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Molecular Spectroscopy

Spectroscopy involves interaction of energy of radiation with matter which is used in physical and analytical chemistry. Molecular spectroscopy is used for identification of the compound and its molecular properties.

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Molecular Luminescence Spectroscopy

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Luminescence is the phenomena of emission of light by a substance. The emission of light is done with loss of excess energy in the form of photon when an electron returns to the electronic ground state from an excited state.

Luminescence spectroscopy is related to three spectroscopic techniques which are,
  1. Molecular fluorescence spectroscopy
  2. Molecular phosphorescence spectroscopy
  3. Chemiluminescence spectroscopy

Molecular Fluorescence Spectroscopy

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Most of the organic molecules have two electronic states. The first is singlet state in which all molecular electrons are in spin-paired state and second is triplet state in which one set of electron spins is unpaired.

Molecular Fluorescence Spectroscopy
It shows the optical emission due to absorption of electromagnetic radiation. The fluorescence detection is of the greater sensitivity because the fluorescence signal has in principle a zero background.

A molecule is excited to one of the vibrational level in the electronic excited state from a vibrational level in the electronic ground state by absorption of UV radiation. This is called first excited singlet state.

Due to the instability of the molecule in the first excited singlet state, it will fall rapidly to the lowest vibrational level of this state by losing energy to other molecules through collision. The molecule can also lose this excess energy to other possible modes of vibration and rotation. Thus, the fluorescence occurs when the molecule comes back to the electronic ground state from the excited singlet state by losing excess energy in the form of emission of a photon. The non-fluoresce molecules lose their energy to some other mode which is known as radiation-less transfer of energy.

Excitation of electron between energy levels


Distribution of energy between possible electronic and vibrational states

The excess energy of the molecule is converted to vibrational energy which is called the internal conversion. This excess vibrational energy, lost by collision with other molecules, is known as vibrational relaxation. This conversion of energies is especially helpful for "loose and floppy" molecules as they can reorient themselves in ways which aid the internal transfer of energy.

A combination of intra and inter-molecular energy redistribution

If the molecule is excited to an excited triplet state due to reversed spin of the electron, this is called inter system crossing. As the triplet state is a state of lower electronic energy than the excited singlet state. The possibility of the excitation of molecule to excited triplet state is high when the vibrational levels of these two states overlap with each other. For example, the lowest singlet vibrational level can overlap with one of the highest vibrational levels of the triplet state. The excess energy of molecule in the excited triplet state is lost by collision with solvent molecules and returns to the lowest vibrational level of the triplet state. It can also undergo a second inter system crossing to a high vibrational level of the electronic ground state. Finally, the molecule comes back to the lowest vibrational level of the electronic ground state by vibrational relaxation.

Molecular fluorescence spectroscopy is widely used in the studies of electronic properties of organic and inorganic molecules in all existing phases, in chemical analysis of various samples, in forensic, environmental, biological and also in industrial operations. It's also used in quantitative measurements of molecules, in solution and fluorescence detection, and in liquid chromatography.
 
Instrumentation

The fluorimeter is used to measure the fluorescence. The excitation source, sample cell, fluorescence detector are the main parts of Fluorimeter. Usually a deuterium or xenon lamp is used for excitation of molecules in solution.
A t-broad-band excitation light passes through a monochromator which allows passing of the light of only a selected wavelength. The fluorescence is detected by a photomultiplier tube. The excitation spectrum and the fluorescence spectrum are found by scanning of the excitation monochromator and the fluorescence monochromator respectively. The pass filters are also used to select the excitation wavelength in simple instruments.

Molecular Phosphorescence Spectroscopy

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The inter system crossing may not be always used by a molecule in the excited triplet state to return to the ground state. The transition from triplet to singlet is very less in existence than a singlet- singlet transition because the lifetime of the excited triplet state is up to 10 seconds in comparison with 10-5 s to 10-8 s average lifetime of an excited singlet state.

