I09: A Two-Color Beamline for Electron Spectroscopies

The first beamline in the world to deliver hard and soft X-rays with independent sources and optics

An article recently published in Synchrotron Radiation News details the development of Diamond’s I09 beamline - the first in the world to deliver both hard and soft X-rays with optimised, independent sources and optics. The extended energy range allows a wide variety of X-ray techniques, which can be combined to maximise the information extracted from a visit to the beamline. For photoelectron spectroscopy, this energy range corresponds to an information depth from 0.5 to over 20 nm, offering both bulk and surface sensitivity. This unique combination has allowed I09 to develop a user community that is interested in a number of interconnected research areas: surface chemistry and structures, electronic structures of oxide hetero-interfaces, electron correlations and metal-insulator transitions, energy materials, functionalised materials, and material design. Most studies benefit from the use of both soft and hard X-rays.

     

The I09 beamline

The I09 beamline is split into three branches. Branch I delivers hard X-rays, and branches J and K deliver soft X-rays. I and J merge in Experimental Hutch 2 (EH2), allowing dual hard and soft X-ray experiments. Additional end-stations are being set up in Experimental Hutch 1 (EH1) for hard X-ray studies, and the Experimental Cabin (EC) for soft X-ray studies. As the sources and optics are independent, one experiment using hard X-rays and one using soft X-rays can be performed in parallel. 

 

Schematic of the beamline layout for I09.
Schematic of the beamline layout for I09.

The beamline uses one of the four 8 m straight sections in the Diamond storage ring. To allow the hard X-ray undulator to reach a suitably small gap, additional magnets were added to create two minima in the vertical β function (βy) over the straight section. Hard X-rays are delivered by a 2 m in-vacuum undulator with a magnetic period of 27 mm and a minimum gap of 5.2 mm. Its first and third harmonics start at approximately 0.9 and 2.4 keV.

The soft X-ray undulator is a 2.3 m modified APPLE II device, with a magnetic period of 60 mm. The lowest energies accessible for the linear horizontal, circular, and linear vertical lights are about 100, 150, and 200 eV, respectively. The hard and soft undulators are canted by 1.3 mrad and each centered with one of the βy minima. 

The energy of the hard X-rays is selected by an in-house-developed Si(111) double-crystal monochromator (DCM) designed to work between 2.1 and 20+ keV. After the DCM, the beam is reflected horizontally by the first and second hard X-ray mirrors (HM1 and HM2, respectively) in optics hutch 1 (OH1). Two of the HM1 mirror optics focus the hard X-rays vertically to the end-station in EH2 for energies below and above 5 keV. HM2 has a flat mirror that keeps the exit beam of HM2 parallel to the incident beam of HM1. After entering EH2, the hard X-ray beam is focused horizontally to the sample by a third mirror, HM3. For HAXPES experiments, three Si channel-cut crystals, oriented to excite the (004), (333), and (202)/(404) reflections at 5.94, 5.95, and 4.07/8.14 keV, respectively, can be inserted in the beam in EH1 to improve the energy resolution.

The design of the soft X-ray branches is based on the well-established concept of a plane grating monochromator (PGM) using a collimated light. Before the PGM, soft X-ray mirror 1 (SM1) collimates vertically and focuses horizontally the synchrotron lights. After the PGM, SM3 focuses the beam vertically to the exit slit of either Branch J or K. After the exit slit, SM4 and SM5 (SM6) refocus the beam to the end-station in EH2 (EC). The beamline optics and diagnostics allow a quick alignment of the focused X-rays of Branches I and J to the focal spot of the electron analyser in EH2.
 
The experimental station in EH2, which became operational in mid-2013, consists of an analysis chamber and two sample preparation chambers. The analysis chamber is equipped with a Scienta EW4000 high-voltage electron analyser, a nano-amp low-energy electron diffraction (LEED) optic, and a monochromatic vacuum ultraviolet light source. The preparation chambers, each fitted with a standard LEED optic, allow ion sputtering, sample heating (up to 1500°C), gas dosing, and physical vapor deposition for in-situ surface preparation. A large manipulator that positions the sample and provides polar and azimuth rotations is used for X-ray measurements at temperatures between 60 and 900 K.
 
To make the best use of the wide energy range available, I09 offers a range of X-ray techniques to the user community. Normal incidence X-ray standing waves (NIXSW) and energy-scanned photoelectron diffraction (PhD) are employed to determine surface adsorbate structures. X-ray photoelectron spectroscopy (XPS) provides chemical information and can be exploited with either soft (SXPS) or hard X-rays (HAXPES) to vary the surface/bulk sensitivity. Resonant photoelectron spectroscopy (ResPES) is a powerful technique for enhancing weak contributions to valence band photoemission from buried or dilute systems with element specificity. X-ray absorption spectroscopy (XAS), currently available in total electron yield and Auger yield modes, probes the unoccupied states and is sensitive to orbital orientation, oxidation state, and hybridization. For studying momentum-resolved electronic structures, angle-resolved photoelectron spectroscopy (ARPES) has been performed on I09 with a He lamp.
 

Future plans

The installation of a new HAXPES end-station in EH1 is underway. It is designed to be more versatile than the end-station in EH2 in accommodating HAXPES experiments with different requirements, with a planned pool of exchangeable sample manipulator feedthroughs, optimised for, e.g., high-throughput screening, in-operando HAXPES of device samples, or hard X-ray ARPES. The plan is for this new facility to welcome its first users in 2019. A third end-station, dedicated for soft X-ray ARPES, is being planned for Branch K in the Experimental Cabin. It will feature a momentum microscope being developed in collaboration with the Schöenhense group at University of Mainz and the Claessen group at University of Würzburg. These developments will allow I09 to offer improved capacity and flexibility for HAXPES studies and new opportunities for investigating momentum-resolved electronic structures.

 
To find out more about the I09 beamline, or to discuss potential applications, please contact Principal Beamline Scientist Tien-Lin Lee: tien-lin.lee@diamond.ac.uk.
 

Related publication:

 Lee, T-L and Duncan DA. A Two-Color Beamline for Electron Spectroscopies at Diamond Light Source. Synchrotron Radiation News 31(4), 16-22 (2018). DOI:10.1080/08940886.2018.1483653.