  Two isotopic chemical reactions, $\mathrm{Ne}^*$ + NH$_3$, and
$\mathrm{Ne}^*$ + ND$_3$, have been studied at low collision energies by means
of a merged beams technique. Partial cross sections have been recorded for the
two reactive channels, namely $\mathrm{Ne}^*$ + NH$_3$ $\rightarrow$ Ne +
NH$_3^+$ + $e^-$, and $\mathrm{Ne}^*$ + NH$_3$ $\rightarrow$ Ne + NH$_2^+$ + H
+ $e^-$, by detecting the NH$_3^+$ and NH$_2^+$ product ions, respectively. The
cross sections for both reactions were found to increase with decreasing
collision energy, $E_{coll}$, in the range 8 $\mu$eV$<E_{coll}<$ 20 meV. The
measured rate constant exhibits a curvature in a log(k)-log($E_{coll}$) plot
from which it is concluded that the Langevin capture model does not properly
describe the $\mathrm{Ne}^*$ + NH$_3$ reaction in the entire range of collision
energies covered here. Calculations based on multichannel quantum defect theory
were performed to reproduce and interpret the experimental results. Good
agreement was obtained by including long range van der Waals interactions
combined with a 6-12 Lennard-Jones potential. The branching ratio between the
two reactive channels, $\Gamma = \frac{[NH_2^+]}{[NH_2^+]+[NH_3^+]}$, is
relatively constant, $\Gamma\approx 0.3$, in the entire collision energy range
studied here. Possible reasons for this observation are discussed and
rationalised in terms of relative time scales of the reactant approach and the
molecular rotation. Isotopic differences between the $\mathrm{Ne}^*$ + NH$_3$
and $\mathrm{Ne}^*$ + ND$_3$ reactions are small, as suggested by nearly equal
branching ratios and cross sections for the two reactions.
