Collisional destruction of four-wave-mixing signals in sodium in the presence of low-pressure buffer gas

Abstract

We present experimental data showing destruction of degenerate four-wave-mixing signals by collisions at extremely low pressures of added buffer gases (\ensuremath{\sim}100 mtorr). These experiments are performed on the $3s^{2}S_{\frac{1}{2}}(F=2)\ensuremath{-}3p^{2}P_{\frac{3}{2}}(F=3)$ transition of the ${D}_{2}$ line in atomic sodium for which, in the absence of buffer gases, the lower state is not optically pumped to the $3s^{2}S_{\frac{1}{2}}(F=1)$ ground state. However, using a simple rate equation model we are able to qualitatively explain the observed behavior by showing that state-changing collisions have the effect of generating a collision-induced optical pumping. This transfers the $F=2$ ground-state population to the $F=1$ ground state, thus reducing the density of atoms available for generating a signal.