Photoelectric effect, photoelectron spectroscopy (XPS) and scanning photoelectron microscopy (SPEM) are explained in this short lecture. The photoelectric effect occurs when the inner shell electrons of the sample atoms are kicked out by high energy electromagnetic radiation. These electrons, that were kicked out, are called photoelectrons and their energy depends on the energy of the exciting radiation, the electrons initial binding energy and on the work function. When having a radiation source with well defined energy and measuring the energy of the emitted electrons, it is possible to get information about the samples composition and chemical state. In the case of photoelectron microscopy the exciting beam is focused into a narrow spot on the sample surface. The sample is then moved in such a way that the spot moves row by row across the sample surface and as a result photoelectrons are emitted from each irradiated spot. By collecting these photoelectrons, it is possible to map the composition or chemical state across the selected sample area (for example 100 x 100 microns area on a 2 x 2 cm sample). Photoelectrons can escape the material only from near the surface and this means that these methods are very surface sensitive, which makes them extremely useful for surface studies. This also means that the surfaces need to be very clean (cleaned with ion bombardment before measurements) and therefore ultra-high vacuum is needed.
X-ray tubes are devices that produce x-ray radiation, which is useful in various applications. For example in medicine this high energy electromagnetic radiation is used for imaging your body. In airports x-rays are used to scan your luggage for prohibited items. X-rays can also be used for materials characterization in techniques such as x-ray fluorescence spectroscopy or photoelectron spectroscopy.
An x-ray tube consists of an anode and a cathode within a casing that can hold vacuum. The cathode is heated to high temperatures, where it starts emitting electrons – this process is known as thermionic emission. A high voltage applied between the cathode and the anode accelerates the emitted electrons towards the anode. When these high energy electrons interact with the anode some of the energy is converted into x-ray radiation and some into heat. Thats why water cooling is needed to prevent the overheating of the anode.The emitted x-ray radiation consists of two components – bremsstrahlung and characteristic x-rays. In the case of bremsstrahlung the electromagnetic radiation is emitted from the negative electron when its trajectory is changed by a positively charged atoms nucleus. This radiation has a very broad energy range. Its energy and intensity depends on the voltage between the anode and the cathode, on the cathode filaments heating current and on the atomic number of the anode material. Characteristic x-rays however have a very specific energy, which strongly depends on the anode material. This radiation is generated when the accelerated electrons excite the anode atoms by kicking out inner shell electrons. In the relaxation process a higher shell electron moves to the vacant spot and the excess energy is emitted in the form of x-rays. The energy of these characteristic x-rays depend on the binding energy of the electron that was kicked out and the binding energy of the electron that occupied the vacant spot. The generated x-rays leave the tube through a beryllium window. Beryllium is used as a window material because it doesnt absorb much of the x-rays as it has a low atomic number. Be sure to follow us in youtube for more awesome videos in the future!
Vacuum can be understood as space from where matter (for example air) has been removed. It naturally exists in outer space but for certain applications, like materials characterization techniques, it needs to be achieved artificially. The desired level of vacuum is obtained with the help of a suitable vacuum pump. For example low vacuum (low quality vacuum with higher pressure) can be generated with a diffusion pump, scroll compressor pump, rotary vane pump, diaphragm pump or a sorption pump. High vacuum (high quality vacuum with very low pressures) however, can be obtained with high vacuum pumps such as the turbomolecular pump, ion pump, titanium sublimation pump and cryopump. The level of vacuum is measured with devices called vacuum gauges (vacuum meters) like the thermocouple gauge, pirani gauge, penning ionization gauge and the quadrupole mass spectrometer (analyzer). The working principle of vacuum pumps and vacuum gauges is explained with 3D animations in the video lecture above.
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.