Neutral matter in planetary nebulae

A review of current studies of neutral envelopes is presented with particular attention given to the use of the envelopes as test cases for understanding the ionization and thermal structure of photodissociation regions. The study of near-IR H2 emission is discussed with detailed spectra given for a few planetary nebulae, and airborne observations of far-IR atomic lines are discussed. These two methods can discern photodissociation regions with warm gas and UV flux is fairly prominent. The use of resonance-absorption-line spectroscopy is also reviewed with respect to the analysis of the Na D lines, and thereby allows the measurement of integrated columns of material through the shell. The methods provide evidence for the notion that planetary nebulae consist of more than just ionized material; large amounts of neutral and molecular material are being confirmed, which has important implications for the mass-loss episode of the nebulae.