Excitation diagrams of shock and PDR model-predicted H₂ line column density distributions as a function of upper energy level. The column density is normalized to the 1-0 S(1) H₂ transition. Diamonds represent lines from v = 1 while the asterisks represent points from v > 1. The solid line is a best fit of the Boltzmann distribution to the v = 1 points, the slope represents the excitation temperature, T_ex. Predictions of shock models from Shull & Hollenbach (1978) with shock speeds 6 km s^-1 (top row) and 14 km s^-1 (second row) and densities n_H = 10³ cm^-3 (left), 10⁴ cm^-3 (middle), and 10⁵ cm^-3 (right). Predictions of PDR models from Black & van Dishoeck (1987) with I_uv = 10⁰ − 10⁴ and densities n_H = 10³ cm^-3, 10⁴ cm^-3, and 10⁵ cm^-3, and are labeled with BvD.