Alignment of the photoelectron spectroscopy beamline at NSRL

TL;DR

This paper details the realignment and optimization of the NSRL photoelectron spectroscopy beamline, restoring spectral resolution and flux throughput, and identifying issues with gratings for future improvements.

Contribution

It presents a comprehensive realignment process and performance recovery of the beamline, including diagnostics and troubleshooting of optical components.

Findings

Spectral resolving power restored to 1000@244eV

Flux throughput for 200 lines/mm grating matches design expectations

Identified contamination and surface roughness issues affecting performance

Abstract

The photoelectron spectroscopy beamline at National Synchrotron Radiation Laboratory (NSRL) is equipped with a spherical grating monochromator with the included angle of 174 deg. Three gratings with line density of 200, 700 and 1200 lines/mm are used to cover the energy region from 60 eV to 1000 eV. After several years operation, the spectral resolution and flux throughput were deteriorated, realignment is necessary to improve the performance. First, the wavelength scanning mechanism, the optical components position and the exit slit guide direction are aligned according to the design value. Second, the gratings are checked by Atomic Force Microscopy (AFM). And then the gas absorption spectrum is measured to optimize the focusing condition of the monochromator. The spectral resolving power is recovered to the designed value of 1000@244eV. The flux at the end station for the 200 lines/mm grating is about 10^10 photons/sec/200mA, which is in accordance with the design. The photon flux for the 700 lines/mm grating is about 5 X 10^8 photons/sec/200mA, which is lower than expected. This poor flux throughput may be caused by carbon contamination on the optical components. The 1200 lines/mm grating has roughness much higher than expected so the diffraction efficiency is too low to detect any signal. A new grating would be ordered. After the alignment, the beamline has significant performance improvements in both the resolving power and the flux throughput for 200 and 700 lines/mm gratings and is provided to users.