The s-process should occur in all but the lower mass progenitor stars of planetary nebulae, and this should be reflected in the chemical composition of the gas that is expelled to create the current planetary nebula shell. Weak forbidden emission lines are expected from several s-process elements in these shells and have been searched for and in some cases detected in previous investigations. Here we extend these studies by combining very high signal-to-noise ratio echelle spectra of a sample of PNe with a critical analysis of the identification of the emission lines of Z > 30 ions. Emission lines of Br, Kr, Xe, Rb, Ba, and Pb are detected with a reasonable degree of certainty in at least some of the objects studied here, and we also tentatively identify lines from Te and I, each in one object. The strengths of these lines indicate enhancement of s-process elements in the central star progenitors, and we determine the abundances of Br, Kr, and Xe, elements for which atomic data relevant for abundance determination have recently become available. As representative elements of the ''light'' and ''heavy''s-process peaks, Kr and Xe exhibit similar enhancements over solar values, suggesting that PN progenitors experience substantial neutron exposure.

The identification of spectral lines can be a tedious process requiring the interrogation of large spectroscopic databases, but it does lend itself to software algorithms that can determine the characteristics of candidate line identifications. We present here criteria used for the identification of lines and a logic developed for a line identification software package called EMILI, which uses the v2.04 Atomic Line List as the basic line database. EMILI considers all possible database transitions within the wavelength uncertainties for observed optical emission lines and computes an approximate intensity for each candidate line. It searches for other multiplet members that are expected to be seen with each candidate line, and rank orders all of the tentative line identifications for each observed line based on a set of criteria. When applied to the spectra of the Orion Nebula and the planetary nebula IC 418, EMILIs recommended line IDs agree well with those of previous traditional manual line assignments. The existence of a semi-automated

[1] The atlas of terrestrial nightglow emission lines from spectra of the night sky obtained from the Ultraviolet and Visual Echelle Spectrograph (UVES) on the 8.2-m UT2 telescope at the Very Large Telescope (VLT), European Southern Observatory, Cerro Paranal, Chile, consists of 2808 line positions, line widths, and intensities over the 314-1043 nm spectral range (Hanuschik, 2003). These lines have been absolute intensity calibrated and measured at a spectral resolution (l/Dl) of $43,000-45,000. Presented here are spectroscopic identifications for 98% of the lines in the atlas, made primarily through comparisons with synthetic spectra of prominent OH and O 2 nightglow emission systems. The ability to simulate these systems successfully has shown that there are many additional lines that could be added to the atlas. We believe that all the O 2 and OH lines in the measured region can now be successfully modeled with an accuracy better than the instrumental spectral resolution.