The search for photocathode materials with low mean transverse energies (MTEs) and, hence, low intrinsic emittance is of crucial importance for various fields of particle and solid-state physics. Here, we demonstrate that polycrystalline multialkali ${\mathrm{Na}}_2\mathrm{KSb}(\mathrm{Cs},\mathrm{Sb})$ photocathodes with negative effective electron affinity (NEA) have MTE values at room temperature by a factor of 2 lower than those of monocrystalline $p$-GaAs(Cs,O) photocathodes. These low MTE values are due to the electron refraction on the jump in mass between a small effective mass in ${\mathrm{Na}}_2\mathrm{KSb}$ and free electron mass in vacuum. It is proved that, at the NEA state, up to half of photoelectrons are emitted in a narrow-angle cone with the fractional MTE of $(9\pm 1)\mathrm{meV}$ at room temperature. We also showed that the transition from NEA to positive effective affinity results in the subthermal total MTE of the ${\mathrm{Na}}_2\mathrm{KSb}(\mathrm{Cs},\mathrm{Sb})$ photocathode, along with quantum efficiency of about ${10}^{-2}$. The physical reasons for the manifestation of the refraction effect in multialkali photocathodes are discussed, opening up opportunities for the development of high-brightness and ultracold robust electron sources.

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• Electron emission
• Surface & interfacial phenomena
• Electron sources
• Interfaces
• Layered semiconductors
• Photoemission spectroscopy