Negative-polarity fast ionization wave in molecular gases: Electric field, electron density, and energy branching

The charge density per unit length, the longitudinal component of the electric field, and the electron density behind the front of a fast ionization wave initiated by a nanosecond negative voltage pulse in air, N₂, and H₂ in the 1-to 24-torr pressure range are reconstructed from the experimental data. It is shown that the electron density behind the wave front depends weakly on the sort of gas used and, at relatively high pressures (8-24 torr), is (2-3) × 10¹² cm^-3. The energy deposited in the internal degrees of freedom is analyzed. It is shown that, for all gases used, most of the deposited energy (40-60%) is spent on the excitation of the electron degrees of freedom. The fraction of the energy deposited in the high-energy degrees of freedom (ionization and dissociation) monotonically decreases with increasing the pressure, whereas the fraction of the energy spent on the excitation of the low-energy degrees of freedom (rotational and vibrational) monotonically increases.