We investigated excess
electron solvation dynamics in (NH
3
)
n
–
ammonia
clusters in the
n
= 8–32
size range by performing finite temperature molecular dynamics simulations.
In particular, we focused on three possible scenarios. The first case
is designed to model electron attachment to small neutral ammonia
clusters (
n
≤ ∼10) that form hydrogen-bonded
chains. The excess electron is bound to the clusters via dipole bound
states, and persists with a VDE of ∼160 meV at 100 K for the
n
= 8 cluster. The coupled nuclear and electronic relaxation
is fast (within ∼100 fs) and takes place predominantly by intermolecular
librational motions and the intramolecular umbrella mode. The second
scenario illustrates the mechanism of excess electron attachment to
cold compact neutral clusters of medium size (8 ≤
n
≤ 32). The neutral clusters show increasing tendency with
size to bind the excess electron on the surface of the clusters in
weakly bound, diffuse, and highly delocalized states. Anionic relaxation
trajectories launched from these initial states provide no indication
for excess electron stabilization for sizes
n
<
24. Excess electrons are likely to autodetach from these clusters.
The two largest investigated clusters (
n
= 24 and
32) can accommodate the excess electron in electronic states that
are mainly localized on the surface, but they may be partly embedded
in the cluster. In the last 500 fs of the simulated trajectories,
the VDE of the solvated electron fluctuates around ∼200 meV
for
n
= 24 and ∼500 meV for
n
= 32, consistent with the values extrapolated from the experimentally
observed linear VDE–
n
–1/3
trend. In the third case, we prepared neutral ammonia cluster configurations,
including an
n
= 48 cluster, that contain possible
electron localization sites within the interior of the cluster. Excess
electrons added to these clusters localize in cavities with high VDEs
up to 1.9 eV for
n
= 48. The computed VDE values
for larger clusters are considerably higher than the experimentally
observed photoelectric threshold energy for the solvated electron
in bulk ammonia (∼1.4 eV). Molecular dynamics simulations launched
from these initial cavity states strongly indicate, however, that
these cavity structures exist only for ∼200 fs. During the
relaxation the electron leaves the cavity and becomes delocalized,
while the cluster loses its initial compactness.