  We report on in situ low-temperature (4 K) scanning tunneling microscope
measurements of atomic and electronic structures of the cleaved surfaces of an
alkali-based kagome metal RbV$_3$Sb$_5$ single crystals. We find that the
dominant pristine surface exhibits Rb-1x1 structure, in which a unique
unidirectional $\sqrt{3}a_0$ charge order is discovered. As the sample
temperature slightly rises, Rb-$\sqrt{3}$x1 and Rb-$\sqrt{3}$x$\sqrt{3}$
reconstructions form due to desorption of surface Rb atoms. Our conductance
mapping results demonstrate that Rb desorption not only gives rise to hole
doping, but also renormalizes the electronic band structures. Surprisingly, we
find a ubiquitous gap opening near the Fermi level in tunneling spectra on all
the surfaces despite their large differences of hole-carrier concentration,
indicating an orbital-selective band reconstruction in RbV$_3$Sb$_5$.
