Mark D. Roper

Direct observation of orbital ordering in La0.5Sr1.5MnO4 using soft X-ray diffraction

We report the first direct resonant soft x-ray scattering observations of orbital ordering. We have studied the low temperature phase of La0.5Sr1.5MnO4, a compound that displays charge and orbital ordering. Previous claims of orbital ordering in such materials have relied on observations at the manganese K edge. These claims have been questioned in several theoretical studies. Instead we have employed resonant soft x-ray scattering at the manganese L(III) and L(II) edges which probes the orbital ordering directly. Energy scans at constant wave vector are compared to theoretical predictions and suggest that at all temperatures there are two separate contributions to the scattering: direct orbital ordering and strong cooperative Jahn-Teller distortions of the Mn3+ ions.

An apparatus for measuring soft X-ray magnetic scattering

Abstract There has been a recent rapid increase in interest in the measurement of soft X-ray magnetic scattering (SoXMaS) to characterize the distribution of magnetic moments in thin films and multilayers. The Daresbury Laboratory team has adapted a high-vacuum two-circle diffractometer for pioneering measurements in this field. The principal features of the diffractometer and the alignment procedure for SoXMaS measurements are presented. A flexible data acquisition system is used, which allows a wide variety of scan modes and control of the various beamlines on which the experiments have been performed.

Synthesis and measurement of normal incidence X-ray multilayer mirrors optimized for a photon energy of 390 eV

Abstract The problems inherent in the fabrication of short period multilayer mirrors are discussed and results of the synthesis of multilayer structures with nanometer period are presented. The shortest period observed is 13 A for WSi and WB 4 C sputtered multilayers. Measurements of near normal incidence reflectivity at λ = 31–32A are presented for WSc multilayers with a period approximately 16 A. The measured reflectivity reaches a maximum of 3.3% and is in good agreement with theoretical modeling after the inclusion of interfacial roughness.

Ultrafast science and development at the Artemis facility

The Artemis facility for ultrafast XUV science is constructed around a high average power carrier-envelope phasestabilised system, which is used to generate tuneable pulses across a wavelength range spanning the UV to the far infrared, few-cycle pulses at 800nm and short pulses of XUV radiation produced through high harmonic generation. The XUV pulses can be delivered to interaction stations for materials science and atomic and molecular physics and chemistry through two vacuum beamlines for broadband XUV or narrow-band tuneable XUV pulses. The novel XUV monochromator provides bandwidth selection and tunability while preserving the pulse duration to within 10 fs. Measurements of the XUV pulse duration using an XUV-pump IR-probe technique demonstrate that the XUV pulselength is below 30 fs for a 28 fs drive laser pulse. The materials science station, which contains a hemispherical electron analyser and five-axis manipulator cooled to 14K, is optimised for photoemission experiments with the XUV. The end-station for atomic and molecular physics and chemistry includes a velocity-map imaging detector and molecular beam source for gas-phase experiments. The facility is now fully operational and open to UK and European users for twenty weeks per year. Some of the key new scientific results obtained on the facility include: the extension of HHG imaging spectroscopy to the mid-infrared; a technique for enhancing the conversion efficiency of the XUV by combining two laser fields with non-harmonically related wavelengths; and observation of D3+ photodissociation in intense laser fields.

Performance of the Daresbury synchrotron radiation source soft x‐ray double‐crystal monochromator

The double‐crystal monochromator on beam line 3.4 at the Daresbury synchrotron radiation source covers the energy region 0.8–4 keV, routinely using crystal pairs of beryl, quartz, InSb, Ge, and Si. The beam line has a unique system to cut off high‐energy radiation. The whole beam line has been built on a swinging arm, which allows the glancing angle on a chromium coated premirror to be varied between 0.5° and 3°. This not only reduces the heat load on fragile crystals, but also reduces the higher order contributions. We have made a thorough study of the alignment problems arising in the double‐crystal geometry. Crystal misalignment, Bragg angular offset, as well as heat‐load effects, can cause severe beam movements. A beam‐position monitor, which has recently been commissioned, is an invaluable diagnostic tool for these beam movements.

