Studies of high-$n$, n$^{\mathrm{1}}$G$_{\mathrm{4}}$ and n$^{\mathrm{1}}$H$_{\mathrm{5}}$ strontium Rydberg states using microwave-optical multiphoton excitation*

We demonstrate that combined microwave/optical four- and five-photon excitation can be used to generate sizable numbers of high-$n$, 120$\le n\le $160, strontium n$^{\mathrm{1}}$G$_{\mathrm{4}}$ and n$^{\mathrm{1}}$H$_{\mathrm{5}}$ Rydberg states, respectively, for use in studies of novel ultralong-range Rydberg molecules, of autoionization, and of planetary atoms. The quantum defects for both these states (and the n$^{\mathrm{1}}$I$_{\mathrm{6}})_{\mathrm{\thinspace }}$were measured using microwave spectroscopy and are in good agreement with theoretical predictions based on a two-active-electron model.