  In this paper, we study a two-dimensional model for the growth of molecular
clusters in superfluid helium at low temperature. In the model, particles of
diameter a follow random ballistic moves of length \delta = a-256a. Upon
attachment on the cluster surface, particles allow one-step relaxation to the
nearest twofold coordinated site. Average coordination numbers of particles
show that in the presence of relaxation the screening for incoming particles
modifies the microscopic structure of the cluster. These results are in
qualitative agreement with experimental aggregation of methane in He droplets,
in which large abundance of fully coordinated sites is observed. The average
coordination number increases with \delta, showing that screening is relevant
when relaxation is present. As the cluster size increases, the corresponding
structure clusters changes from a compact to a fractal, typical of ballistic
and diffusion limited models, respectively. A scaling ansatz describing the
crossover between the two regimes is proposed.
