  The diffusion Monte Carlo technique is used to calculate and analyze the
excitation spectrum of a single 3He atom bound to a cluster with N 4He atoms,
with the aim of establishing the most adequate filling ordering of
single-fermion orbits to the mixed clusters with a large number of 3He atoms.
The resulting ordering looks like the rotational spectrum of a diatomic
molecule, being classified only by the angular momentum of the level, although
vibrational-like excitations appear at higher energies for sufficiently large
N.
