SXSS Group (Infrared): Facilities

The SXSS Group: Fourier transform infrared (FTIR) spectroscopy.

Beamline Description

The MET beamline is based on a dipole bending magnet as the source for synchrotron infrared light. The beam radius is 25 meters. The collected emission spans 50 mrad horizontally and from about 30 mrad to 50 mrad vertically. The vertical collection varies continuously along the source arc. The light is delivered using a toroidal mirror that re-images the source at the exit window. While the central arc segment of the source is re-imaged at 1:1, the large source depth results in 25% magnification and de-magnification of the arc ends.

The collected and focused light forms a beam waist, as it passes through a diamond window, having a 15 mm diameter clear aperture with a 0.5 degree wedge and an average thickness of about 0.35 mm. From there, the light is transported to the endstation instruments using aluminium-coated mirrors. The energy range of the beamline's transport optics spans from below 0.25 meV to 4 eV.

The beam size available for measurements varies with photon energy. At the diamond window focus, the visible beam (~ 2 eV) is about 7 mm by 3 mm, while for the far-infrared (~ 4 meV) it is 15 mm in diameter. The spot size available at a sample is usually limited with apertures to a value near the diffraction-limit of 1.2λ/NA, where NA=0.14 for conventional optics and NA=0.6 for microscope optics.

The standard spectrometer at the beamline is a Bruker Vertex 80v vacuum FTIR interferometer. It is equipped with beam splitters and detectors to span the spectral range from 1 meV to above 4 eV. The highest available spectral resolution is about 8 µeV. This instrument can be used with its standard sample compartment including optics for transmission and reflection spectroscopies. The spectrometer can also be used in combination with a Spectra-Tech/Nicolet Nic-Plan microscope configured for use with most of the Bruker detectors. The low-frequency limit for this instrument is about 20 cm^-1 (2.5 meV). An Oxford Microstat open-flow cold-finger cryostat is being implement for providing sample temperatures down to ~10 K.

For infrared spectroscopy with a spatial resolution below the diffraction limit, the MET beamline provides an Infrared Near-Field Spectroscopy endstation, which uses a commercial scattering-type scanning near-field optical microscope (s-SNOM) by NeaSpec. This setup combines a spectral resolution of 0.4 meV (3.3 cm^-1) with a spatial resolution of a few 10 nm. The accessible spectral range is currently limited by the available detectors to about 80 to 250 meV (650 -2000 cm^-1). An extension of this spectral range towards both higher and smaller energies is planned.

Infrared Lab (1L-L10)

The Infrared Lab (1L-L10) is equipped with additional spectroscopic instruments for measurements not requiring high spectral radiance for sample dimensions approaching the diffraction-limit. The facilities for optical spectroscopy currently consist of a Bruker IFS 113v Fourier transform infrared (FTIR) spectrometer (shown below) and a Bruker Vertex 80v FTIR spectrometer; both of these instruments are vacuum benches. The Bruker 113v is a Genzel-type interferometer that is optimized for the far-infrared and THz regions (~1 meV to 1 eV). One of the primary advantages of this bench is that the light is focused at the beamsplitter, unlike a conventional Michelson interferometer where it is collimated; as a result the beamsplitters in the 113v are quite small (the working surface is typically 1 cm²) and several may be placed on a wheel inside spectrometer allowing them to be changed while the instrument is under vacuum. This optical bench also represents the last generation of spectrometer in which laser interferometer is not integrated into the beamsplitter, allowing us to build and test our own beamsplitters.

A reflectance unit has been custom-designed for this optical bench allowing a compact flow cryostat to attach directly to the spectrometer. An optical window separates the rough instrument vacuum from the high vacuum maintained in the cryostat. Coaxial transfer lines allow a range of temperatures from ~6 to over 400 K to be examined.

The Vertex 80v is a modern FTIR spectrometer that is a modified Michelson design with a dynamically stabilized scanner, allowing interferometry from about several meV to over 5 eV. A laser driven light source is being evaluated as an ultraviolet source to replace the current deuterium lamp. A reflectance unit using a redundant aperture has been designed for this bench; the configuration of the cryostat is similar to that used with the 113v.

Last modified: Friday, June 17, 2022 03:23 PM.

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