Production of very-high-$n$ strontium Rydberg atoms*

The production of very-high-$n$, $n \sim $ 300, strontium Rydberg atoms is explored using a crossed laser-atom beam geometry. $n^{1}$S$_{0}$ and $n^{1}$D$_{2}$ states are created by two-photon excitation via the 5s5p $^{1}$P$_{1}$ intermediate state using radiation at 461 and 413 nm. Rydberg series in the different strontium isotopes present in the beam can be selectively excited by tuning the 461 nm light allowing measurements of the isotope shifts in the series limits, that for $^{88}$Sr-$^{86}$Sr being $\sim +$210MHz, and that for $^{88}$Sr-$^{84}$Sr being $\sim +$440 MHz. Well-resolved Rydberg series can be seen for values of $n$ approaching $\sim $ 500. Photoexcitation spectra calculated using a two-active-electron model reproduce the relative strengths of the measured $^{1}$S$_{0}$ and $^{1}$D$_{2}$ lines. The surprisingly large photoexcitation rates provide new opportunities to study quasi-stable two-electron excited states and to explore the behavior of strongly coupled Rydberg systems at high $n$.