A critical review of H₃⁺recombination studies

Abstract

After more that 30 years of experimental and theoretical work it now appears that theory and experiment on H 3 + recombination have finally converged. Since the storage ring results come very close to the latest theoretical calculations one might conclude that "the case is closed". However, some troublesome issues remain and should not be dismissed too quickly. For instance, some afterglow measurements showed faster decays at early afterglow times. It is now clear that the fast recombining species cannot be vibrationally excited ions since the same rates were found for spectroscopically identified H 3 + ions in v=0. On the other hand, there is no convincing explanation for the very small rates obtained in the Prague experiments at low H 2 densities. It is also puzzling that some measurements find nearly the same recombination coefficients for H 3 + and D 3 + , while others indicate that H 3 + recombines much faster. It has often been stated that vibrational excitation of the H 3 + ions tends to enhance recombination, but the evidence for that is far from solid; some observations suggest that the opposite is just as likely. IntroductionThe history of recombination studies of the H 3 + ion still presents a confusing picture, even to someone who has worked on experimental studies of this ion for more than 30 years and has closely followed the "ups and downs" of experimental results and the interplay between experiment and theory. It is most gratifying that we are now at the point where the best available theory [1, 2] agrees rather well with the latest results of ion-storage-ring experiments [3]. Has the time come to close the books and to say that no further work is needed? I do not think so.If one looks at the experimental results that were obtained over the years (see table 1 and table 2), one notices that the published thermal rate coefficients at temperatures near 300 K have varied by more than one order of magnitude. What shall we make of this large variation? Discarding the older data as "flawed" is not acceptable and may deprive us of useful insights. The variations do not seem to due to ordinary experimental errors such as calibration errors of probes etc. Almost all experimenters also studied the "benchmark ion" O 2 + to test their procedures and in that case the 300 K rate coefficients agree rather well (see e.g. the references to O 2 + data in [4] and [5]). It is far more likely that the poorly controlled and usually unknown internal energy of the H 3 + ion (vibrational, rotational) had an effect on the measured recombination rate. Of course, the vibrational energy of the "benchmark" O 2 + ions also was generally poorly controlled, but fortunately it seems to have only a small effect on the total recombination rate [6] and hence did not lead to significant discrepancies between results.

Pressurized rf cavities in ionizing beams

“…The recombination rates for both H þ 3 and H þ 5 have been measured extensively using a variety of methods (H þ 3 [53][54][55][56][57], H þ 5 [55][56][57]). There is approximately an order of magnitude difference in the recombination rates of H þ 3 and H þ 5 .…”

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

“…The discrepancies between measurements of the binary dissociative reaction rate and the theoretical complexity of this seemingly simple reaction led to a great deal of fruitful research on this process. The history of H + 3 recombination studies has been adequately covered in several review articles [3][4][5][6][7][8][9]. Recently, both theory and experiment have converged to a value for the rate of this particular reaction.…”

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

“…The theoretical developments and improvements in storage ring experiments have resulted in a reconciliation of theory and experiment for the binary electron-H + 3 recombination process. However, the H + 3 and D + 3 recombination experiments carried out in afterglow plasmas [3,4,5,12,13,14,15,16,17] have repeatedly given rate coefficients very different from the ones obtained in the aforementioned storage ring experiments and the theoretical calculations. Until now, the plasma studies have not been fully understood and, as a result, they have been frequently rejected because they do not mesh with the present understanding of the binary DR process (see, e.g., the very recent review discussing this subject [18]).…”