Elettra | BEAR, BACH Experimental photon e- yield from Rashid lunar rover at BABE-AHEAD: input for space charge modeling The Emirates Lunar Mission (ELM), led by the Mohammed Bin Rashid Space Centre (MBRSC) in Dubai, marked the United Arabs Emirates (UAE) first contribution to the Moon exploration. The mission was based on Rashid, a compact lunar rover equipped with a suite of instruments specifically designed to collect data at the Moon’s surface and in its environment exploring during the 14day lunar daytime of April 2023 the Atlas north east Moon’s Crater. The international institutions involved in the mission included the European Space Agency (ESA), and the Norwegian industrial and public institutions: Eidel, the Physics Department of Oslo University, the Department of Computational Materials Processing at the Institute for Energy Technology of Kjeller, and SINTEF Industry (Oslo). ESA, through its Exploration Preparation, Research and Technology (ExPeRT), addressed abrasion, discoloration, and dust sticking issues of the magnesium alloy wheels. Eidel was responsible for the 4 Langmuir probes assembly (4-LNG, Fig. 1 a), aiming at providing the first extensive, high-resolution in situ measurements of the bulk parameters of plasma sheath forming at lunar surface under solar and cosmic radiation at different altitudes. The Physics Department of Oslo University, the Institute for Energy Technology, and SINTEF contributed to the mission goal of modeling the plasma sheath using as input the experimental values from the vertically arranged 4-LNG. The Rashid overall appearance is shown in Fig. 1 (a), on board instrumentation in the caption. The vehicle surfaces consist of different materials (Fig. 1 a and caption). Solar irradiance (Fig. 1 b) induces a charge density generated from the Moon’s surface, to which the emission from the rover surface adds, introducing a systematic measuring error that must be corrected when using 4-LNG signals as input for the sheath numerical simulation. To this end an off-site laboratory campaign of Photon Energy Resolved Electron Yield, Y(ℏω,…), defined as number of electrons emitted per photon, was carried on. Data were collected at the AHEAD2020 project BABE facility of IOM-CNR using radiation from the Elettra storage ring (Trieste, Italy) in the visible to 30 eV incident photon energy range, corresponding to the dominant portion of the solar irradiance spectrum (see Fig. 1 b). Y(ℏω,…) is eminently a surface process depending on material parameters together with surface properties. To get an idea of the mechanisms producing the electron emission under photon irradiation the following essential scheme is proposed. After photon absorption and once the photo-electron is created, it spherically diffuses, and, following a free-collisionpath, undergoes scattering events with the electron charge. Neglecting energy gains and radiative de-excitation for simplicity, after scattering, the electron emerges with reduced energy and altered momentum, accompanied by a created quasi-particle – typically a phonon/atomic vibration or a plasmon – stored in their respective reservoirs. As a result, the energy and momentum of the electromagnetic field are distributed according to the relative cross sections along these excitation paths. The scattered electron keeps on propagating, thus triggering further scattering events. Meanwhile, the system tends to get rid of these excitations. Among them, vibrations cannot excite electrons above the vacuum level, and thus do not cause electron emission, but just transfer the excitation to the "thermal bath". On the other side, plasmons – whose energy ranges from a few to a couple of tens of eV – can couple with valence band (VB) electrons, transferring energy and momentum to generate e-h pairs in the VB, some of which involve electrons with energy and momentum suitable to cross the solid-vacuum barrier. In other words, a dynamically excited region forms near the surface, constituted by plasmons that may interact with the VB, generating a Low Electron Energy Charge Density (LEECD) that acts as a source of secondary electrons into vacuo. Although with different arrival times and at a lower rate, an additional contribution to the LEECD – and the resulting emission – comes from the tail of the inelastically scattered electrons, the socalled electron cascade, kinematically linked to the primary photo-electron and adding up to this distribution. In addition, the integrated current contribution from the highest-kinetic-energy electrons— shown in Fig. 1 (c)—must be added (these are the primary photoelectrons exploited in Ultraviolet Photoelectron Spectroscopy (UPS) that are emitted without undergoing scattering). This electron distribution has a kinetic energy cutoff at 0, peaking close to zero as dramatized in Fig. 1 (c). The emitted electrons, integrated over energy and momentum, reach by ballistic transport the chamber wall electrically connected to sample through an ammeter, generating a current – known as the drain current – as illustrated in the inset of Fig. 1 (d). The current measured by the ammeter can be directly converted into the number of emitted electrons per time unit. As the incident photon rate is also measured, the ratio of these two quantities provides the photon electron yield Yω(ℏω,…). Yω(ℏω,…) data for the Rashid’s materials, listed in the inset, are shown in Fig. 1 (d). Unfortunately, the contact with the private Japanese-made lunar lander Hakuto-R was lost on 25 April 2023, just seconds before its attempted Moon landing. The mission team confirmed that the spacecraft had crashed. However, the experience gained was not lost, just a day later, on 26 April 2023, a new mission featuring the Rashid 2 rover was announced. Elettra Highlights 2024–2025 138
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