Nonadiabatic excitation of iodine molecules in the translational disequilibrium zone of a shock wave

Abstract

Short-lived peaks of nonequilibrium emission are detected at 320–350 nm in shock-wave fronts in He, Ne, Ar, and H2 containing from 0.1 to 3% iodine molecules. The effect is observed in the range of Mach numbers from 3.2 to 6.3 for initial pressures of the mixtures ranging from 133 to 2660 Pa. The emission observed is assigned to the electronic I2(D3Σ→B3Π) band, which is located at excitation energies 5.45→1.8 eV, i.e., significantly above the dissociation threshold of iodine molecules (1.54 eV). An analysis of the results shows that the leading role in the excitation of iodine molecules is played by high-energy collisions in the translational disequilibrium zone of the shock wave. The best description of the experimental data is achieved for the value of the effective collision energy in the front calculated on the basis of a numerical solution of the Boltzmann equation by a modified Tamm-Mott-Smith method. The absolute values of this energy under the conditions of the experiments performed are roughly 10 times greater than the mean collision energy in the equilibrium zone behind the shock wave. The probability of nonadiabatic supercollisions of the type I2+I2→I2(D3Σ)+I2−6.4eV exceeds the adiabatic values by a factor of 1015–1020.