A Submicron Soft X-ray Active Grating Monochromator Beamline for Ultra-High Resolution Angle-Resolved Photoemission Spectroscopy

We have constructed a new soft X-ray beamlinc, namely the Taiwan Photon Source (TPS) 45A NSRRC-MP1 beamline, to facilitate suhmicron angle-resolved photoemission spectroscopy (ARPES) experiments with ultra-high energy resolution. The beamline uses an elliptically polarized undulator with 46 min magnet period (EPU46) that provides photon energies from 280 to 1500 eV with horizontal and vertical linear polarization, as well as left and right circular polarization. The vertical focusing mirror (VFM) and the active grating monochromator (AGM) utilize the novel 25-actuator optical surface bender developed for ultra-high resolution soft X-ray spectroscopies. With the surface slope error being reduced down to 0.03 jirad root-mean-square (mis) by the bender as verified by the long trace profiler (LIP) measurements, the ray-tracing simulation shows that an energy resolution of 5 meV can be achieved at 750 eV photon energy and the beam spot size can reach 0.5 jun x 0.4 tun at the sample position. By adjusting only the 6 actuators to control the surface major profile, our preliminary results show that the VFM is able to focus 70% of the photon flux from EPIJ46 through an entrance slit set at 1.9 1.un opening. As deduced from the measured ARPES data at the Fermi-level of Au, the energy resolving power of the monochromiitor \vas found to reach 34,000 full-width-half-miiximum (FWI1M) at 850 eV photon energy. By using the in-vacuum LTP measurement system cunently under development and all 25 actuators of the surface bender, we anticipate to further reduce greatly the intrinsic and thermal-induced slope errors of the VFM and AGM. It is feasible that the overall ARPES energy resolving power can reach the 140,000 target in the soft X-ray spectral region.