Temperature dependence of binary and ternary recombination ofH3+ions with electrons

We study binary and the recently discovered process of ternary He-assisted recombination of $\text{H}_{3}{}^{+}$ ions with electrons in a low-temperature afterglow plasma. The experiments are carried out over a broad range of pressures and temperatures of an afterglow plasma in a helium buffer gas. Binary and He-assisted ternary recombination are observed and the corresponding recombination rate coefficients are extracted for temperatures from 77 to 330 K. We describe the observed ternary recombination as a two-step mechanism: first, a rotationally excited long-lived neutral molecule $\text{H}_{3}{}^{\ensuremath{\ast}}$ is formed in electron-$\text{H}_{3}{}^{+}$ collisions. Second, the $\text{H}_{3}{}^{\ensuremath{\ast}}$ molecule collides with a helium atom that leads to the formation of a very long-lived Rydberg state with high orbital momentum. We present calculations of the lifetimes of $\text{H}_{3}{}^{\ensuremath{\ast}}$ and of the ternary recombination rate coefficients for para- and ortho-$\text{H}_{3}{}^{+}$. The calculations show a large difference between the ternary recombination rate coefficients of ortho- and para-$\text{H}_{3}{}^{+}$ at temperatures below 300 K. The measured binary and ternary rate coefficients are in reasonable agreement with the calculated values.