Ionization Rates behind Shock Waves in Argon

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Abstract
The ionization rate behind shock waves in argon has been recalculated allowing for excitation and ionization by atom‐atom collisions. Good agreement is obtained between these calculations and the experimental results of others for very pure argon. For pure argon, the initial rate of ionization is controlled by the rate of excitation from the ground state by means of atom‐atom collisions. The atom‐atom cross sections are several percent of the corresponding electron‐atom cross sections. When the degree of ionization builds up to some value greater than 10−4, electron‐atom collisions dominate, and then the ionization rate is controlled by the rate of energy transfer between ions and electrons by means of elastic collisions. The effect of impurities is considered and a relatively crude theory is proposed to explain the experimental observations. It is shown that one must be careful to distinguish between ionization times and luminosity delay times.