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Unit 3 :Electronic Spectroscopy Contents UNIT 3 :ELECTRONIC SPECTROSCOPY CONTENTS • PHOTOELECTRON SPECTROSCOPY-INTRODUCTION • PRINCIPLE • INSTRUMENTATION INTRODUCTION • Photoelectron spectroscopy (PES) is the energy measurements of photoelectrons emitted from solids, gases, or liquids by the photoelectric effect. • Depending on the source of ionization energy, PES can be divided accordingly into Ultraviolet Photoelectron Spectroscopy (UPS) and X- ray Photoelectron Spectroscopy The photoelectric effect is a phenomenon in which electrons are ejected from the surface of a metal when light is incident on it. These ejected electrons are called photoelectrons. It is important to note that the emission of photoelectrons and the kinetic energy of the ejected photoelectrons is dependent on the frequency of the light that is incident on the metal’s surface. The process through which photoelectrons are ejected from the surface of the metal due to the action of light is commonly referred to as photoemission. Explaining the Photoelectric Effect: The Concept of Photons The photoelectric effect cannot be explained by considering light as a wave. However, this phenomenon can be explained by the particle nature of light, in which light can be visualized as a stream of particles of electromagnetic energy. These ‘particles’ of light are called photons. The energy held by a photon is related to the frequency of the light via Planck’s equation: E = h휈 = hc/λ Where, •E denotes the energy of the photon •h is Planck’s constant •휈 denotes the frequency of the light •c is the speed of light (in a vacuum) •λ is the wavelength of the light Threshold Energy for the Photoelectric Effect For the photoelectric effect to occur, the photons that are incident on the surface of the metal must carry sufficient energy to overcome the attractive forces that bind the electrons to the nuclei of the metals. The minimum amount of energy required to remove an electron from the metal is called the threshold energy (denoted by the symbol Φ). For a photon to possess energy equal to the threshold energy, its frequency must be equal to the threshold frequency (which is the minimum frequency of light required for the photoelectric effect to occur). The threshold frequency is usually denoted by the symbol 휈th and the associated wavelength (called the threshold wavelength) is denoted by the symbol λth. The relationship between the threshold energy and the threshold frequency can be expressed as follows. Φ = h휈th = hc/λth Relationship between the Frequency of the Incident Photon and the Kinetic Energy of the Emitted Photoelectron Therefore, the relationship between the energy of the photon and the kinetic energy of the emitted photoelectron can be written as follows. Ephoton = Φ + Eelectron 2 ⇒ h휈 = h휈th + ½mev Where, •Ephoton denotes the energy of the incident photon, which is equal to h휈 •Φ denotes the threshold energy of the metal surface, which is equal to h휈th •Eelectron denotes the kinetic energy of the 2 photoelectron, which is equal to ½mev (me = mass of electron = 9.1*10-31 kg) Minimum Condition for Photoelectric Effect Threshold Frequency (γth) It is the minimum frequency of the incident light or radiation that will produce a photoelectric effect i.e. ejection of photoelectrons from a metal surface is known as threshold frequency for the metal. It is constant for a specific metal but may be different for different metals. If γ = frequency of incident photon and γth= threshold frequency, then, •If γ < γTh, there will be no ejection of photoelectron and, therefore, no photoelectric effect. •If γ = γTh, photoelectrons are just ejected from the metal surface, in this case, the kinetic energy of the electron is zero •If γ > γTh, then photoelectrons will come out of the surface along with kinetic energy Laws Governing the Photoelectric Effect 1.For a light of any given frequency; (γ > γ Th) photoelectric current is directly proportional to the intensity of light 2.For any given material, there is a certain minimum (energy) frequency, called threshold frequency, below which the emission of photoelectrons stops completely, no matter how high is the intensity of incident light. 3.The maximum kinetic energy of the photoelectrons is found to increase with the increase in the frequency of incident light, provided the frequency (γ > γ Th) exceeds the threshold limit. The maximum kinetic energy is independent of the intensity of light. 