N$_2$H$^+$ recombination with electrons in low temperature afterglow plasma*

A study on N$_2$H$^+$ recombination with electrons in afterglow plasma is presented. The cavity ring-down absorption spectroscopy in continuous wave modification was used as a principal diagnostics tool to probe number densities of several rotational states of the ground and the lowest excited vibrational state of the N$_2$H$^+$ ion. The recombination rate coefficients were measured at 300 K in afterglow of a microwave discharge ignited in a mixture of He/H$_2$/N$_2$ and of H$_2$/N$_2$ with typical number densities on the order of 10$^{17}$/10$^{14}$/10$^{14}$ cm$^{-3}$ and 10$^{17}$/10$^{14}$ cm$^{-3}$, respectively. The dependencies of the measured recombination rate coefficient on number densities of He and H$_2$ were evaluated to obtain upper limit for three body recombination rate coefficient of N$_2$H$^+$.