  Spectroscopy of planetary nebulae (PNe) provides the means to investigate
s-process enrichments of neutron(n)-capture elements that cannot be detected in
asymptotic giant branch (AGB) stars. However, accurate abundance determinations
of these elements present a challenge. Corrections for unobserved ions can be
large and uncertain, since in many PNe only one ion of a given n-capture
element has been detected. Furthermore, the atomic data governing the
ionization balance of these species are not well-determined, inhibiting the
derivation of accurate ionization corrections. We present initial results of a
program that addresses these challenges. Deep high resolution optical
spectroscopy of ~20 PNe has been performed to detect emission lines from
trans-iron species including Se, Br, Kr, Rb, and Xe. The optical spectral
region provides access to multiple ions of these elements, which reduces the
magnitude and importance of uncertainties in the ionization corrections. In
addition, experimental and theoretical efforts are providing determinations of
the photoionization cross-sections and recombination rate coefficients of Se,
Kr, and Xe ions. These new atomic data will make it possible to derive robust
ionization corrections for these elements. Together, our observational and
atomic data results will enable n-capture element abundances to be determined
with unprecedented accuracy in ionized nebulae.
