Dissociative recombination of $NH4+$ and $ND4+$ ions: Storage ring experiments and ab initio molecular dynamics

The dissociative recombination (DR) process of $NH4+$ and $ND4+$ molecular ions with free electrons has been studied at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). The absolute cross sections for DR of $NH4+$ and $ND4+$ in the collision energy range 0.001–1 eV are reported, and thermal rate coefficients for the temperature interval from 10 to 2000 K are calculated from the experimental data. The absolute cross section for $NH4+$ agrees well with earlier work and is about a factor of 2 larger than the cross section for $ND4+.$ The dissociative recombination of $NH4+$ is dominated by the product channels $NH3+H$ $(0.85±0.04)$ and $NH2+2H$ $(0.13±0.01),$ while the DR of $ND4+$ mainly results in $ND3+D$ $(0.94±0.03).$ Ab initio direct dynamics simulations, based on the assumption that the dissociation dynamics is governed by the neutral ground-state potential energy surface, suggest that the primary product formed in the DR process is $NH3+H.$ The ejection of the H atom is direct and leaves the $NH3$ molecule highly vibrationally excited. A fraction of the excited ammonia molecules may subsequently undergo secondary fragmentation forming $NH2+H.$ It is concluded that the model results are consistent with gross features of the experimental results, including the sensitivity of the branching ratio for the three-body channel $NH2+2H$ to isotopic exchange.