Foreign-gas-induced relaxation of RydbergSandDstates in atomic sodium

We have measured the He-, Ne-, and Ar-induced relaxation properties of the $3S-nS$ ($3S-nD$) superposition states of atomic sodium, where $n$ ranges from 5 to 20 (4 to 34). The measurements were made using the trilevel echo effect. We find that the effective collisional cross section first increases (as a function of $n$) at low $n$, and then decreases to a lower asymptotic limit at high $n$. This behavior is similar to that previously observed for the $3S-nP$ series. We also find that the difference between the collisional cross sections for $S$ and $D$ series superpositions closely approximates the inelastic collision cross sections measured elsewhere for the $d$ states. A theoretical description of the trilevel echo effect used to make the measurements is presented.