 microns), N (NII at 121.9 and NIII at 57.4 microns), O (OIII at 51.8 & 88.4 microns and OIV at 25.9 microns), Ne (NeII at 12.8 and NeIII at 15.6 & 36.0 microns) and Ar (ArII at 6.99 and ArIII at 8.99 & 21.8 microns) in 5 HII regions in the inner 0.5 degrees of the Galaxy. These observations will be used with existing radio continuum data to construct models for the temperature, density, and radiation field within each HII region so that all ionization states will be represented in the models. Ar and Ne have been chosen as probes of the heavy element abundance because they are likely to be negligibly depleted onto grains. In addition, the Pf-alpha line of H at 7.45 mu is included for direct comparison with the Ar lines and to provide an estimate of the H column density for comparison with those derived from radio continuum data. Br-alpha is included in order to constrain the mid-IR extinction by comparison with Pf-alpha. The HII regions chosen for study are all relatively isolated and their structures are all reasonably well understood from radio observations. All are very strong sources of near- and far-infrared radiation. On the basis of existing velocity information, mapping observations, and clues from absorption line studies, one can be confident that these objects are close to the galactic center, rather than being fortuitously superimposed. They have a spread of galactocentric distances, so that any sharp metallicity gradients near the nucleus might be probed. Sgr B2 and Sgr A are not on our list, but the spectral surveys of Sgr B2 and Sgr A West to be undertaken in the core program can also be used as measures of the metallicity at 0 and about 100 pc, respectively. In case of the Sagittarius hole, the objects will not be visible, and no time will be allocated to this proposal. The available time will in that case shift to the proposal "Large Scale Shocks in the Galactic and Other Regions" to map the H2O distribution in IC 443. OBSERVATION SUMMARY We propose to observe the fine structure lines of N II, N III, O III, O IV, Ar II, Ar III, Ne II, Ne III and recombination lines of H I in 5 H II regions in the Galactic Center region. Because the continuum is strong, we will use the Fabry-Perot wherever feasible in order to distinguish the lines (SWS AOT07, LWS AOT04). The remaining lines will be done with the SWS grating, AOT02. The requested S/N is 50-100 on the continuum for the grating and 10-20 on the line for the FP. The integration times would typically be 80s for each line in each source. Concatenation of the SWS AOT2 and AOT7 observations is required in order to have the same orientation of the slit on the source. It is recommended for the LWS observations in order to save overhead for slewing.