Collisional Radiative Recombination Ar$^{+}$ + e + e at low temperatures from 77K to 180 K

The extremely strong (T $^{-4.5})$ temperature variation of collisional radiative recombination (CRR) makes it difficult to extend experimental tests of the frequently used ``Stevefelt formula'' to low temperatures, since the electron temperature must be precisely known and CRR adds heat to the electron gas. Using a newly developed low-temperature helium-buffered flowing afterglow (Cryo-FALP), we have determined CRR rate coefficients for of Ar$^{+}$ ions with electrons at electron temperatures from 77 K to 180 K at electron densities n$_{e}$ from~10$^{8}$ to 10$^{10}$~cm$^{-3}$. The measured ternary rate coefficient $K_{CRR}$~at 77 K of (1.0 $\pm $~0.4) $\times $~10$^{-17}$ cm$^{6}$s$^{-1}$ and the observed variation with electron temperature $T_{e}$ agree well with the $T_{e}^{-4.5}$ dependence predicted by the Stevefelt formula