Pressure Broadening of Gas Phase Molecular Ions at Very Low Temperature

A novel apparatus has been developed to produce large quantities of thermal molecular ions at temperatures well below the freezing points of their precursor neutrals. This apparatus has been used to study the pressure broadening of the $J=3\mathrm{}-2$ rotational transition of ${\mathrm{HCO}}^{+}$ in ${\mathrm{H}}_2$ gas at temperatures in the range 11–30 K.