Temperature dependence of binary and ternary recombination of H3+ ions with electrons

We study binary and the recently discovered process of ternary He-assisted recombination of 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 H-3* is formed in electron-H-3+ collisions. Second, the H-3* 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 H-3* and of the ternary recombination rate coefficients for para- and ortho-H-3+. The calculations show a large difference between the ternary recombination rate coefficients of ortho- and para-H-3+ at temperatures below 300 K. The measured binary and ternary rate coefficients are in reasonable agreement with the calculated values.