Change in the states of optically excited Rb atoms near sapphire surface

The changes in the states of excited Rb atoms approaching a single-crystal sapphire surface have been investigated by methods of laser-excitation spectroscopy and luminescence of Rb vapor in a cell with sapphire windows, the gap between which was varied from 250 to 500 nm. Upon resonant excitation of Rb atoms by two semiconductor lasers with powers of 20 and 40 mW, luminescence from optically excited 5D _3/2 and 5D _5/2 states and optically unexcited 6P _1/2 and 6P _3/2 states is observed. It is established that the luminescence intensity from unexcited states is only a few times lower than that from excited states, with allowance for the fact that excited atoms are rapidly and almost completely quenched on the sapphire surface. The found anomalously strong luminescence from optically unexcited 6P _J states is explained by their nonradiative occupation near the sapphire surface from optically excited 5P _J states, in which atoms fail to reach the sapphire surface because of the repulsion from it. This repulsion is due to the polarization interaction between sapphire and the atoms in the 5P _J states near the surface. Nonradiative transition from the 5P _J state to the 6P _J ^?1 state is accompanied by excitation of two optical phonons in sapphire.