The electron microscope relies on electron beam imaging. However, the electron beam is highly prone to colliding and scattering with molecules in the air, resulting in blurred or even failed imaging. Therefore, a high vacuum environment is the core prerequisite for the performance of the electron microscope, and vacuum diffusion pump oil is the key material to achieve this environment. 
The vacuum diffusion pump oil builds a high vacuum through the "evaporation - ejection - condensation" cycle process. Under the action of the diffusion pump heating device, the pump oil is heated and evaporates to form oil vapor. When it is ejected through the nozzle at high speed, it forms a directional airflow. This airflow will carry impurities such as air and water vapor in the vacuum chamber to the cooling area on the pump wall. The oil vapor will condense upon cooling and flow back to the heating zone for reuse, while the impurities are removed by the preceding vacuum pump. Eventually, the vacuum degree inside the electron microscope tube reaches an ultra-high vacuum standard of 10⁻⁴ to 10⁻⁸ pascals. 
In addition to creating a vacuum, the performance of the pump oil directly affects the stability of the electron microscope's operation. High-quality diffusion pump oil has an extremely low saturated vapor pressure, which can prevent its own evaporation from polluting the tube and samples; its good thermal stability and chemical inertness can withstand long-term high-temperature operation without decomposition, reducing the damage caused by oil deposits to the pump body and components of the electron microscope. If the performance of the pump oil is poor, not only will it be difficult to maintain a high vacuum, but the evaporated oil vapor may also adhere to the sample surface or the electronic optical system, resulting in a decrease in image signal-to-noise ratio, resolution reduction, and even damage to precision electronic components. 
It can be said that although vacuum diffusion pump oil does not directly participate in imaging, it is the "invisible cornerstone" for the electron microscope to obtain clear microscopic images and ensure the reliable operation of the equipment.