## 1 Introduction  
Secondary electron emission (SEE) refers to the physical phenomenon that some electrons escape from a material after the particle bombardment. SEE phenomenon has been utilized in various sophisticated scientific instruments and electronic devices, such as scanning electron microscope (SEM) [1], photomultiplier tube (PMT) [2; 3], microchannel plate (MCP) [4; 5; 6], etc. In addition, SEE can also induce adverse effects in some typical working occasions. For instance, SEE is one of the inducements of multipactor in space of a high-power microwave system [7; 8]. In particle accelerators, SEE may lead to an e-cloud effect, which could further influence the energy and direction stabilities of the passing proton beams [9; 10]. Therefore, in spacecrafts and accelerators, SEE phenomenon usually needs to be suppressed to lower the risk of detrimental effects [11; 12]. SEE characteristics of various materials are of significant distinctions, and for different working occasions, the suitable materials ought to possess a certain SEE level to match the working environments [13; 14; 15]. SEE is widely applied in the modern electronic industry to accomplish diverse functions for various electron devices. For instance, the coating material possessing a high SEE ability can be used in ultra-fast detectors to improve its time resolution and spatial resolution [16; 17]. In addition, the coatings with high secondary electron yield (SEY, also denoted as \(\delta\)) can also be used for the pattern recognition in particle physics experiments to enhance the performance of time resolution and track separation [18]. Furthermore, for transmission dynodes (tynodes) device, SEE also play a significant role on its multiplying efficiency [19; 20]. To be specific, for the electron multiplier devices, such as typodes, PMT, and MCP, the inner dynodes ought to possess a high SEY level so as to improve the electron gain and lower the noise, which makes considerable sense for various detectors employing multipliers as amplifier unit [21]. For a variety of multipliers, electron multiplication efficiency of the device can be greatly improved by depositing high SEY coatings on the dynode surface. Magnesium oxide (MgO) is an excellent candidate for dynode coating material since it possesses relatively high SEY. As early as in 1973, [PERSON] et al. acquired the MgO coating of 300 nm thickness via sputtering technology, measurements showed that the SEY peak value (also denoted as \(\delta_{\rm m}\)) of the MgO coating equaled to 8 [22]. [PERSON] et al. studied the SEE behavior of MgO single crystals with different crystal orientations under Ne ion bombardment in 1999, and found that MgO single crystals with crystal orientations <111> had the highest electron emission efficiency under ion bombardment [23]. Although SEY of MgO in various published literatures are different, it is commonly higher than the SEY of many other materials. Based on the related studies, \(\delta_{\rm m}\) of MgO reported in various literatures is relatively broad, ranging from 5 to 12 [19; 22; 24; 25; 26]. The high SEY property of MgO enables itself to be applied in many devices. For example, in 1981, [PERSON] et al. carried out the study of coating MgO film on the inner surface of a channel electron multiplier to improve the electron multiplier efficiency and applied the multiplier in the detector. The study showed that the MgO coating increased the efficiency of the detector by more than 3 times [27]. In 2002, [PERSON] et al. studied the influence of MgO coating on the surface of emission cathode on the opening voltage of plasma display, and the test results showed that the high electron emission efficiency of MgO film reduced the opening voltage of display by more than 50% [28]. In 2004, [PERSON] et al. used MgO to replace the aluminum electrode in the capacitive coupling RF plasma source, which improved the breakdown voltage generated on the electrode and increased the plasma density generated at high RF voltage by tens of times [29].