The Ionization of HCl by Electron Impacts

Analysis of the nature of the ions formed in HCl by electron impact.—For the purpose of supporting the Born crystal grating theory the assumption has generally been made that the observed ionization potential of HCl was a measure of the amount of energy required to split an HCl molecule into an ${\mathrm{H}}^{+}$ and a ${\mathrm{Cl}}^{-}$ ion. Recent indirect experiments have cast doubt on the correctness of this assumption. The work here described was undertaken to determine with the aid of a mass spectrograph, the actual products of electron impact ionization in HCl. In the first experiments, the following types of ion were observed: ${\mathrm{H}}^{+}$, ${\mathrm{H}}_2^{+}$, ${\left({\mathrm{H}}_2\mathrm{O}\right)}^{+}$, ${\mathrm{}\left(\mathrm{HCl}\right)\mathrm{}}^{+}$ and ${\mathrm{Cl}}_2^{+}$. The last type, ${\mathrm{Cl}}_2^{+}$ was only observed with high pressures and is thought to be a secondary product. It was found that the hot tungsten filament in the HCl vapor produced molecular hydrogen in quantities sufficient to account for the hydrogen ions. To be certain no ${\mathrm{H}}^{+}$ ions were produced by impacts with HCl molecules, and also to establish the identity of the ${\mathrm{}\left(\mathrm{HCl}\right)\mathrm{}}^{+}$ ions, a new apparatus was built. The resolving power obtained was sufficient to show that the peak ascribed to ${\mathrm{}\left(\mathrm{HCl}\right)\mathrm{}}^{+}$ consisted, in reality, of four peaks corresponding to the ions ${\left({\mathrm{Cl}}^{35}\right)}^{+}$, ${\left(\mathrm{H}{\mathrm{Cl}}^{35}\right)}^{+}$, ${\left({\mathrm{Cl}}^{37}\right)}^{+}$, and ${\left(\mathrm{H}{\mathrm{Cl}}^{37}\right)}^{+}$, the isotopes being for the first time evident. The ${\mathrm{Cl}}^{+}$ ions and the hydrogen ions were observed to decrease in number relative to the ${\mathrm{}\left(\mathrm{HCl}\right)\mathrm{}}^{+}$ ions with time after each evacuation in the same manner as the ${\left({\mathrm{H}}_2\mathrm{O}\right)}^{+}$ ions decreased. It was concluded that the water vapor coming off the walls was in some way responsible for these ions, probably aiding the production of molecular hydrogen at the filament. In test runs with pure water vapor, and with HCl after several weeks time had been given the water vapor to disappear, no hydrogen ions were observed.

In experiments with negative ions, it was found that ${\mathrm{Cl}}^{-}$ ions were formed in numbers relatively large, chiefly near the filament. They did not require impacts of more than 3 volts for their formation, if they required impacts at all. There seemed to be no connection between the ${\mathrm{Cl}}^{-}$ ions and the 13.8 volt ionization potential. The ${\mathrm{}\left(\mathrm{HCl}\right)\mathrm{}}^{+}$ ions appeared at a minimum potential of 13.3±1.5 volts, considered to be in agreement with the 13.8 volt value given by others. The ${\mathrm{H}}^{+}$ ions appeared at 16±1.5 volts, about the same voltage as is required for their appearance in pure molecular hydrogen.

The results all point to the conclusion that $W+\mathrm{HCl}\to {\left(\mathrm{HCl}\right)}^{+}+e$ is the process by which electron impacts ionize this gas, and that the process $W+\mathrm{HCl}\to {\mathrm{H}}^{+}+{\mathrm{Cl}}^{-}$ does not occur. A general discussion of the breakdown of molecules is given.