Electron Energy Loss Spectroscopy

Electron Energy Loss Spectroscopy is a analytical technique used in Transimssion Electron Microscopy. (This article is a stub, please add more information)

Monochromator / Filter Types

Monochromator types can be grouped into in-column monochromators and post column monochromators. In column monochromators where first commercially introduced by Carl Zeiss with the Castaing-Henry Monochromator based EM 902 EFTEM (Energy Filtered Transmission Electron Microscope).

In Column

In Column Filters are those that are inline with the Microscope column and usually located between two sets of Projector lens systems, and typically feature the ability to create energy filtered images allowing for element mapping, among other energy loss techniques. On some microscopes like the Zeiss Libra 200 the filter can be bypassed in image mode, by opening a beam stop and changing optical parameters.

Castaing-Henry Filter

Castaing-Henry Monochromator diagram as seen in the Manual of the Zeiss EM 902.

First implemented by Castaing and Henry within a Siemens Elmiskop 102 to then later be adapted onto the newly developed Zeiss EM 900 TEM, thus creating the EM 902 EFTEM. It uses a Dipole Magnet to bend the electron beam by 90°, then reflecting it back into the Diopole via an Electrostatic Mirror, bending it by another 90° back onto the optical axis. Electrons with energies below or above the selected energy colide against the exet aperture in Image mode, and are bent away from the optical axis creating a Spectrogram on the Viewscreen / Camera in Spectrometer mode. Further the energy can be scanned using the exit slit and a PMT allowing digital aquesition of the spectrum via a PMT and Microcomputer (such as in the CEM 902). The Electrostatic Mirror is powered by the same High Voltage as the Electron Gun, thus a second Highvoltage cable originating from the Highvoltage tank goes into the side of the column.

Omega Filter

Omega Filter as seen in the Zeiss EM 912 EFTEM.

First implemented in the Zeiss EM 912 EFTEM, now using 4 Dipoles to bend the into the shape of a sideways Omega, this filter design allowed higher resolutions then where achievable with the Castaing-Henry Filter, and also no longer requiring the use of a second High Voltage cable. This filter design was later licensed to JEOL.

Corrected Omega Filter

The corrected Omega Filter as seen in the Zeiss Libra 200 EFTEM. Featuring more multipoles used for correcting higher order aberrations then in the normal Omega Filter.

First Implemented in the Zeiss SESAM 1, refined in the Zeiss SESAM 2 and brought to market in the Zeiss Libra 200 EFTEM. It features more Multipole lenses to correct higher order Aberration, and also features a bypass Methode allowing imaging without transmitting the image through the filter system. In order to get the best energy resolution out of the Corrected Omega filter, the use of a Omega Filtered FEG is required, as is with all high resolution EELS filters. The Zeiss Libra 200 did not come standard with a Filtered Electron Gun, instead having it as an optional upgrade.

The Corrected Omega Filter also allowed for a higher acceptance angle then the filter designs preceding it, allowing for larger energy filtered electron diffraction patterns to be produced.

Mandoline Filter

Schematic Diagram of the Mandoline Fitler used in the Zeiss SESAM.

This one of a kind energy filter was implemented in 2007 within the Zeiss SESAM (SESAM 3) and is one of the highest energy resolution filters yet conceived, it also happens to be the largest mechanically speaking. It offers a very large acceptance angle allowing for large energy filtered diffraction patterns to be produced. It was only built a single time in the unique SESAM Electron Microscope, based on the Libra 200. This Microscope is the only surviving SESAM project microscope and is preserved in semi functional (in restoration) condition at the Museum of Electron Microscopy.

It features 3 Dipoles with even more multipole lenses then the corrected Omega filter, allowing for correction of even higher order aberrations then the corrected Omega.

Wien Filter

A coaxial in column Filter using crossed Electrostatic and Magnetic Fields to filter electrons. Implemented by JEOL in machines such as the Monochromated ARM200F. (add more information here)

Alfa Filter

This filter is another pure Magnetic filter, with the electron path fallowing the general shape of a sideways Alpha.

Post Column

This type of filter is mounted below the column, typically on the camera port of a standard TEM. Unlike the in column filter, this type of EELS filter dose not require the creation of special purpose machine.

Serial 90° Scofield Monochromator

Serial EELS model 607 from Gatan featuring a 90° Sector Field Dipole monochromator.

The Serial 90° Sector Field Monochromator can be thought of, as analogous to a prism based light spectrometer with variable prism refractive index. The electron beam enteres the monochromator via the entrance aperture, and is relayed and focused by a projector lens mounted in front of the 90° dipole Analyzer. A Dipole Magnet bends the beam by 90° and into an Exit Slit, where a Szintillator converts the electron beam current into light, which is then measured by a Photomultiplier tube. A spectrum is recorded or displayed on a Cathode Ray tube by changing the excitation of the 90° magnet which is analogous to changing the refractive index of a light prism. Further addons allow the use of a Electrostatic field to vary the output energy by a hundred eV or so. The Spectrometer was at times, and is now controlled by the use of Minicomputers or in modern times Microcomputers. The first commercial Implementation of a post column 90° sector field monochromator based EELS system was the Model 607 Serial EELS by Gatan.

Parallel 90° Sector field Spectrometer

This is a variation of the above monochromator, and uses a multipole corrector to project a full spectrum through the sector field magnet, which is then recorded onto the Szintillator of a CCD camera mounted at the end of the filter projective column. The first Commercial implementation of this Parallel EELS design was the Gatan Model 607 with Model 666 CCD Camera. This spectrometer also features a PMT for Serial data recording like the older Serial version. Modern 90° monochromators can still operate in this Fassion.

90° Sector Field Imaging Spectrometer / Monochromator

Similar to the Parallel 90° Spectrometer, this type of Filter uses a 90° Dipole sector field magnet as its dispersive element, but with a number of multipole lenses in front and after the sectorfield, allowing the transmission of images like those of the In-Column Filters mentioned above. The position of the spectrometer on the Bottom mount camera port of a none purpose built machine comes with the downside of not having as high of an acceptance angle, and also not showing the filtered image on a normal viewscreen, relying on a Digital camera. This spectrometer design has now reached similar energy resolutions to the In-Column designs, while being adaptable to a very large amount of machines.