Photodissociation experiments are carried out for single HBr molecules absorbed on the surface
of large Arn clusters with the average size of n = 159 at the dissociation wavelength of 193 nm.
The kinetic energy of the H atom fragments is measured exhibiting peaks at zero and two energies
which mark completely caged and unperturbed fragments going into the two spin orbit channels of
Br, respectively. The results are compared with Molecular Dynamics simulations which account for
the quantum librational delocalization of the H atom in the HBr molecule and the substitutional
surface position of the molecule at 27 K. The latter result is obtained from a trajectory study of the
pick-up process under realistic conditions. Both channels are populated by mainly perpendicular
transitions in agreement with the results for the bare molecule. The high intensity close to the
energy of the excited Br. state mainly originates from slowed down dissociation products from the
ground state. Calculations based on these realistic initial state conditions give also good agreement
with the experimental results for the dissociation wavelength of 243 nm.