  The physical mechanisms of the laser-induced periodic surface structures
(LIPSS) formation are studied in this paper for single-pulse irradiation
regimes. The change in the LIPSS period with wavelength of incident laser
radiation is investigated experimentally, using a picosecond laser system,
which provides 7-ps pulses in near-IR, visible, and UV spectral ranges. The
experimental results are compared with predictions made under the assumption
that the surface-scattered waves are involved in the LIPSS formation.
Considerable disagreement suggests that hydrodynamic mechanisms can be
responsible for the observed pattern periodicity.