Thus, the loss of energy can be done by emission of a photon. So, the loss of energy in the form of photon is know as phosphorescence. Internal conversion and radiation less transfers of energy can be successfully done with phosphorescence at low l.00/ temperatures or in highly viscous media.

Molecular Absorption Spectroscopy

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These spectroscopic techniques are used to measure the absorption of radiation in terms of frequency or wavelength. The sample absorbs energy in the form of photons from the radiating field. The intensity of the absorption varies with frequency. The variation in the intensity is known as the absorption spectrum. Absorption spectroscopy is related to the electromagnetic spectrum.

Various experimental techniques are used to measure absorption spectra but the common arrangement is the one in which a beam of radiation passes through a sample and the intensity of the radiation is measured. This transmitted intensity can be used to calculate the absorption. The correct choice of the source, a sample arrangement and detection technique depends on the frequency range and the aim of the experiment. Thus, the absorption spectroscopy is used in analytical chemistry to detect the presence and amount of a particular substance in a sample, in astronomy (astronomical spectroscopy), in remote sensing, and to study molecular physics.

Absorption spectrum


Absorption spectrum of any material is absorbed radiation of different frequencies, which is determined by the atomic and molecular composition of the material. Only those radiation of particular frequencies are generally absorbed which match with the energy difference between two quantum mechanical states of the molecules. This absorption between two states is known as an absorption line and a spectrum has many lines.

The frequencies and intensities of absorbed radiations depend on the electronic and molecular structure of the molecule, also on the interactions between molecules, and on many factors like temperature, pressure, electromagnetic field. The width and shape of the lines are measured by the spectral density. The substance such as H2, CO2 and KMnO4 give band absorption spectrum and atomic state of substance generate line absorption spectra.

The classification of absorption lines depends on the nature of the quantum mechanical change in the molecule or atom.
  1. Rotational lines - These lines are generated due to change in the rotational state of a molecule. For example, microwave spectral region.
  2. Vibrational lines - These lines are generated due to change in the vibrational state of the molecule and are generally found in the infrared region.
  3. Electronic lines - These lines are generated due to change in the electronic state of an atom or molecule and are found in the visible and ultraviolet region. X-ray absorptions are related to rotation-vibration transitions.

The frequency of the absorption is determined by the energy of the quantum mechanical change line but the frequency can vary with many interactions. For example- electric and magnetic field, interactions with neighboring molecules etc. The spectrometer is used to record the width of absorption lines. It depends on the environment of the absorber. Both liquid and solid absorber have broad absorption lines than a gas due to strong interaction of neighboring molecules with one another in liquid or solid absorber. The line width will also tend to increase with increase in the temperature or because of the pressure of the absorbing material.

Absorption and transmission spectra both show the same information. A transmission spectrum will have its maximum intensities at wavelengths whereas the absorption is weakest as more light is transmitted through the sample.

Absorption and Emission Spectra

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In the emission spectra, substance releases energy in the form of electromagnetic radiation. Both the absorption and emission spectra can occur at same frequency, so the absorption lines can be easily determined from an emission spectrum. The emission spectrum will have a different intensity pattern from the absorption spectrum so they are not same. The one can be calculated from other using appropriate theoretical models and information about the quantum mechanical states.

The scattering and reflection spectra- The approximation absorption spectrum also can be derived from a scattering or reflection spectrum which requires assumptions or models.

Instrumentation

The most commonly used instrument to measure the absorption spectra is the one in which radiation is generated with a source and a detector is used to measure the spectrum of that radiation. Then the sample spectrum is measured with placing the material between the source and detector. These two measured spectra are combined to determine the material's absorption spectrum. The combination of these two spectra is used because individually these spectra are affected by the environmental conditions. Moreover, individually these also have effects on the source and the detectors that are used to measure them.

