Collisional angular-momentum mixing of Rydberg states of Na by He, Ne, and Ar

The collisional angular-momentum mixing of $n=5-15\mathrm{}$ Na $d$ states with the $l>\mathrm{}2\mathrm{}$ states of the same $n$ has been studied using He, Ne, and Ar as collision partners. At rare-gas pressures of ∼1 Torr, the $l\ge 2$ states are thoroughly mixed and the mixture decays at the average radiative lifetime of all the $l\ge 2$ states of the same $n$. Using ∼1 Torr of neon to collisionally mix the $l\ge 2$ states, the average lifetime for $l\ge 2$ was measured for each $n$ and compared to the theoretical prediction for hydrogen, with which it is in excellent agreement. The cross section for $l$ mixing by each of the gases is found to increase sharply from $n=5\mathrm{} \mathrm{to} 10$ and decrease above $n=10\mathrm{}$. The magnitudes of the $l$-mixing cross sections are found to correlate with the low-energy scattering lengths of the collision partners.