Estimate of the barrier to and rate of dissociation of dense nitrogen under shock conditions

A simple method is introduced to estimate the barrier to dissociation of nitrogen under shock conditions. A free-energy curve is constructed from quantum-mechanical calculations of the ground-state potential well for ${\mathrm{N}}_2$, the model exponential-six potential for the dissociated state, and the additional chemical potential (due to the surrounding dense media) in the hard-sphere approximation. The curves are adjusted to yield behavior in agreement with equilibrium thermodynamic calculations. The barriers are several electron volts, decreasing with pressure. Estimates for the prefactor in an Arrhenius rate expression are based on the recent computer simulations of nitrogen vibrational relaxation [B. Holian, J. Chem. Phys. 84, 3138 (1986)]. Time scales for reaction vary from microseconds to tenths of nanoseconds with increasing shock pressure.