Clusters of para-hydrogen and ortho-deuterium have been assembled around (p
H(2)) (oD(2)) an OCS chromophore molecule inside He droplets in a molecular
beam and studied via IR diode laser depletion spectroscopy (v approximate
to 2060 cm(-1)). The superfluid He-4 droplets provide a gentle host ensurin
g a constant low temperature of either T = 0.38 K for He-4 droplets or T =
0.15 K for both the pure He-3 and mixed He-4-He-3 droplets. The spectra sho
w well resolved rotational structure of the vibrational bands for each atta
ched hydrogen molecule in the range n = 1-8. With only one (n = 1) attached
pH(2), HD or oD(2) an molecule the best fit rotational constants were used
to determine the structure of the complex, which was found to be in surpri
singly good agreement with quantum chemical calculations for the free compl
ex. With n = 5 and 6 the Q-branch disappears for the pH(2) clusters but not
for the oD(2) clusters which is consistent with a donut model. The moments
of inertia of the pH(2) and the oD(2) complexes are explained by a new mod
el in which each of the 18 attached helium atoms in a shell surrounding the
OCS molecule are assigned a mass of 0.55, while each attached H-2 and D-2
molecule has an effective mass of about 10 and 12 u, respectively.