Reactions of Interstellar Ions O$^+$ and N$_2$H$^+$ with Electrons and Molecules*

We present the experimental results of study of reaction rate coefficients for selected reactions of atomic (O$^+$) or molecular (N$_2$H$^+$) ions with hydrogen isotopologues or electrons, respectively. The recombination of N$_2$H$^+$ ions with electrons was studied in the temperature range of 80 -- 350 K using cryogenic stationary afterglow apparatus equipped with cavity ring-down spectrometer. The thermal recombination rate coefficients were evaluated from the time evolutions of number densities of selected rovibrational states of N$_2$H$^+$. The cryogenic 22-pole radiofrequency ion trap was utilized to study the reactions of the ground electronic state of O$^+$ ions with HD in the temperature range of 15 -- 200 K and with D$_2$ in the temperature range of 15 -- 300 K. For the reaction of O$^+$ ions with HD, the product branching ratios for the production of OH$^+$ and OD$^+$ ions were obtained. A careful attention was given to ensure that the obtained reaction rate coefficients pertained to the ground electronic state of O$^+$ ion.