Absorption spectrum and Emission spectrum are graphical representations of the measurement of the amount of light absorbed and emitted by a material respectively. The absorption spectrum shows the wavelengths of light absorbed by a material, while the emission spectrum displays light emission.
When a molecule absorbs energy in the form of electromagnetic radiation, there are a number of different ways by which the molecule emits this radiation and return to its ground state. The processes that occur between the absorption and emission of light are illustrated by the Jablonski diagram.
Jablonski diagram
A Jablonski diagram is an energy diagram that shows the possible transitions between the electronic states and the vibrational levels of a photo-excited molecule. The diagram is arranged vertically by increasing energy levels of a molecule and horizontally by spin multiplicity.
The electronic states are the electron configuration of a molecule and are characterized based on each state's spin and orbital angular momentum configuration. These are represented as singlet states ‘S’, where the total spin angular momentum of zero, and triplet states ‘T’, with a total spin angular momentum of one. Each of these electronic energy levels can exist in a number of vibrational energy levels such as 0, 1, 2, etc. These energy levels are represented as singlet ground state S0, first excited singlet state S1, second excited singlet state S2, first excited triplet state T1, etc.
An excited molecule de-excites to lower molecular states by transferring energy, which can be either radiative or non-radiative.
The two main transitions during this absorption and emission process are Radiative transitions and Non-radiative transitions. Radiative transitions are transitions between two molecular states where the energy difference is emitted or absorbed as photons, whereas Non-radiative transitions are transitions without absorption or emission of photons.
Once a molecule has absorbed energy in the form of electromagnetic radiation, there are several paths to return to its ground state. Consequently, various actions such as vibrational relaxation, fluorescence, intersystem crossing & phosphorescence occur between this absorption and emission.
Absorption starts from the lowest energy level of the ground state, S0, as it is densely populated. Upon incidence of photons, the molecule absorbs the photon's energy and gets excited to a higher energy state, either S1 or S2. This transition occurs in the order of 10-15 s and is the fastest transition.
Absorption is only possible when the incident photon has energies corresponding to the energy difference between two electronic states molecule.
The excited molecule is in a non-equilibrium condition and will eventually release the energy it has received and return to the ground state. The excess energy is transferred to other vibrational modes in the form of a non-radiative transition, namely vibrational relaxation. Upon transferring energy, the molecule reaches the lowest vibrational level of the electronic state in the order of 10-12 – 10-10 s.
If vibrational energy levels strongly overlap electronic energy levels, the molecule in a higher singlet electronic state de-excites to a lower singlet electronic state. This transition is also non-radiative and is identical to vibrational relaxation. This non-radiative process is called internal conversion and is immediately followed by vibrational relaxation. The internal conversion rate is inversely proportional to the energy gap between the two electronic states. Internal conversion will happen quickly on a timescale of 10-11 to 10-9 s for the closely separated higher-lying singlet excited states (S3→S2, S2→S1, etc.). At the same time, internal conversion between S1 and S0 states occurs slowly. It will compete with transitions like fluorescence and intersystem crossing because the energy difference between these states is larger.
Fluorescence is a radiative transition as the excited photon de-excites from S1 to S0 by emitting the absorbed energy as photons. The process occurs slowly in the order of 10-9 to 10-7 s. Even though the transition is slow, it is an allowed transition. The emitted photon has energy equal to the energy difference between the two electronic states involved in the transition. The energy of the emitted photon is typically lower than the energy of the absorbed photon due to energy losses during internal conversion and vibrational relaxation.
Intersystem crossing is a non-radiative transition from S1 to the T1 upper state, which is forbidden due to the conservation of spin angular momentum. However, factors such as spin-orbit coupling, which arises from the interaction between the electronic and nuclear spins, make this transition allowed. Immediately following intersystem crossing, the molecule will experience vibrational relaxation to the T1 ground vibrational state.
The radiative transition from the T1 to S0 with the emission of photons is known as phosphorescence. Like intersystem crossing, phosphorescence is also a forbidden transition but is allowed through spin-orbit coupling.
Absorption Spectrum
An Absorption spectrum is an essential spectroscopic graphical representation used to identify and analyze the chemical composition of materials. It is the pattern of light absorption by a material when exposed to different frequencies of light. The material absorbs energy at a specific frequency of light. The different frequencies of light correspond to different colors, and the absorption spectrum can be used to identify which colors are absorbed and reflected. The amount of incident electromagnetic radiation, temperature, the solid crystalline structure of the material, and inter-molecular interactions affect a material’s absorption level.
