Luminescence is an emission phenomenon of light that does not result from high temperature. It can be said that it is a cold body radiation process. Luminescence is a light emission that represents an excess of thermal radiation and lasts for a time exceeding the period of an electromagnetic oscillation.

In luminescence, there are intermediate processes between absorption and emission duration which exceed the period of a single electromagnetic oscillation. A luminescent analysis is performed using the intrinsic Luminescence of materials under observation, or special markers called luminophores are added when the material itself does not demonstrate luminescent properties. 

There are numerous types of luminescence however two commonly used techniques are photoluminescence and chemiluminescence. 

Photoluminescence is the exhibition of luminescence by the absorption of photons. This phenomenon is further divided into fluorescence and phosphorescence.

Chemiluminescence is the emission of light as a result of a chemical reaction in the analyte. This also can be further divided into- bioluminescence and electrochemical luminescence.

Process of Luminescence

Pauli’s Exclusion Law explains how the Electrons in molecular orbitals are paired. When an electron absorbs enough energy it will be excited to a higher energy state, but will keep the orientation of its spin.

The molecular electronic state in which electrons are paired is called a singlet transition whereas the molecular electronic state in which the two electrons are unpaired is called a triplet state.

When there is a transition of electrons from ground states to these singlet and triplet states, there is the emission of light from the analyte molecules, which is called luminescence. 

When there is a transition of electrons between two singlet states, it is called fluorescence. On the other hand, the transition of electrons between two triplet states is called phosphorescence. 

Whenever electromagnetic radiation interacts with matter there are a number of phenomena occurring as a result. Fluorescence and phosphorescence are the two of them. Other phenomena are absorption, vibrational relaxation, internal and external conversions, and intersystem crossings. 

All these phenomena are the deactivation processes in which an excited molecule returns to the ground state by minimizing the lifetime of the electronic state.

What is Luminescence Spectroscopy?

Luminescence spectroscopy is a term given to three types of spectroscopy, collectively. They are:

  • Molecular fluorescence spectroscopy
  • Molecular phosphorescence spectroscopy
  • Chemiluminescence spectroscopy

The above three spectroscopic methods have a slight difference in the detection principles but the instrumentation is almost similar for these methods. 

Instrumentation of Luminescence Spectroscopy

1. Radiation Source

The most basic radiation source is the xenon arc lamp, mercury lamp, or laser. The radiation source produces the light beam of wavelengths 230-1000 nm, across a wide range of UV-visible-IR energies. 

2. Monochromators

The light from the radiation source is passed through the monochromator which converts the polychromatic light into the desired monochromatic light. This can be achieved by employing filters and diffraction gratings. The monochromator diffracts the light and disperses it into different wavelengths. 

3. Sample Cell

A sample cell or cuvette made up of quartz is used to hold the sample. It is preferred over glass as the quartz can pass both UV and visible radiations through it.

4. Detectors 

The light emitted by the sample is collected perpendicular to the excitation, to reduce the excitation light being detected. The light is passed through another monochromator, and the intensity of light at each wavelength is recorded, as the monochromator rotates and scans through a range of wavelengths (relating to a range of angles of the grating), to create a spectrum.

The intensity of the light is detected by the detectors. The most commonly used detectors are photomultiplier tubes and charge-coupled devices. 

The above instrument is known as a spectrofluorometer and is dedicated to fluorescence. 

While for phosphorescence a rotating shutter called a phosphoroscope (a mechanical device) is fitted in the instrument which separates phosphorescence from fluorescence. Also, the sample system is maintained at liquid nitrogen temperature. A Dewar flask with quartz windows is often used to hold the sample.

In chemiluminescence, no radiation source is required as the luminescence is produced by the reaction of an analyte and a strongly oxidizing reagent gas such as fluorine or ozone, etc. The reaction occurs on a time scale such that the production of light is essentially instantaneous; therefore, most analytical systems simply mix analytes and the reagent in a small volume chamber directly in front of a PMT. 

Advantages of Luminescence Spectroscopy

  • It is a highly sensitive and specific technique.
  • A small sample is required for the analysis.
  • It is a quantitative technique.

Limitations of Luminescence Spectroscopy

  • The sample preparation is quite time-consuming.
  • In phosphorescence spectroscopy temperature maintenance is difficult. The phosphorescence lifetime is too short and it is quite difficult to separate phosphorescence from other simultaneous phenomena occurring due to the absorption of light. 
  • In chemiluminescence, the sample should be in a gaseous phase. 
  • The spectrum bands are quite wide compared to atomic spectra.

Applications of Luminescence Spectroscopy

  • Luminescence can be used in the analysis of metallic poisons.
  • In forensic serology, the concentration of metals in biological fluids and other biochemical reactions can be traced by this spectroscopy.
  • It is applicable for the determination of metals in hair and fiber samples.
  • Metallic constituents in petrochemical products, various dyes, and coloring agents or powders can be easily determined by this technique.
  • Ink analysis and questioned document examination can be performed.

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Frequently Asked Questions

1. What is Luminescence in Molecular Spectroscopy?

It is an emission of light by a substance not resulting from heat therefore is also known as cold-body radiation. The reason for these emissions could be chemical reactions, motions by atoms, or electrical energy.

2. What is The Formula For Luminescence?

L = L0 exp (−t/τ)

Here, L is the intensity of emission at a time,
L0 is the initial intensity of emission at a time,
t is the time and,
τ is decay time of luminescence

3. Difference Between Luminescence & Fluorescence?

Luminescence is a mechanism in which heat is not required to generate photons while Fluorescence is a mechanism where a higher energy photon is required to generate a photon.


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