Nuclear Spin Effect on Recombination ofH3+Ions with Electrons at 77 K

Abstract

Utilizing different ratios of para to ortho H₂ in normal and para enriched hydrogen, we varied the population of para-H₃⁺ in an H₃⁺ dominated plasma at 77 K. Absorption spectroscopy was used to measure the densities of the two lowest rotational states of H₃⁺. Monitoring plasma decays at different populations of para-H₃⁺ allowed us to determine the rate coefficients for binary recombination of para-H₃⁺ and ortho-H₃⁺ ions: (p)α(bin)(77 K) = (1.9 ± 0.4) × 10⁻⁷ cm³ s⁻¹ and (o)α(bin)(77 K) = (0.2 ± 0.2) × 10⁻⁷ cm³ s⁻¹.

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

“…The experimental apparatus is the same as that in our previous studies [20,26]. A pulsed microwave discharge generates a plasma in a tube (inner diameter of approx.…”

“…Near-infrared cavity-ringdown absorption spectroscopy (NIR-CRDS) enabled in situ determination of the spin states, together with the kinetic and the rotational temperatures of the recombining ions. A similar approach was used in our recent study of binary recombination of para-H + 3 and ortho-H + 3 ions at temperatures close to 77 K [20]. In the present studies, we have extended the range of temperatures up to 200 K. Our previous measurements in H + 3 -and D + 3 -dominated plasmas at conditions similar to those in the present experiment [4,14,15,21,22] have shown that the H + 3 ions recombine by both a binary process with a rate coefficient a bin , and a ternary helium-assisted recombination mechanism, with a rate coefficient K He .…”

“…A measurement of a eff for two or more different values of p f 3 , but under otherwise identical conditions (temperature, and density of He and H 2 ), then permits a determination of the individual recombination rate coefficients p a eff and o a eff . The fraction p f 3 can be enhanced from about 0.5 to 0.8 by using para-enriched hydrogen instead of normal hydrogen (for details, see [20,26]). …”

“…The measured electron density decay curves were analysed to obtain a eff for two particular values of p f 3 (further details can also be found in the study of Varju et al [20]). We carried out a systematic set of measurements that differed only in the value of p f 2 ( p f 2 = 0.25 when using n H 2 and p f 2 = 0.87 using e H 2 ), but under otherwise very similar conditions.…”

“…To obtain a eff from the measured electron density decay curves, we used direct fits to the data and, in addition, the more advanced 'integral analysis' technique [31]. We deliberately excluded the first 50-150 ms of the afterglow decay from the data analysis because some new ions were probably still being formed (for details see [4,20,26,31]). The observed dependences of n a eff and e a eff on [He] at 140 K are shown in figure 2a.…”