  The Landau--Zener (LZ) type classical-trajectory surface-hopping algorithm is
applied to the nonadiabatic nuclear dynamics of the ammonia cation after
photoionization of the ground-state neutral molecule to the excited states of
the cation. The algorithm employs the recently proposed formula for
nonadiabatic LZ transition probabilities derived from the adiabatic potential
energy surfaces. The evolution of the populations of the ground state and the
two lowest excited adiabatic states is calculated up to 200 fs. The results
agree well with quantum simulations available for the first 100 fs based on the
same potential energy surfaces. Four different time scales are detected for the
nuclear dynamics: Ultrafast Jahn--Teller dynamics between the excited states on
a 5 fs time scale; fast transitions between the excited state and the ground
state within a time scale of 20 fs; relatively slow partial conversion of a
first-excited-state population to the ground state within a time scale of 100
fs; and nearly constant populations after roughly 120 fs due to a dynamical
equilibrium between all three states. The latter provides a possible
explanation of the experimental evidence that ammonia cation is nonfluorescent.
