Positive Ions in Nitric Oxide Afterglows

Reported in this paper are results of measurements performed on a stationary nitric oxide afterglow. The nitric oxide was contained in a large gold-plated cylinder in which it was ionized by krypton resonance radiation. The ions from the afterglow were sampled through an electrically insulated orifice; it was observed that the potentials (in the mV range) applied to the orifice had a pronounced effect on the measured time constant of the afterglow decay. After the ions emerged from the sampling orifice, they were quantitatively detected by time-resolved mass spectrometry. An extensive study was made of the assumptions involved in the data-analysis model in order to establish the validity of the measurement techniques. In pure nitric oxide the only reaction observed over the pressure range investigated (10-300 m Torr) was NO⁺+NO+NOk→NO⁺·NO+NO. The rate constant k was determined to be 5 ±{} 1 ×{} 10^-30 cm⁶/sec. Measurements were also made of the diffusion coefficients for NO⁺ and NO⁺. NO in nitric oxide gas. The afterglow plasma exhibits a sharp transition from positive-ion-electron ambipolar-diffusion domination D+,e to positive-ion-negative-ion ambipolar-diffusion domination D_+,-. The negative ion participating in this latter diffusion process is NO₂^-. These data represent the first reported observation of such a transition. For NO⁺ the experimental values are D+,ep=85±10 cm² Torr/sec and D_+,-p=42±5 cm² Torr/sec. Similarly, for NO⁺. NO the values are D+,ep=84±13 cm² Torr/sec and D_+,-p=41±5 cm² Torr/sec.