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

Short-lived peaks of nonequilibrium emission are detected at 320-350 nm in shock-wave fronts in He, Ne, Ar, and H-2 containing from 0.1 to 3% iodine m olecules. 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 e mission observed is assigned to the electronic I-2(D(3)Sigma --> B(3)Pi) ba nd, which is located at excitation energies 5.45 --> 1.8 eV, i.e., signific antly above the dissociation threshold of iodine molecules (1.54 eV). An an alysis of the results shows that the leading role in the excitation of iodi ne molecules is played by high-energy collisions in the translational diseq uilibrium zone of the shock wave. The best description of the experimental data is achieved for the value of the effective collision energy in the fro nt calculated on the basis of a numerical solution of the Boltzmann equatio n by a modified Tamm-Mott-Smith method. The absolute values of this energy under the conditions of the experiments performed are roughly 10 times grea ter than the mean collision energy in the equilibrium zone behind the shock wave. The probability of nonadiabatic supercollisions of the type I-2+I-2- -> I-2(D(3)Sigma)+I-2-6.4eV exceeds the adiabatic values by a factor of 10( 15)-10(20). (C) 1999 American Institute of Physics.