discussed mechanisms of 2excitation in PNe; see also Kastner et al. (1996). If detected in sufficient number, the observed 2 line ratios are an excellent diagnostic for determining the relative importance of shocks and UV photons in a photodissociation region (PDR) for the excitation of the 2emission. Even if the excitation mechanism cannot be determined, the presence of 2 emission is important to understanding the conditions in PNe and how they evolve through wind interactions and photodissociation.

An analysis of near-IR 2 emission can determine an ortho-to-para (O/P) ratio, the rotational excitation temperature $T_{ex}(J)$, and the vibrational excitation temperature $T_{ex}(v)$ of the molecules. If the rotational and vibrational excitation temperatures differ, then UV excitation is indicated. This is most readily determined by comparing the column densities in the upper state vibration-rotation levels with the upper state energy (in temperature units; see Hora & Latter 1994, 1996 for a full discussion; see also Black & van Dishoeck 1987; Sternberg & Dalgarno 1989). When many 2 lines are detected, especially those from highly excited levels that fall in the J band, a much stronger case can be made for the importance of UV excitation than does the traditional $v = 2\to 1$ 