Nonequilibrium rovibrational energy distributions of hydrogen isotopologues in an expanding plasma jet.

State resolved densities of high rovibrationally excited hydrogen isotopologues H(2), HD, and D(2) in the electronic ground state have been measured in a supersonically expanding plasma jet. The obtained state distributions differ substantially from thermal equilibrium. Moreover, the distributions are not the same for H(2), HD, and D(2) indicating different formation and relaxation rates for each isotopologue. Mechanisms for this deviation from a Boltzmann distribution are given and compared to hydrogen reactions in other environments. The difference between the measured highest occupied rovibrational states in H(2), HD, and D(2) is ascribed to an isotope effect in the dissociation process.