Subthermal Mean Transverse Energies Induced by Electron Refraction on the Jump in Mass at the Surface of Multialkali Photocathodes

Abstract

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 Na$_{2}$KSb(Cs,Sb) photocathodes with negative effective electron affinity (NEA) have MTE values at room temperature by a factor of 2 lower than those of monocrystalline \textit{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 Na$_{2}$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\,meV at room temperature. We also showed that the transition from NEA to positive effective affinity results in the subthermal total MTE of the Na$_{2}$KSb(Cs,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.

Figures (3)

• Figure 1: (a) Electron refraction on the jump in mass at the ideal crystalline solid-vacuum interface and the reduction of the electron transverse energy $E_{\text{tr}}$ in vacuum due to the conservation of transverse component of electron momentum $\textit{p}_{\text{tr}} = \textit{p}_{\text{tr0}}$. (b) Energy-band diagram of a semiconductor photocathode with NEA ($\chi^{*} < 0$) and a three-step photoemission process: (1) photoexcitation, (2) transport to the emitting surface and (3) escape into vacuum. $E_{\text{c}}$ and $E_{\text{v}}$ are the conduction band bottom and the valence band top in the active layer, respectively, $E_{\text{g}}$ is the band gap, $E_{\text{F}}$ is the Fermi level, $E_{\text{vac}}$ is the vacuum level.
• Figure 2: (a) Longitudinal energy distribution curves of Na$_{2}$KSb(Cs,Sb) and p-GaAs(Cs,O) photocathodes measured at $T = 80$ K and $\hbar\omega = E_{\text{g}}$. Vacuum level $E_{\text{vac}}$ is marked with an arrow. (b) A schematic representation of a vacuum tube and apparatus for the measurements of both longitudinal and transverse electron distributions. $d$ is the photocathode-MCP gap, $U$ is the photocathode-MCP voltage, $J_{\text{ph}}$ is the photoemission current. The measured images of the electron beam emitted by the Na$_{2}$KSb(Cs,Sb) photocathode and the corresponding radial intensity distributions $I(r)$ are shown for two accelerating voltages. (c) Transverse energy distribution curves of Na$_{2}$KSb(Cs,Sb) and p-GaAs(Cs,O) photocathodes measured at $T = 295$ K and $\hbar\omega = E_{\text{g}}$. The thermal transverse distribution and refraction model distribution [Eq.(\ref{['Eq.1']})] for $m^{*} = 0.35 m_{\text{e}}$ are shown by dashed and dash-dotted lines, respectively. Inset: Schematic angular distributions of electrons emitted partly into a narrow cone due to the refraction on the jump in mass and partly into a wide solid angle due to diffuse scattering.
• Figure 3: (a) Transverse energy distribution curves of the Na$_{2}$KSb(Cs,Sb) photocathode measured at $T = 295$ K and $\hbar\omega = E_{\text{g}}$ before and after annealing. The thermal transverse distribution is shown by the dashed line. (b) Photoemission quantum efficiency spectra of the Na$_{2}$KSb(Cs,Sb) photocathode measured at $T = 295$ K before and after annealing. Band gap $E_{\text{g}}$ is marked with an arrow.