We identify [ Rb iv ] 1.5973 and [ Cd iv ] 1.7204 \mu m emission lines in high-resolution ( R = 40 , 000 ) near-infrared spectra of the planetary nebulae ( PNe ) NGC 7027 and IC 5117 , obtained with the IGRINS spectrometer on the 2.7-m telescope at McDonald Observatory .
We also identify [ Ge vi ] 2.1930 \mu m in NGC 7027 .
Alternate identifications for these features are ruled out based on the absence of other multiplet members and/or transitions with the same upper levels .
Ge , Rb , and Cd can be enriched in PNe by s -process nucleosynthesis during the asymptotic giant branch ( AGB ) stage of evolution .
To determine ionic abundances , we calculate [ Rb iv ] collision strengths and use approximations for those of [ Cd iv ] and [ Ge vi ] .
Our identification of [ Rb iv ] 1.5973 \mu m is supported by the agreement between Rb ^ { 3 + } /H ^ { + } abundances found from this line and the 5759.55 Å feature in NGC 7027 .
Elemental Rb , Cd , and Ge abundances are derived with ionization corrections based on similarities in ionization potential ranges between the detected ions and O and Ne ionization states .
Our analysis indicates abundances 2–4 times solar for Rb and Cd in both nebulae .
Ge is subsolar in NGC 7027 , but its abundance is uncertain due to the large and uncertain ionization correction .
The general consistency of the measured relative s -process enrichments with predictions from models appropriate for these PNe ( 2.0–2.5 M _ { \odot } , [ Fe/H ] = -0.37 ) demonstrates the potential of using PN compositions to test s -process nucleosynthesis models .