When a beam of light passes through a sample, it will absorb certain wavelengths of light, depending on its chemical composition and structure. The absorbed wavelengths are determined by the energy differences between different electronic or vibrational states of the molecules in the sample. The electrons from the ground state reach higher energy levels and transmit light frequencies with dark bands. When light travels through the chemical solution, a pattern of black lines is visible in the spectrum. This pattern can be examined further using a spectrophotometer, which measures the amount of light transmitted through a sample as a function of wavelength. The resulting spectrum shows the wavelengths of light absorbed by the sample and the absorption intensity at each wavelength.
The absorption spectrum of a substance can provide information about its chemical structure, composition, and physical properties. In particular, they can be used to identify specific molecules or functional groups, determine the concentration of a substance in a mixture, and study the interaction of molecules with light.
Types of Absorption Spectrum
There are three main types of absorption spectra:
Applications of Absorption Spectrum
The absorption spectrum is used in various fields, from astronomy and geology to physiology and biochemistry.
The absorption spectra can also be used to determine the optical properties of materials, medical imaging, pollution monitoring, and material analysis.
Emission Spectrum
The Emission spectrum is the spectrum of wavelengths of electromagnetic radiation emitted by atoms, molecules, or other materials when excited by an external energy source such as heat, electricity, or light. These spectra are unique light patterns for each material and can be used to identify the substance emitting the light. The composition of matter can be determined by analyzing the colors and intensities of the light emitted. It is done by comparing the emission spectra of the sample with the known spectra of elements and compounds.
When an atom or molecule from the lower energy levels (ground state) is excited by using an external energy source, such as heat or electricity, the electrons move to higher energy levels (excited state). As these electrons de-excite to their lower energy levels, they release energy in the form of photons. The emitted light has a specific frequency or color corresponding to the energy difference between the higher and lower energy levels.
The energy of the emitted photon is given as:
where Ephoton is the energy of the photon, ν is the frequency, and h is Planck’s constant.
The emission spectra of a material can be observed using a spectroscope, which separates the emitted light into its different wavelengths. Each element or compound has its characteristic emission spectra, which can be used to identify its presence in a sample or to study its properties.
Types of Emission Spectra
This spectrum refers to the pattern of wavelengths of light emitted by atoms when they are thermally excited and then return to their ground state. The wavelengths of this emitted light can be used to identify the atom. The wavelength of the atomic spectral line gives the element’s identity, while the intensity of the emitted light is proportional to the number of atoms of the element. This spectroscopy is a method of chemical analysis that uses the intensity of light emitted from a flame, plasma, arc, or spark to determine the quantity of an element in a sample. The study of atomic emission spectra is important in many areas of science, including astronomy, where it is used to identify the chemical composition of stars, and in chemistry, where it is used to identify unknown substances.
The molecular emission spectrum, known as molecular fluorescence, results from light emitted by molecules when excited and then return to their ground state. This spectrum is a unique pattern of wavelengths specific to the analyzed molecule. It composes a series of lines corresponding to the different energy transitions within the molecule. These lines can be used to identify the molecule and provide information about the electronic and vibrational states of the molecule. It has many practical applications, including studying chemical reactions, the detection of environmental pollutants, and medical diagnostics.
The spectrum arises from the light emission by a metal ion complex and a ligand when excited by the energy, typically in the form of light. Ligand is an ion or molecule that binds to a central metal ion or atom, typically forming a coordination complex. This spectrum represents a series of peaks and valleys corresponding to different energy levels within the complex. The specific wavelengths and intensities of the peaks and valleys provide information about the electronic structure of the complex and the nature of the chemical interactions between the metal ion and the ligand. It is commonly used in spectroscopy to study the properties of metal-ligand complexes, which play an important role in various chemical and biological processes.
An ion emission spectrum is a graphical representation of the wavelengths of light emitted by ions when the transition occur from higher energy levels to lower energy levels.
When an ion or a gas when excited by the energy, typically in the form of heat or electricity, its electrons can be excited to higher energy levels. As these excited electrons return to lower energy levels, they emit photons with specific energies and wavelengths. The spectrum shows a series of discrete lines corresponding to different energy levels within the ion or gas. The ion spectrum provides information about the electronic structure of the ion or gas. It is commonly used in spectroscopy to study the properties of ions and gases, which play an important role in various chemical and physical processes.
