Destruction of very-high-$n$ Rydberg atoms in Rydberg-Rydberg collisions*

The destruction of very-high-$n$ strontium Rydberg atoms in Rydberg-Rydberg collisions is examined in an atomic beam. ~Rydberg blockade is exploited to produce a string of Rydberg atoms with approximately equal initial spatial separations but a distribution of velocities leading to subsequent collisions and Rydberg atom destruction through Penning ionization, which is monitored by measuring the time evolution of the high-$n$ population. ~The data are analyzed using a Monte Carlo collision code that models Rydberg atom production together with their subsequent motions and collisions. ~Comparisons between model predictions and experimental data point to large collisional loss cross sections, on the order of 10$^{\mathrm{-5}}$ cm$^{\mathrm{2}}$, that match those expected for hard-sphere collisions, i.e., $\sigma =$ 4$\pi $R$_{n}^{\mathrm{2}}$ where R$_{n}=$ 2$n^{\mathrm{2}}$ is the atomic radius.