  The material emitted from the target surface during laser ablation generates
a net thrust (propulsion) in the opposite direction. The energetic efficiency
of this laser-driven propulsion is given by the mechanical coupling coefficient
(Cm). In this work we considered nanosecond UV laser ablation of the aluminum
6061 alloy to study the Cm behaviour with different irradiating conditions.
This is done by systematically changing: fluence, uniform/nonuniform intensity,
and incident angle of the laser beam. In particular we found that, when dealing
with nonuniform laser intensity, characterizing Cm exclusively in terms of
fluence is not fully satisfactory because the energy distribution over the
irradiated area plays a key role in the way material is removed -- interplay
between vaporization and phase-explosion -- and thrust is generated.
