  We demonstrate that a sodium dimer, Na$_{2}$($1^3\Sigma_{u}^+$), residing on
the surface of a helium nanodroplet, can be set into rotation by a nonresonant
1.0 ps infrared laser pulse. The time-dependent degree of alignment measured,
exhibits a periodic, gradually decreasing structure that deviates qualitatively
from that expected for gas phase dimers. Comparison to alignment dynamics
calculated from the time-dependent rotational Schr\"{o}dinger equation shows
that the deviation is due to the alignment dependent interaction between the
dimer and the droplet surface. This interaction confines the dimer to the
tangential plane of the droplet surface at the point where it resides and is
the reason that the observed alignment dynamics is also well-described by a 2D
quantum rotor model.
