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Galaxies in the reionization-era have been shown to have prominent[OIII]+H$\beta$ emission. Little is known about the gas conditions andradiation field of this population, making it challenging to interpret thespectra emerging at $z\gtrsim6$. Motivated by this shortcoming, we haveinitiated a large MMT spectroscopic survey identifying rest-frame opticalemission lines in 227 intense [OIII] emitting galaxies at $1.3<z<2.4$. Thissample complements the MOSDEF and KBSS surveys, extending to much lower stellarmasses ($10^7-10^8 M_\odot$) and larger specific star formation rates ($5-300$Gyr$^{-1}$), providing a window on galaxies directly following a burst orrecent upturn in star formation. The hydrogen ionizing production efficiency($\xi_{\rm{ion}}$) is found to increase with the [OIII] EW, in a manner similarto that found in local galaxies by Chevallard et al. (2018). We describe howthis relationship helps explain the anomalous success rate in identifyingLy$\alpha$ emission in $z\gtrsim7$ galaxies with strong [OIII]+H$\beta$emission. We probe the impact of the intense radiation field on the ISM usingO32 and Ne3O2, two ionization-sensitive indices. Both are found to scale withthe [OIII] EW, revealing extreme ionization conditions not commonly seen inolder and more massive galaxies. In the most intense line emitters, the indiceshave very large average values (O32 $=9.1$, Ne3O2 $=0.5$) that have been shownto be linked to ionizing photon escape. We discuss implications for the natureof galaxies most likely to have O32 values associated with significant LyCescape. Finally we consider the optimal strategy for JWST spectroscopicinvestigations of galaxies at $z\gtrsim10$ where the strongest rest-frameoptical lines are no longer visible with NIRSpec.

MMT/MMIRS spectroscopy of $z=1.3-2.4$ extreme [OIII] emitters: Implications for galaxies in the reionization-era