Knowledge Bank

The chemical reaction H$\mathrm{3+}$ + H$2$ $\to$ H$2$ + H$\mathrm{3+}$ is the simplest bimolecular reaction involving a polyatomic, and is possibly the most common such process occurring in the universe. Recent measurements of interstellar clouds have shown that the temperatures derived from the lowest rotational levels of H$2$ and H$\mathrm{3+}$ do not agree, and it is expected that this reaction plays a key role in this deviation. To investigate this process, we have measured the $ortho$/$para$ ratio of H$\mathrm{3+}$ produced in this reaction by performing high resolution spectroscopy on its $\nu 2$ fundamental band in plasmas formed from various mixtures of $ortho$ and $para$ H$2$. These measurments have been performed in a supersonic expansion discharge source and in a cooled hollow cathode cell to probe the reaction at a variety of temperatures at and below 300 K. Our results provide experimental evidence that the population distribution of the lowest levels of H$\mathrm{3+}$ is governed by the steady state of the H$\mathrm{3+}$ + H$2$ $\to$ H$2$ + H$\mathrm{3+}$ reaction, not by thermalization.