  Lyman and Werner band absorption features from interstellar molecular
hydrogen in rotational levels J = 0, 1, 2, 3 and 5 were observed in the
spectrum of zeta Ori A by the Interstellar Medium Absorption Profile
Spectrograph (IMAPS). Most of the molecules are found in two distinct velocity
components, with one of them showing profile shapes that become broader and
show small displacements toward more negative velocities as J increases. This
change in profile shapes, along with evidence from atomic lines at high
negative velocities, supports an interpretation that the molecules are
initially created by associative detachment of H- in a partially ionized, warm
gas that is cooling and recombining behind a stationary shock front. The front
probably arises from an interaction of high speed gas from the Orion stars and
the ambient medium. These molecules are most conspicuous in the high J levels.
Later, when the gas becomes much cooler, mostly neutral, and more compressed as
it comes nearly to a halt, the H2 is primarily created by reactions on the
surfaces of dust grains and is more easily seen in the lowest J levels.
