  Recently, kagome lattice metal AV$_3$Sb$_5$ (A = K, Rb, Cs) family has
received wide attention due to its presence of superconductivity, charge
density wave (CDW) and peculiar properties from topological nontrivial
electronic structure. With time-resolved pump-probe spectroscopy, we show that
the excited quasiparticle relaxation dynamics can be explained by formation of
energy gap below the phase transition being similar to a usual second-order CDW
condensate, by contrast, the structure change is predominantly first order
phase transition. Furthermore, no CDW amplitude mode is identified in the
ordered phase. The results suggest that the CDW order is very different from
the traditional CDW condensate. We also find that weak pump pulse can
non-thermally melt the CDW order and drive the sample into its high temperature
phase, revealing the fact that the difference in lattice potential between
those phases is small.
