Excitation and dissociation of molecular hydrogen in shock waves at interstellar densities

The rate constant (kD) for the thermal dissociation of molecular parahydrogen by collisions with helium at kinetic temperatures of 2000-10,000 K is determined for the entire range of densities appropriate to interstellar molecular clouds. Included in a solution of the master equation for the dissociation reaction are collisional transitions among the rotation-vibration levels, collision induced dissociation out of all levels, and infrared quadrupole emission. The rate constant decreases strongly over the density range of 1 million-10,000/cu cm due to the competition between radiative and collisional processes. At low densities, kD approaches a constant value corresponding to direct collisional dissociation out of the (v = 0, J = 0) level. It is noted that the population distribution among the rotation-vibration levels can show large deviations from a Boltzmann distribution and that, under certain circumstances, there are even population inversions.