Soft x‐ray spectroscopy in atmospheric pressure helium

We report on an environmental chamber, which is attached to a UHV beamline, in which soft x‐ray measurements can be done at atmospheric pressure in helium. X‐ray measurements in air can only be performed at energies above about 3 keV because of the strong absorption of soft x rays by oxygen and nitrogen. However, a low‐Z scatterer such as helium has a long absorption length for soft x rays even at atmospheric pressure. Thus, this new chamber allows soft x‐ray experiments to be performed on samples with physical properties that are incompatible with UHV conditions, e.g., liquid and frozen aqueous solutions, corrosive materials, etc. A helium‐tight tank has been installed behind the vacuum experimental chamber of the double crystal beamline 3.4 at the Daresbury SRS. The tank is purged with helium at atmospheric pressure and the gas in the tank is isolated from the high vacuum of the rest of the beamline by a thin mylar window which is supported on a capillary array. The tank contains a sample stage, two ion...

Grazing incidence reflectivity and total electron yield effects in soft x-ray absorption spectroscopy

We report on a study of grazing incidence absorption and reflection spectra of NiO in the region of the Ni 2p edge. The aim is to evaluate the distortion of the near edge spectrum by the critical angle behavior of individual components within the spectrum. This can be used to improve the separation of multiplets and enhance low spectral weight line shapes like charge transfer satellites. The measured spectra have been compared with calculations using an optical model.

A ReflEXAFS apparatus for use with soft X-rays in the sub-4-keV energy range

Abstract Grazing angle reflection EXAFS (ReflEXAFS) is a useful tool for determining the surface layer and interlayer structures of thin film samples. At X-ray energies above about 4 keV it is practicable to design a ReflEXAFS apparatus which uses standard optical micro-positioning equipment and can be operated in air. At soft X-ray energies below this atmospheric X-ray absorption becomes a significant problem and it is normal to measure EXAFS with the sample in vacuum which would generally preclude the use of commercially available goniometer stages. We describe an apparatus for soft X-ray ReflEXAFS which can be operated at either reduced atmospheric pressure or in a helium atmosphere and has been built using mostly off-the-shelf components.

EXAFS measurements on the structure of Mo/Si multilayers produced using ion bombardment and increased deposition temperature

Abstract This study focuses on explaining differences in soft X-ray reflectivity observed for Mo/Si multilayers produced by e-beam evaporation with two different additional methods: ion etching of the Si layer and optimization of the substrate temperature during deposition. A 32-period multilayer made with ion etching has a near normal incidence reflectivity of 50%, while the one made at an optimum deposition temperature shows only 41%. The values for the interface roughness or layer thickness errors obtained from the analysis of small angle reflectivity measurements cannot explain this difference, but a different morphology of the Si and Mo layers could. Extended X-ray absorption fine structure (EXAFS) experiments have been carried out at the SiK and the MoK edge to obtain information on the multilayer structure and chemical composition. The results of these EXAFS measurements on the two types of multilayers indicate that both the Si and the Mo have an amorphous structure and that no MoSi compounds are formed at the interfaces. However, it is found that multilayers made with ion etching have a reduced fraction of oxygen (≤ 0.5%) in the Si layer compared to layers produced without etching (4.7%). The latter fraction of oxygen, orginating from the background gas in the deposition system, results in higher absorption of soft X-ray radiation and therefore affects the near normal incidence reflectivity. Although both the Si and Mo layers produced with the three different deposition methods are amorphous, some structural differences have been found using the EXAFS analysis.

Spectrometer Based on a VLS Grating for Diagnostics of a Vacuum‐Ultraviolet Free Electron Laser

Photon beam diagnostics for vacuum-ultraviolet free electron lasers (VUV FEL) are critical to monitoring and understanding their performance characteristics. Due to the shot-to-shot fluctuations inherent in FELs based on the self amplified spontaneous emission (SASE) process, it is mandatory to use pulse-resolved diagnostics. We have designed a spectrograph based on a variable-line-spacing (VLS) plane grating and a phosphor/CCD to monitor single shot spectra of the free electron laser at DESY. The basic concept is to allow most of the beam to be reflected towards an experimental station while the first order light is dispersed and focused by the VLS grating onto the CCD. The spectrograph will cover the wavelength range 6.4-60 nm with the CCD accepting a bandwidth of ~10%. The grazing angle of incidence on the grating is 2o, the central line density is 1200 l/mm, and the distance grating-CCD is approximately 2 m. The linear variation of the grating line spacing combined with positioning the detector at the focal curve, allows zeroing the defocus in the full spectrograph wavelength range. The correction of higher order grating aberrations yields a theoretical resolving power greater than 20000 over the full length of the 20 mm CCD when the CCD is positioned tangent to the focal plane. Based on power considerations, a shallow blazed grating is the preferred profile. Efficiency calculations over the spectrograph range show that with a carbon coating the absolute efficiency for zeroth order is higher than 0.85 and the first order efficiency varies between 0.5% and 8%.