Opposite effects of the rotational and translational energy on the rates of ion-molecule reactions near 0 K: the D_2^++NH_3 and D_2^++ND_3 reactions

The ion-molecule reactions D_2^++NH_3 and D_2^++ND_3 are studied at low collision energies (E_coll from zero to ∼ k_B· 50 K), with the D_2^+ ions in the ground rovibrational state and for different rotational temperatures of the ammonia molecules, using the Rydberg-Stark merged-beam approach. Two different rotational temperatures (∼ 15 K and ∼ 40 K), measured by (2+1) resonance-enhanced multiphoton-ionization spectroscopy, are obtained by using a seeded supersonic expansion in He and a pure ammonia expansion, respectively. The experimental data reveal a strong enhancement of the rate coefficients at the lowest collision energies caused by the charge-dipole interaction. Calculations based on a rotationally adiabatic capture model accurately reproduce the observed kinetic-energy dependence of the rate coefficients. The rate coefficients increase with increasing rotational temperature of the ammonia molecules, which contradicts the expectation that rotational excitation should average the dipoles out. Moreover, these reactions exhibit a pronounced inverse kinetic isotope effect. The difference is caused by nuclear-spin-statistical factors, and the smaller rotational constants and tunneling splittings in ND_3.