Schönhense G, Kutnyakhov D, Pressacco F, Heber M, Wind N, Agustsson S Y, Babenkov S, Vasilyev D, Fedchenko O, Chernov S, Rettig L, Schönhense B, Wenthaus L, Brenner G, Dziarzhytski S, Palutke S, Mahatha S K, Schirmel N, Redlin H, Manschwetus B, Hartl I, Matveyev Yu, Gloskovskii A, Schlueter C, Shokeen V, Duerr H, Allison T K, Beye M, Rossnagel K, Elmers H J, Medjanik K
Johannes Gutenberg-Universität, Institut für Physik, D-55099 Mainz, Germany.
Deutsches Elektronen-Synchrotron DESY, D-22607 Hamburg, Germany.
Rev Sci Instrum. 2021 May 1;92(5):053703. doi: 10.1063/5.0046567.
The performance of time-resolved photoemission experiments at fs-pulsed photon sources is ultimately limited by the e-e Coulomb interaction, downgrading energy and momentum resolution. Here, we present an approach to effectively suppress space-charge artifacts in momentum microscopes and photoemission microscopes. A retarding electrostatic field generated by a special objective lens repels slow electrons, retaining the k-image of the fast photoelectrons. The suppression of space-charge effects scales with the ratio of the photoelectron velocities of fast and slow electrons. Fields in the range from -20 to -1100 V/mm for E = 100 eV to 4 keV direct secondaries and pump-induced slow electrons back to the sample surface. Ray tracing simulations reveal that this happens within the first 40 to 3 μm above the sample surface for E = 100 eV to 4 keV. An optimized front-lens design allows switching between the conventional accelerating and the new retarding mode. Time-resolved experiments at E = 107 eV using fs extreme ultraviolet probe pulses from the free-electron laser FLASH reveal that the width of the Fermi edge increases by just 30 meV at an incident pump fluence of 22 mJ/cm (retarding field -21 V/mm). For an accelerating field of +2 kV/mm and a pump fluence of only 5 mJ/cm, it increases by 0.5 eV (pump wavelength 1030 nm). At the given conditions, the suppression mode permits increasing the slow-electron yield by three to four orders of magnitude. The feasibility of the method at high energies is demonstrated without a pump beam at E = 3830 eV using hard x rays from the storage ring PETRA III. The approach opens up a previously inaccessible regime of pump fluences for photoemission experiments.
在飞秒脉冲光子源上进行时间分辨光电子发射实验的性能最终受到电子-电子库仑相互作用的限制，这会降低能量和动量分辨率。在此，我们提出一种有效抑制动量显微镜和光电子发射显微镜中空间电荷伪像的方法。由特殊物镜产生的减速静电场排斥慢速电子，保留快速光电子的k图像。空间电荷效应的抑制程度与快速和慢速光电子的速度比成正比。对于能量为100 eV至4 keV的直接二次电子和泵浦诱导的慢速电子，场强范围为-20至-1100 V/mm可将它们重新引导回样品表面。光线追踪模拟表明，对于能量为100 eV至4 keV的情况，这发生在样品表面上方最初的40至3μm范围内。优化的前置透镜设计允许在传统加速模式和新的减速模式之间切换。使用来自自由电子激光FLASH的飞秒极紫外探测脉冲在能量为107 eV下进行的时间分辨实验表明，在入射泵浦通量为22 mJ/cm（减速场-21 V/mm）时，费米边的宽度仅增加30 meV。对于加速场为+2 kV/mm且泵浦通量仅为5 mJ/cm的情况，它增加0.5 eV（泵浦波长1030 nm）。在给定条件下，抑制模式可使慢速电子产率提高三到四个数量级。在E = 3830 eV时，使用来自储存环PETRA III的硬x射线，在没有泵浦光束的情况下证明了该方法在高能量下的可行性。该方法为光电子发射实验开辟了一个以前无法达到的泵浦通量范围。