When cathode is heated or subjected to an electric field, the light emission by a cathode or a cathode material results in a cathode emission spectrum. This spectrum is characterized by a series of discrete lines corresponding to different energy levels within the cathode material. Each line in the spectrum corresponds to a specific transition between two energy levels, and the specific wavelengths of the lines provide information about the electronic structure of the cathode material. This type of spectrum is commonly used in spectroscopy to study the properties of cathode materials, which play an important role in various technological applications, including lighting, electronics, and energy storage. One example of a cathode emission spectrum is the spectrum of a tungsten filament, which is commonly used in incandescent light bulbs.
Glow discharge emission spectra refer to the emission spectrum generated by a low-pressure electrical discharge between two electrodes in a gas chamber. It is a series of discrete lines corresponding to different energy levels within the gas and the electrodes. Each line in the spectrum corresponds to a specific transition between two energy levels. The specific wavelengths of the lines provide information about the composition and properties of the gas and the electrodes. This type of emission spectrum is commonly used in spectroscopy to study the properties of gases and solids, which play an important role in various technological applications, including lighting, surface modification, and material synthesis. One example of a glow discharge emission spectrum is the spectrum of a neon gas discharge, used in neon signs.
An analytical technique produced by a high-temperature ionized gas in an inductively coupled plasma spectrometer. The ICP emission spectrum is characterized by discrete lines corresponding to different energy levels within the ionized gas. Each line in the spectrum corresponds to a specific transition between two energy levels. The specific wavelengths of the lines provide information about the composition and concentration of the elements. ICP emission spectra are commonly used in analytical chemistry to identify and quantify the elements present in a sample. The sample is introduced into the plasma, where it is vaporized and ionized, and the resulting emission spectrum is analyzed using a spectrometer. The intensity of the spectral lines is proportional to the concentration of the corresponding element in the sample, allowing for quantitative analysis. It is widely used in various fields, such as environmental monitoring, material analysis, metallurgy, geology, and biochemistry. It has the advantages of high sensitivity, low detection limits, and the ability to analyze a wide range of elements simultaneously.
When a gas, such as hydrogen or helium, is excited by an external energy source, such as an electrical discharge or a flame, it results in a line emission spectrum. In this spectrum, the gas emits light only at specific wavelengths, producing bright lines on a dark background. This spectrum contrasts with continuous emission spectra, which show a continuous range of colors. The sources of line emission spectra are sodium in the sodium vapor lamp, mercury in a mercury vapour lamp, and gases in a discharge tube. This type of emission spectra is important in many areas of science, including astronomy, where they are used to identify the chemical composition of stars and other celestial objects. They are also used in analytical chemistry to identify and quantify the elements present in a sample.
This light emission spectrum is generated when a solid or a dense gas, such as a star, emits light. In a continuous emission spectrum, the emitted light covers a broad range of wavelengths, producing a continuous rainbow of colors. This contrasts with other spectra, such as emission and absorption spectra, showing only certain wavelengths of light. The colors in a continuous spectrum are determined by the temperature of an emitting object and are independent of the characteristic of the source. The spectrum shifts to shorter wavelengths as the temperature increases, producing a blue color. It is referred to as blackbody radiation and is important in astrophysics and other fields where the temperature of distant objects needs to be determined based on the color of the light they emit. The spectrum contains all the colors from violet to red. Examples of sources of continuous spectra are Incandescent solids, liquids, Carbon arcs, electric filament lamps, etc.
A gas or vapor emits light in a range of wavelengths rather than at specific wavelengths, as in a line emission spectrum. These emissions are produced when molecules in the gas or vapor are excited and undergo changes in their vibrational and rotational energy levels. The resulting spectrum consists of broad bands of light rather than discrete lines and can have a complex structure with overlapping bands.
Band emission spectra are typically produced by molecules rather than atoms and are commonly observed in the spectra of gases such as nitrogen, oxygen, and carbon dioxide. Examples of band spectra are Calcium or Barium salts in a bunsen flame and gases like carbon dioxide, ammonia, and nitrogen in a molecular state in the discharge tube. This type of emission spectra is vital in atmospheric science, where they study the composition and behavior of the Earth's atmosphere. They are also used in industrial processes, such as laser technology and plasma physics, where the properties of gases and plasmas are essential.
Applications of Emission Spectrum
Emission spectra have a wide range of applications in science, from astronomy to chemistry and beyond. They are an invaluable tool for studying the composition and behavior of matter.
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