Various types of sources are used to measure the spectrum like black body sources in the infrared, mercury lamps, synchrotron radiation for all spectral regions, etc. The measurement of the radiation power of detector depends on wavelength range, so, the detectors like heterodyne receivers, bolometers, cooled semiconductor detectors, etc., are used. Generally, a spectrograph is used to separate the wavelengths of radiation for correct measurement of the power of each wavelength. The Beer-Lambert Law is used to measure the concentration of an absorbing species in a sample.

Chemiluminescence Spectroscopy

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It occurs when an electronically excited species, produced in a chemical reaction, emits a photon as its excess energy. These types of reactions are seen in biological systems which are known as bio luminescence. Especially it’s used in the determination of nitric oxide.

NO + O3 NO2* + O2
NO2*, NO2 + hv (l = 600 - 2800 nm)

Beer-Lambert Law

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It is based on the linear relationship between absorbance and concentration of an absorbing species. The general equation of Beer-Lambert law is written as:

A = a($\lambda $)* b * c

where A is the measured absorbance, a($\lambda $) is an absorptivity coefficient which depends on wave-length, b is the length of path, and c is the concentration of analyte.

If the concentration units is molarity then Beer-Lambert law is written as

A = $\epsilon $ x b x c

where epsilon $\epsilon $ is the molar absorptive coefficient, depends on wavelength and units is M-1 cm-1.

In the absorption spectrometer, a whole series of wavelengths of light passes through a solution of a substance of the sample cell and also through the reference cell. Thus the intensity of the light passing through the reference cell is
measured, as Io. "I" is the intensity of the light passing through the sample cell.

Absorption Spectrometer

So transmittance (T) = $\frac{I}{I_{o}}$, where I is the intensity of light passes through the sample and Io is the initial light intensity.

A = - log T = - log $\frac{I}{I_{o}}$

Derivation of the Beer-Lambert law


The derivation of Beer-Lambert law is based on the approximation of the absorption coefficient of a molecule. This is done by approximation of the molecule with the use of an opaque disk. The cross-sectional area of an opaque disk is σ and the frequency of photon is w. The area is a maximum if w is close to resonance but it's approximately equal to 0 if the frequency of the light is far from resonance.

Opaque Disk
Where,
Io = the intensity of light entering in the sample at z = 0,
Iz =the intensity of light entering the infinitesimal slab at z,
dI is the intensity of absorbed light of the slab, and
I is the intensity of light which cross through the sample

So, the total opaque area on the slab due to the absorbers = $\sigma $ *N * A * dz

The fraction of absorbed photons = $\sigma $* N * A * dz / A so,
dI/ Iz = - $\sigma $ *N * dz
After taking Integrating in the limit from z = 0 to z = b,
ln(I) - ln(Io) = - $\sigma $*N * b
- ln$\frac{I}{I_{o}}$ = $\sigma $ *N * b.

As N (molecules/cm3) x (1 mole / 6.023x1023 molecules) x 1000 cm3 / liter = c (moles/liter)
Or log $\frac{I}{I_{o}}$ = $\sigma $ *(6.023x1020 / 2.303) * c *
Or log $\frac{I}{I_{o}}$ = A = $\epsilon $ *b * c

So $\epsilon $ = $\sigma $ (6.023x1020 / 2.303) = $\sigma $ (2.61x1020)

Limitations of the Beer-Lambert law

There are some limitations in the linearity of the Beer-Lambert law. There are some reasons for the non linearity which are given below:
  1. Due to disorder in the absorptivity coefficients at high concentrations or less than 0.01M which is because of electrostatic interactions between molecules with closeness.
  2. Light scattering can be the reason because of particulates present in the sample.
  3. Fluorescence or phosphorescence and non-monochromatic radiation also can be the reasons.
  4. The deviations in refractive index at high analyte concentration, the stray light, shifting in chemical equilibria, the function of concentration etc. may be cause of the non linearity.

