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

Abstract

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 ‫ء‬ is formed in electron-H 3 + collisions. Second, the H 3 ‫ء‬ 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 ‫ء‬ 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.

Binary recombination of para- and ortho-H 3 + with electrons at low temperatures

“…The agreement between the experimental values ( n a bin , p a bin and o a bin ) and the theoretical values ( n a DR , p a DR and o a DR ) [18] is very good. Moreover, though the electron number density used in the present experiment was by an order of magnitude higher than in our previous FALP experiments using Langmuir probes [14], the agreement between the present n a bin values and those from the FALP experiments is very good over the whole temperature range. Because of this agreement at higher temperatures, where CRR is negligible, we conclude that the measured rate coefficients at 77 K do not depend on the electron density.…”

“…This resulted in a convergence with experimental DR data reported from ion storage rings where experimentalists had realized the impact of rotational excitation of the H + 3 ions on the DR reaction rate, especially with respect to the importance of the DR process in interstellar molecular clouds [11][12][13]. Final convergence between theory and the remaining important experimental techniques, stationary and flowing afterglow, was reached after recognizing that a fast third-body reaction, not previously considered, stabilizes the recombination process in such plasmas [4,14,15]. However, recently it has been shown [16,17] that the assumption of rotationally cold H + 3 ions in storage rings was not entirely correct [17].…”

“…The measured values of p a bin , o a bin , n a bin and values of n a bin from previous FALP experiments [4,14,15] are plotted in figure 3. The displayed errors of rate coefficients are 1s errors, and systematic errors (mainly from determination of ion number densities) were estimated to be less than 10 per cent.…”

Kinetic processes in recombining H 3 + plasmas

“…While the 'raw' afterglow data are larger by factors of 2-4, the 'discrepancy' is greatly reduced if one corrects for third-body effects, as was carried out by Glosík et al [3]. The quotes around the word 'discrepancy' are meant to indicate that the negative connotation of the term is undeserved.…”

“…Afterglow plasmas at higher densities, however, explore additional mechanisms to return to the neutral state, as is immediately apparent when one examines historical recombination data. Figure 1 compares selected afterglow rate coefficients [2,3,5,6] to the thermal recombination coefficient inferred from earlier ISR experiments [4].…”