Dissociative recombination of ${\mathrm{H}}_3$${\mathrm{O}}^{+}$ and ${\mathrm{D}}_3$${\mathrm{O}}^{+}$ at elevated electron and gas temperatures

The rate coefficients for the dissociative ion-electron recombination of ${\mathrm{H}}_3$ ${\mathrm{O}}^{+}$ and ${\mathrm{D}}_3$ ${\mathrm{O}}^{+}$ have been measured in shock-heated afterglow plasmas. A low-pressure, pressure-driven shock-tube apparatus enabled gas temperatures in the range $800\le T_g\le 3000$ K to be investigated. A focused microwave beam which intersected the shock-heated plasma permitted the separate, controlled variation of the electron temperature over the range $800\le T_e\le 9000$ K. The recombination coefficient for ${\mathrm{H}}_3$ ${\mathrm{O}}^{+}$ was found to vary approximately as $T^{-\frac{1}{2}}$ over both the ranges of electron and gas temperature covered, with the power-law exponent increasing slightly with temperature. The coefficients for ${\mathrm{H}}_3$ ${\mathrm{O}}^{+}$ and ${\mathrm{D}}_3$ ${\mathrm{O}}^{+}$ were found to be equal within the experimental error. The results indicate that the effect of vibrational excitation on the recombination coefficient is small for these ions. The present results for ${\mathrm{H}}_3$ ${\mathrm{O}}^{+}$ extrapolated to 540 K agree reasonably well with a previous measurement by the well-established pulsed-afterglow microwave-cavity technique. The absolute magnitude of the measured coefficient decreased from a value of 7.5±1.0×${10}^{-7\mathrm{}}$ ${\mathrm{cm}}^3$ ${\sec }^{-1\mathrm{}}$ at $T_g=T_e=800\mathrm{}$ K to 4.2±0.3×${10}^{-7\mathrm{}}$ ${\mathrm{cm}}^3$ ${\sec }^{-1\mathrm{}}$ at $T_g=T_e=3000\mathrm{}$ K for ${\mathrm{H}}_3$ ${\mathrm{O}}^{+}$.