  Molecular dynamics computer simulations of collective excitations in a system
of hard disks confined to a narrow channel of the specific width, that
resembles 2D triangular lattice at disk close packing, are performed. We found
that transverse excitations, which for hard-disk system are absent in the limit
of 1D and are of acoustic nature in the limit of 2D, in the case of q1D
hard-disk system emerge in the form of transverse optical excitations and could
be considered as a tool to detect the structural transition to a zigzag
ordering. By analyzing density evolution of longitudinal static structure
factor and pair distribution function we have shown that driving force of
zigzag ordering is caging phenomenon that in the case of hard-disk system is
governed by excluded volume interaction with first and second neighbors and is
of entropic origin.
