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We investigate the ionization state of the damped Lya system at z=2.62 towardGB1759+7539 through an analysis of ionic ratios sensitive to photoionization:ArI/SII, FeIII/FeII, NII/NI, AlIII/AlII. Approximately half of the metals arisein a mostly neutral velocity component with HI/H > 0.9, based on FeIII/FeII <0.013. In contrast, the remaining half exhibits FeIII/FeII~0.3 indicative of apartially ionized medium with HI/H~0.5. These conclusions are supported by theobserved NII/NI, AlIII/AlII, and ArI/SII ratios.  We assess ionization corrections for the observed column densities throughphotoionization models derived from the CLOUDY software package. In the neutralgas, the ionization corrections are negligible except for ArI. However for thepartially ionized gas, element abundance ratios differ from the ionic ratios by0.1-0.3 dex for (SiII, SII, NiII, AlII)/FeII ratios and more for (NI,ArI)/FeII. Independent of the shape of the photoionizing spectrum andassumptions on the number of ionization phases, these ionization correctionshave minimal impact (<0.1dex) on the total metallicity inferred for this dampedLya system. Measurements on the relative elemental abundances of the partiallyionized gas, however, have a greater than ~0.15 dex uncertainty which hides theeffects of nucleosynthesis and dust depletion.  We caution the reader that this damped system is unusual for a number ofreasons (e.g. a very low ArI/SII ratio) and we believe its ionizationproperties are special but not unique. Nevertheless, it clearly shows the valueof examining photoionization diagnostics like FeIII/FeII in a larger sample ofsystems.

The UCSD HIRES/Keck I Damped Lyα Abundance Database. III. An Empirical Study of Photoionization in the Damped Lyα System toward GB 1759+7539