are not transparent to low-energy electrons; In order to separate the incident and elastically scattered low energy electrons, scientists use magnetic “electron prism” beam separators which focus electrons both in and out of the plane of the beampath (to avoid distortions in the image and diffraction patterns); In electrostatic immersion objective lens brings the sample close to that of the gun, slowing down the high energy electrons to a desired energy only just before interacting with the sample surface; The instrument must be able to work under ultra-high vacuum (UHV), or 10−10 torr (760 torr = 1 atm, atmospheric pressure), although "near-ambient pressure" (NAP-LEEM) instruments have been developed by adding a higher pressure compartment and differential pumping stages, allowing for sample room pressures up to 10−1 mbar. Surface diffraction Kinematic or elastic backscattering occurs when low energy (1-100 eV) electrons impinge on a clean, well-ordered crystalline specimen. It is assumed that each electron undergoes only one scattering event, and incident electron beam is described as a plane wave with the wavelength: Inverse space is used to describe the periodicity of the lattice and the interaction of the plane wave with the sample surface. In inverse (or "k-space") space, the wave vector of the incident and scattered waves are and , respectively, and constructive interference occurs at the Laue condition: where (h,k,l) is a set of integers and is a vector of the reciprocal lattice. Experimental setup A typical LEEM setup consists of electron gun, used to generate electrons by way of thermionic or field emission from a source tip. In thermionic emission, electrons escape a source tip (usually made of LaB6) by resistive heating and application of an electric field to effectively lower the energy needed for electrons to escape the surface. Once sufficient thermal vibrational energy is attained electrons may overcome this electrostatic energy barrier, allowing them to travel into vacuum and accelerate down the lens column to the gun potential (because the lenses are at ground). In field emission, rather than heating the tip to vibrationally excite electrons from the surface, the source tip (usually tungsten) is sharpened to a small point such that when large electric fields are applied, they concentrate at the tip, lowering the barrier to escape the surface as well as making tunneling of electrons from the tip to vacuum level more feasible. Condenser/illumination optics are used to focus electrons leaving the electron gun