Rempfer G F, Griffith O H
Department of Physics, Portland State University, OR 97207.
Ultramicroscopy. 1992 Nov;47(1-3):35-54. doi: 10.1016/0304-3991(92)90184-l.
A unified treatment of the resolution of three closely related techniques is presented: emission electron microscopy (particularly photoelectron microscopy, PEM), low energy electron microscopy (LEEM), and mirror electron microscopy (MEM). The resolution calculation is based on the intensity distribution in the image plane for an object of finite size rather than for a point source. The calculations take into account the spherical and chromatic aberrations of the accelerating field and of the objective lens. Intensity distributions for a range of energies in the electron beam are obtained by adding the single-energy distributions weighted according to the energy distribution function. The diffraction error is taken into account separately. A working resolution is calculated that includes the practical requirement for a finite exposure time, and hence a finite non-zero current in the image. The expressions for the aberration coefficients are the same in PEM and LEEM. The calculated aberrations in MEM are somewhat smaller than for PEM and LEEM. The resolution of PEM is calculated to be about 50 A, assuming conventional UV excitation sources, which provide current densities at the specimen of 5 x 10(-5) A/cm2 and emission energies ranging up to 0.5 eV. A resolution of about 70 A has been demonstrated experimentally. The emission current density at the specimen is higher in LEEM and MEM because an electron gun is used in place of a UV source. For a current density of 5 x 10(-4) A/cm2 and the same electron optical parameters as for PEM, the resolution is calculated to be 27 A for LEEM and 21 A for MEM.
发射电子显微镜（特别是光电子显微镜，PEM）、低能电子显微镜（LEEM）和镜像电子显微镜（MEM）。分辨率计算基于有限尺寸物体在像平面中的强度分布，而非点源的强度分布。计算过程考虑了加速场和物镜的球差和色差。通过将根据能量分布函数加权的单能量分布相加，可得到电子束中一系列能量的强度分布。衍射误差则单独考虑。计算出的工作分辨率考虑了有限曝光时间的实际要求，因此也考虑了像中有限的非零电流。PEM和LEEM中的像差系数表达式相同。计算得出MEM中的像差略小于PEM和LEEM中的像差。假设使用传统紫外激发源，其在样品处提供的电流密度为5×10⁻⁵ A/cm²，发射能量高达0.5 eV，则PEM的分辨率计算约为50 Å。实验已证明其分辨率约为70 Å。LEEM和MEM中样品处的发射电流密度更高，因为使用了电子枪而非紫外光源。对于5×10⁻⁴ A/cm²的电流密度以及与PEM相同的电子光学参数，计算得出LEEM的分辨率为27 Å，MEM的分辨率为21 Å。
