  We performed the experimental and theoretical study of the heating and
damaging of ruthenium thin films induced by femtosecond laser irradiation.
Results of an optical pump-probe thermoreflectance experiment with rotating
sample allowing to significantly reduce heat accumulation in irradiated spot
are presented. We show the evolution of surface morphology from growth of a
heat-induced oxide layer at low and intermediate laser fluences to cracking and
grooving at high fluences. Theoretical analysis of pump-probe signal allows us
to relate behavior of hot electrons in ruthenium to the Fermi smearing
mechanism. The analysis of heating is performed with the two-temperature
modeling and molecular dynamics simulation, results of which demonstrate that
the calculated melting threshold is higher than experimental damage threshold.
We attribute it to heat-induced surface stresses leading to cracking which
accumulates to more severe damage morphology. Our results provide an upper
limit for operational conditions for ruthenium optics and also direct to
further studies of the Fermi smearing mechanism in other transition metals.
