Tag Archives: spectroscopy

Infrared (IR) Spectroscopy

Infrared (IR) #spectroscopy is a powerful technique that allows to obtain information about the chemical structure of a variety of substances by utilizing infrared electromagnetic radiation. In this science video we will discuss the basics of this technique with the help of 3D animations and also perform a practical demonstration, where we study a Teflon substrate with ATR technique. The video was made by Captain Corrosion OÜ in collaboration with Uno Mäeorg (Associate Professor in Organic Chemistry, Institute of Chemistry, University of Tartu).

Educators can download some of the animations used in the video from our online library to enhance their courses.

X-Ray Fluorescence Spectroscopy (XRF)

X-ray fluorescence spectroscopy (XRF) is one of the most common techniques used for studying the elemental composition of different materials. In this materials characterization method the sample is irradiated with x-ray radiation, which knocks out electrons from atoms, leaving them in an excited state. During the relaxation of these atoms the excess energy is released in the form of x-ray radiation. The energy and intensity of this radiation however depends directly on the composition of the material. Therefore it is possible to study a materials composition by detecting the x-rays that come out of the sample. Watch our video to learn more!

Low-Energy Ion Scattering Spectroscopy (LEIS)


Low-energy ion scattering spectroscopy (LEIS) is an exciting technique that allows to study the structure and chemical composition of a materials surface.

In this materials characterization method the sample is bombarded with a stream of ions and the positions, velocities and energies of the scattered ions are observed. The energy of scattered ions depends on the mass of the target, so there are distinct peaks in the energy spectrum of the scattered ions. These peaks give information about the samples elemental composition. The uniqueness of this technique lies in its sensitivity to the very first atomic layer on a sample and with forward scattering setup it is even capable of directly observing hydrogen atoms.

One of the main components of the system is the ion gun, that shoots ions at the studied substrate. The most widely used ions for that purpose are ionized noble gas or alkali atoms. Noble gas such as helium, neon or argon is ionized with electrons, giving them a positive charge. Alkali ion beams can be created by heating alkali wafers. In low-energy ion scattering spectroscopy the ions usually have an energy from 500 eV to 10 000 eV. The precise desired energy of the ions is obtained by applying a suitable accelerating voltage.

Before interacting with the substrate, the ions first need to pass through the ion beam manipulator, that narrows the beam and also filters the ions based on mass and velocity. For some experiments the ion beam is also chopped with an unipolar electrical chopper – a pulsed-wave generator, that lets through ions only when no voltage is applied. As a result the ion beam leaves the ion beam manipulator in pulses. By using short ion pulses, one can separate backscattered primary ions, for example He, from sputtered ions of different masses by time gating. This makes it possible to detect signals that would otherwise be buried in the background that is caused by ions sputtered from the sample surface.

The sample itself is attached to a special holder that allows the operator to adjust the position and angle of the sample for different experiments. When the ions hit the substrate, different interactions take place. Some ions are scattered at a certain angle and also their energy will be different after the impact. Some ions however become neutral as they pick up electrons from the substrate. The ions may also be implanted into the material or deposited on the substrate surface. The primary beam ions may also kick out electrons or atoms from the substrate and the atoms may even be ionized in the process. Radiation may also be emitted from the substrate as the excited atoms undergo a relaxation process.

The electrostatic analyzer is commonly used to detect the velocities and energies of the scattered ions. In this hemispheric device an electrical potential is applied between the inner and outer wall. The outer wall with positive potential repels the positive ions and the inner wall with negative potential attracts the positive ions. Neutral particles are unaffected by the field and hit the wall and thus never reach the detector. Positive ions with too low energy are pulled to the inner wall and also don’t reach the detector. If the cations energy is too high however then it simply hits the outer wall. Only if the ions energy is just right, it can pass through the analyzer and create a signal by interacting with the detector. By changing the potential between the walls, the operator can scan through a wide energy range in order to find out the energy of the emitted particles. In newer systems however a double toroidal analyzer is preferred as it integrates the signal over the scattering azimuth, so the intensity is some orders of magnitude higher compared to a hemispherical analyzer. The drift tube is used in TOF experiments in order to detect the energies and velocities of the scattered ionic and also neutral particles. Neutrals can easily be seperated from ions with the accelerator. There are two types of detectors that are commonly used – channel electron multipliers and microchannel plates. If the ion or a neutral particle with sufficient energy hits the detector then a cascade of secondary electrons is created and the signal significantly amplified. Microchannel plates also give information about the particles position but that comes at the cost of sensitivity.

Measurements with low-energy ion scattering spectroscopy are performed in ultra-high vacuum in order to avoid interactions with the surrounding gas. Having a good vacuum also ensures that the studied substrate and system parts are clean. Ultra-high vacuum is achieved with turbomolecular and ion pumps with the help of rough vacuum pumps.

Samples that have been exposed to open air are always contaminated for this type of surface sensitive characterization method and therefore they need to be cleaned inside the system in vacuum with appropriate equipment. Common ways to remove the contaminated top layer are sputtering, annealing or exposing to atomic oxygen.

There are of course few other surface sensitive materials characterization techniques such as XPS, AFM and SEM but each of them has distinct advantages and disadvantages. Therefore they are often used together when studying novel nanomaterials as they compensate each others weaknesses and allow to get a better overview.


Auger Effect

The Auger electron is generated during an excited atoms relaxation process, where excess energy is transfered to an outer shell electron, which leaves the atom and becomes the Auger electron. The energy of this electron depends on the binding energies of the participating electrons and is unique to the element where it occurs. As the Auger electrons energy is very low, it can escape the material from only near the surface (few nanometers). This means that this signal is highly surface sensitive and can be used to obtain information from only the surface, not from the bulk material as it is common with other material characterization methods.

How an X-Ray Tube Works

X-ray tubes are widely used for generating X-ray radiation. This radiation has a shorter wavelength than visible light and can easily penetrate through different materials. It can be used in different applications such as materials characterization (XRF, XPS, XRD etc), medicine (x-ray tomography) or security in airports.

The radiation is generated with the help of accelerated electrons. These electrons are first generated on a tungsten cathode via thermoionic emission. Then these electrons are accelerated towards the anode due to a high electric potential between the anode and the cathode. When the electrons interact with the anode, x-rays are emitted. The radiation consists of two components – characteristic x-rays and bremsstrahlung. Characteristic x-rays are generated during the relaxation process of excited anode atoms. This radiation has a specific energy. Bremsstrahlung with a broad range of energy however is emitted from the primary electrons when they slow down or change trajectory during interaction with the anode.

The generated x-rays leave the tube through a beryllium window. This material is used as it has a low atomic number and doesnt absorb much of the emitted radiation.

There are also other types of x-ray tubes, such as the twin anode x-ray tube and the rotating anode x-ray tube.

In the case of twin-anode system, the anodes are made from different materials and only one of them is bombarded with electrons at the same time. This allows fast and easy switching between two excitation energies. The other anode will also serve as a backup if one should fail.

Using a rotating anode allows the heat to distribute on a larger surface area and therefore it is possible to get x-rays with much higher energies and intensities.