Measured temperature dependences of the reaction rate coefficients and of the reactions (3a) and (3b), respectively. The data were binned and averaged for presentation. The raw data without binning are indicated with the smaller points. The stars (at 20 K) indicate the reaction rate coefficients obtained from the dependences of and on [H₂] (Fig. 4). The horizontal solid line (k_L) indicates the value of the Langevin collisional rate coefficient. Results of Kim et al. (1975), Adams et al. (1980), and Gerlich (1993) are indicated with the open symbols. The values taken from the KIDA (Wakelam et al. 2012) and UMIST (McElroy et al. 2013) databases are also plotted.