ANALYTICAL TECHNIQUES Essay Example

ANALYTICAL TECHNIQUES

Analytical Techniques

Neutral Impact Collision Ion Scattering Spectroscopy

NICISS is a procedure used in chemistry to determine the element concentration profiles in the soft matter’s interfacial region. The method is applied to the soft matter whose depth has a resolution of about 0.2 nm regarding closeness to the surface. The bombardment of inert gas ions is the major target of the NICISS method. Such inert gases include helium. Helium ions are hence often used for the NICISS experiments.

When Helium projectiles are backscattered, energy is lost through the collision of the atoms contained in the target. The time that the projectiles used to fly from the target to the detector is measured and it is the representation of the amount of energy which is lost in the NICISS process. The energy can be lost in two different ways. The first way through which energy is lost is by energy transfer from the ion during collision to the target (Ridings 2014, p.65). The elements of backscattering the projectiles are identified through the energy release and which is dependent on the mass of the atom being targeted (Hu 2017, p. 26). Energy is hence lost as the ions pass through the target.

Secondly, energy is lost when the target atom experience electronic excitation and scattering at a small angle. The depth of the atoms which backscatters projectiles is identified through this energy loss (Chauhan 2012, p. 23). The energy lost is continuous as electronic excitation takes place throughout the NICISS process. The deeper layer of the target atom loses energy through a large single angle. In addition, the loss of energy in the two broad ways leads to the determination of the depth of concentration in materials as well as the profiles of the materials which makes up the target atom from which the energy is released through NICISS.

Ultraviolet Photoemission Spectroscopy

Ultraviolet photoelectron spectroscopy UPS is the measuring of the kinetic energy which is the spectra of the photoelectrons. The kinetic energy is measured from molecules that absorb ultraviolet photons. The calculation of ultraviolet photoemission spectroscopy is done through Einstein’s photoelectric law. The equation that is used in the calculation is Ek=hv-I whereby Planck’s is represented by ‘h’, “v” represents the ionizing light frequency while the ionizing energy is denoted by ‘I’ (Jiang 2005, p. 174). The charged ion can either be in the excited state or in the ground state. The energy is contained in an orbital which is molecular in nature can be helpful in identifying the ionizing energy according to Koopmans’ theory. When an electron is removed from the molecular orbital which is highest occupied, an ion which is a ground state is formed. The excited ions on the other hand, are formed when the electrons are removed from molecular orbital which has the lowest occupation (Osbourne 2003, p. 1153).

UPS has been used in the fields of sciences, especially in physics and chemistry since its establishment in the 1960s. The spectrum of the photoelectron has peaks which are arranged in series and each peak corresponds to each region of valence in molecular orbital levels of energy (Michels 2008, p. 443). The vibration levels in molecular ion help in fostering bonding and anti-bonding molecular orbitals. The method has continually been advanced and is used to study solids. The determination of the work function of a material is also determined through UPS. Four factors are necessary for the UPS computation to be successful. These include emission line, gas, energy relative intensity as well as the wavelength.

Metastable Induced Electron Spectroscopy MIES

MIES applies Helium atoms which electronically excite atoms and molecules from surfaces and the energy spectrum of the electrons which have been emitted are measured. The method has various similarities to XPS as well as UPS in some areas. For instance, just as the XPS and the USP, MEIS maps the electronic structure and the composition of surfaces. MEIS helps in the calculation of probability electron valence structure. As in a UPS, MEIS uses the energy of ion excitation.

MEIS sensitivity is influenced by the energy released when ions are excited. The metastable ion is unique since it cannot penetrate the analyte material when the material has been excited. The sensitivity of the surface in MEIS does not initiate from the average free path, but rather from the electrons which are in the outermost layers since it is the only one that can be excited (Houssiau 2014, p.21). The region important for interaction reactions on the surface and outside the surface is probed through the use of MEIS. The density of states is used in the directly in the evaluation of spectra in MEIS (Yu 2015, p.19). Density of states is majorly used in the assessment of the electron valences.

A surface which is covered by a monolayer is used for the impingement of helium atoms in a schematic metastable. The outermost layer of the sample is formed by the molecule in the left panel with various orientations and the outermost layer molecule has the same orientation. Metastable atoms in the left panel reaches all molecule parts, hence form the outermost layer (Meis 2015, p.503). The electrons from different orbitals can hence be found within the spectrum. However, on the right side only the electrons which form one orbital side are found in the spectrum.

XPS X-Ray Photoelectron Spectroscopy

XPS X-ray photoelectron spectroscopy works in the same principle as the UPS and the MEIS in the determination of the energy lost in the excitation of molecules as well as the content of a material in terms of depth. It is a technique which is primarily used for the spectroscopic calculations which involve the sensitivity of a surface. The XPS X-ray photoelectron spectroscopy measures the composition of elements in a surface in parts per thousands. There are various factors which are considered for the empirical formulae to be applied including the electronic state and the chemical state of an object that is empirically tested using the XPS X-ray photoelectron spectroscopy (Yoshida 2008, p. 191). UPS on the other hand leads to the determination of the work function of a material.

XPS X-ray photoelectron spectroscopy technique is useful for the electronics which have escaped into the vacuum of the instrument from the sample. The electrons escape from the sample after a photoelectron passes through the sample. As the photoelectron travels via the sample, sample reactions such as recombination, inelastic collisions and excitation of the sample occur to the molecules and ions which make up the sample (Castle 2014, p. 950). Various states can also be recaptured and be trapped within the material as the photoelectron moves through the sample. The process of trapping and recapturing of the states within the material leads to a reduction of the number of photo electrons which are emitted from the sample through XPS X-ray photoelectron spectroscopy. As the depth increases, the results of photoelectrons emitted appear to attenuate exponentially. The analytes are hence detected by the signals. The surface weighted signal which is exponential is measured by XPS (Jurgensen 2012, p. 1100). The depth of the material that has been layered can hence be calculated through XPS X-ray photoelectron spectroscopy hence it’s functioning.

Difference between UPS, XPS and MEIS

UPS, XPS and MEIS are all used in the identification of the ion excitation and the determination of the surface content. However, all the techniques are different in terms of what they measure in the material of a solid and more importantly in the surface sensitivity. MEIS uses the helium atoms which are metastable, so as to excite the molecules electronically as well as the surface atoms. MEIS is also used in measuring the energy spectrum produced by the electron emissions.

XPS is used in the determination of the core structure of the electrons. The radiation used for excitation of ions into the sample and the depth is determined through the path which is free and average according to the electrons which have been emitted. In the UPS, the signal from the electron emission from both sub-surface and surface layers is usually collected. The surface sensitivity in MEIS is dependent on the object which carries the energy which is applied in exciting the target material. MEIS is completely different from UPS and XPS since the spectra in MEIS is analyzed quantitatively and revealed directly regarding the density of the states. UPS and the XPS on the other hand have various similarities that make them different from MEIS. For instance, unlike the MEIS, XPS and UPS leads to mapping of the composition within a given material. However, XPS and UPS also have differences. The XPS X-ray photoelectron spectroscopy measures the composition of elements in a surface in parts per thousands.

Therefore, although the techniques have similarities with one another, they also have difference which makes them unique and hence used for different purposes. All the techniques are of great essence in the fields of physics and chemistry involving material composition and surface sensitivity.

References