4.The photo-emission is an instantaneous process. From the image, it can be observed that: •The photoelectric effect does not occur when the red light strikes the metallic surface because the frequency of red light is lower than the threshold frequency of the metal. •The photoelectric effect occurs when green light strikes the metallic surface and photoelectrons are emitted. •The photoelectric effect also occurs when blue light strikes the metallic surface. However, the kinetic energies of the emitted photoelectrons are much higher for blue light than for green light. This is because blue light has a greater frequency than green light. Definition of Photoelectric Effect The phenomenon of metals releasing elec trons when they are exposed to the light o f the appropriate frequency is called the photoelectric effect, and the electrons emitted during the process are called photoelectrons. Photoelectron Instrumentation. The main goal in either UPS or XPS is to gain information about the composition, electronic state, chemical state, binding energy, and more of the surface region of solids. ... XPS sources from x-rays while UPS sources from a gas discharge lamp. Photoelectron spectroscopy involves the measurement of kinetic energy of photoelectrons to determine the binding energy, intensity and angular distributions of these electrons and use the information obtained to examine the electronic structure of molecules. It differs from the conventional methods of spectroscopy in that it detects electrons rather than photons to study electronic structures of a material. Photoelectron spectroscopy (PES) is the energy measurements of photoelectrons emitted from solids, gases, or liquids by the photoelectric effect. Depending on the source of ionization energy, PES can be divided accordingly into Ultraviolet Photoelectron Spectroscopy (UPS) and X-ray Photoelectron Spectroscopy (XPS). The source of radiation for UPS is a noble gas discharge lamp, usually a He discharge lamp. For XPS, also referred to as Electron Spectroscopy for Chemical Analysis (ESCA), the source is high energy X-rays (1000-1500 eV). Furthermore, depending on the source of the ionization energy, PES can probe either valence or core electrons. UPS, which uses the energy of ultraviolet rays (<41 eV) will only be sufficient to eject electrons from the valence orbitals, while the high energy X-rays used in XPS can eject electrons from the core and atomic orbitals Photoelectron Instrumentation All photoelectron spectrometers must have three components. The first is an excitation source used to irradiate the sample into releasing electrons. The second is an electron energy analyzer which will disperse the emitted photoelectrons according to their respective kinetic energy. Lastly, a detector. In addition, the spectrometer needs to have a high vacuum environment, which will prevent the electrons from being scattered by gas particles. Photoelectron Instrumentation The main goal in either UPS or XPS is to gain information about the composition, electronic state, chemical state, binding energy, and more of the surface region of solids. The key point in PES is that a lot of qualitative and quantitative information can be learned about the surface region of solids. The focus here will be on how the instrumentation for PES is constructed and what types of systems are studied using XPS and UPS. The goal is to understand how to go about constructing or diagramming a PES instrument, how to choose an appropriate analyzer for a given system, and when to use either XPS or UPS to study a system. There are a few basics common to both techniques that must always be present in the instrumental setup. 1.A radiation source: The radiation sources used in PES are fixed-energy radiation sources. XPS sources from x-rays while UPS sources from a gas discharge lamp. 2.An analyzer: PES analyzers are various types of electron energy analyzers 3.A high vacuum environment: PES is rather picky when it comes to keeping the surface of the sample clean and keeping the rest of the environment free of interferences from things like gas molecules. The high vacuum is almost always an ultra high vacuum (UHV) environment. Radiation sources While many components of instruments used in PES are common to both UPS and XPS, the radiation sources are one area of distinct differentiation. The radiation source for UPS is a gas discharge lamp, with the typical one being an He discharge lamp operating at 58.4 nm which corresponds to 21.2 eV of kinetic energy. XPS has a choice between a monocrhomatic beam of a few microns or an unfocused non- monochromatic beam of a couple centimeters. The main thing to consider when choosing a radiation source is the kinetic energy involved. The source is what sets the kinetic energy of the photoelectrons, so there needs to not only be enough energy present to cause the ionizations, but there must also be an analyzer capable of measuring the kinetic energy of the released photoelectrons. In XPS experiments, electron guns can also be used in conjunction with x-rays to eject photoelectrons. There are a couple of advantages and disadvantages to doing this, however. With an electron gun, the electron beam is easily focused and the excitation of photoelectrons can be constantly varied. Unfortunately, the background radiation is increased significantly due to the scattering of falling electrons. Also, a good portion of substances that are of any experimental interest are actually decomposed by heavy electron bombardment such as that coming from an electron gun.
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