ECAMP 15

An association reaction of H$_3^+$ ions with H$_2$ forming H$_5^+$ was studied in the temperature range of 15 – 35 K with either helium or hydrogen acting as a third body to understand the role of rotational excitation of colliding bodies.
A 22-pole radio-frequency ion trap apparatus [1] was employed to study the influence of the internal excitation of the reactants on the measured ternary reaction rate coefficient for H$_5^+$ formation. The population of the rotational ground state $J$ = 0 of H$_2$ was varied from 25 % up to more than 99 % of all hydrogen molecules in these experiments.
The measured values of the ternary reaction rate coefficients for H$_3^+$ + H$_2$ + H$_2$ association reaction are in agreement with the previously reported ion trap study by Paul [2]. The results indicate that the stability of the initial adduct formed in the collision of H$_3^+$ with H$_2$ is strongly dependent on the rotational state of the H$_2$ molecule. On the other hand, the character of the third body exerts little influence on the measured value of the ternary reaction rate coefficient.

[1] R. Plašil et al., Astrophys. J. 948, 131, (2023)
[2] W. Paul et al., Int. J. Mass Spectrom. Ion Processes 149–150, 373, (1995)