Light amplification in the ultraviolet region is confirmed in mixed rubidiu
m-cesium chloride crystals at room temperature. The probe laser light, whic
h falls on the 275-nm band of Auger-free luminescence (AFL) arising from ra
diative transition of the Cl 3p valence electrons into the Cs 5p core holes
, is enhanced in intensity when the deep-lying Rb 4p core electrons are pum
ped into the conduction band by undulator radiation from an electron storag
e ring. The obtained enhancement factor roughly corresponds to an amplifica
tion coefficient of 7 x 10(3) cm(-1), which is much higher than those of ty
pical solid-state lasers. It is emphasized that the amplification of AFL oc
curs in a surface layer as thin as about 20 nm, and that the inverted popul
ation between the valence and core bands is realized with any pump power. T
he present observation provides us with a new possibility of nanolaser fabr
ication.