Raman Scattering

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Raman scattering also known as Raman Effect shows the inelastic scattering of a photon. The blue color of the sky is due to inelastic scattering. The incident photons can interact with the molecules to gain or lose the energy so that the frequency of scattered photons is shifted. This inelastic scattering is known as Raman scattering.

Stock and Anti Stock Lines


The Raman scattering depends upon the polarizability of the molecules just like Rayleigh scattering. The incident photon energy can excite vibrational modes and make scattered photons. These scattered photons are diminished in energy by the amount of the vibrational transition energies. The analysis of the scattered light under these circumstances will show spectral lines below the Rayleigh scattering peak at the incident frequency.

These spectral lines are known as stokes lines. But some excitations to vibrational excited states of the scattering molecules make the scattering at frequencies above the incident frequency because the vibrational energy is added to the incident photon energy. These lines are known as anti-Stokes lines which are weaker then stock lines. The molecules which have no net dipole moment, do not give pure rotational spectrum because the Raman Effect depends upon the polarizability.

Raman Amplification


The stimulated Raman scattering (SRS) is used to obtain Raman amplification. Stimulated Raman scattering (SRS) is a combination of a Raman process with stimulated emission. It is used in telecommunication fibers. The broadness of amplification band can be up to 100 nm which depends on the availability of allowed photon states. The rotational transitions of the scattering molecule are also involved with Raman scattering. In the two-photon process, the excess energy is released in the form of a photon of lower energy. So, the selection rule is DJ = +/-2 for rotational Raman transitions.

Raman Spectrum Generation


To produce broad band width spectra for high-intensity continuous wave lasers, SRS is used. The initial Raman spectrum is generated with spontaneous emission and then with amplifier. The higher-order Raman spectra can be obtained by using the Raman spectrum at high pumping levels in long fibers with building a chain of new spectra with decreasing amplitude.

Applications


An intense monochromatic light source (laser) can be used for scattered light in Raman scattering in which one or more "side-bands" are present with offset by rotational and or vibrational energy differences. This set up is very useful for remote sensing. Raman scattering is used for identification of mineral forms on Mars, remote sensing, remote monitoring for pollutants etc. Such remote sensing could become a major tool in planetary exploration.

For example, a laser beam scattering, directed on the plume from an industrial smokestack, can be used to evaluate the effluent for levels of molecules which will produce recognizable Raman lines. Raman spectrum is used for materials analysis, high complex materials, human tissues, detecting high frequency photon etc.

Emission Spectroscopy

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It's a kind of chemical analysis method in which the quantity of an element in a sample is analyzed by emitted intensity of light from a flame, a spark, etc., at a particular wavelength. The identity of the element is determined by wavelength of the atomic spectral line and the intensity of the emitted light is for the number of atoms in the element.

Flame Emission Spectroscopy

Flame Emission Spectroscopy


In this emission spectroscopy, a sample of a material is taken in the contact with flame in the form of gas or sprayed solution.
The solvent evaporates from the flame heat and free atoms are produced. These atoms get excitation to excited electronic states with the absorption of thermal energy and emit excess energy in the form of light of a particular wavelength, back to the ground electronic state. This wavelength is detected in the spectrometer. The emission measurement with the flame is mostly used for alkali metals.

Inductively coupled plasma atomic emission spectroscopy


In this type of emission spectroscopy, excited atoms and ions are produced by using inductively coupled plasma. These atoms or ions emit electromagnetic radiation at particular wavelengths. These emission spectroscopies give a stable and reproducible signal and also have multi-element capability but it's very expensive to operate.

Spark and arc atomic emission spectroscopy


This is especially used for metallic elements in solid. In this method, a spark is passed through the destroyed grounded sample. Thus the excited atoms emit light of particular wave length which is detected from monochromator. Generally, modern spark sources with controlled discharges under an argon atmosphere are used to control the spark conditions in quantitative analysis.
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