Spectroscopy Meaning
Spectroscopy (a noun) is a branch of science that concerns the investigation and measurement of spectra produced when matter interacts with or gives off electromagnetic radiation.
In simple terms, spectroscopy is the dispersion of light into component colours.
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We find the application of spectroscopy in Beer-Lambert’s law.
In this law, we use electromagnetic spectroscopy to find the quality and quantity of chemicals in a drug.
Here, we are studying atomic and molecular spectroscopy. When atoms and molecules absorb or radiate electromagnetic radiation, we call this study atomic and molecular spectroscopy, respectively.
On this page, we will understand the atomic and molecular spectroscopy with the types of molecular spectroscopy in detail.
Atomic Spectroscopy Meaning
Electrons exist in various energy levels within an atom. We know that atomic orbitals are discrete in number; also, they have discrete energy levels.
When electrons oscillate between these orbitals, they absorb or emit energy.
The emitted energy is equal to the energy difference between their atom's specific discrete orbital energy levels.
Optical Spectroscopy
In optical spectroscopy, an electron absorbs energy to reach the higher energy level or the higher orbital.
Further, electrons emit energy when it comes back to a lower energy level or a lower orbital.
Wherefore, electrons absorb or emit energy in the form of photons (light particles).
Since each element has a unique number of electrons; therefore, an atom absorbs/releases energy in a pattern unique to its elemental identity, for instance, Ca, Na.
Thus the absorption and emission of photons also occur in a unique pattern.
The identification of the type of atoms present in a sample or the number of atoms present in a sample can be done by measuring changes in light wavelength and light intensity.
Types of Optical Spectroscopy
Optical spectroscopy further divides itself into the two following parts:
Atomic absorption spectroscopy
Atomic emission spectroscopy
(We have discussed these two in the above context).
Molecular Spectroscopy
From the above text, we understand that likewise atoms, molecules also oscillate from a lower energy level to a higher energy one, and vice versa.
Do You Know?
Atomic and molecular spectrum in the VUV (Vacuum Ultraviolet) region is a vital supporter of the experimental features of the structures of excited electronic states.
These spectra also have significance in identifying species that participate in physical and chemical processes in environments ranging from terrestrial to astrophysical.
What is a Molecule?
A molecule is a bundle of positively charged atomic nuclei surrounded by a cloud of electrons.
As a rule, molecular stability is the result of the balance among the attractive and repulsive forces of the nuclei and electrons.
A molecule is featured by the total energy that results from these interacting forces. The case is similar to atoms, in molecules, the allowed energy states are quantized.
Further, we will give an Introduction to Molecular Spectroscopy with the types of molecular spectroscopy:
What is Molecular Spectroscopy?
We know that when a matter is exposed to electromagnetic radiation, the electromagnetic spectrum of a series of wavelengths produces.
Molecules absorb certain wavelengths to the higher electronic, vibrational, and rotational energy levels.
So, the series of wavelengths, a molecule absorbs gives off a distinct molecular spectrum. A distinct molecular spectrum lies in the specific region of the electromagnetic spectrum.
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Types of Molecular Spectra
The three types of molecular spectra are:
Pure rotational spectra
Vibrational rotational spectra
Electronic band spectra
Pure Rotational Spectra
When a molecule absorbs a lower amount of energy that it makes a transition from one rotational level to another within the same vibrational level.
Rotational spectra are observable in the spectral region of Far Infrared and Microwaves.
Also, the energies in these spectral regions are exceedingly small. Therefore, we call rotational spectra the microwave spectra.
Vibrational Rotational Spectra
When a molecule absorbs sufficient energy that causes the transition of a molecule from one vibrational level to another within the same electronic level.
Therefore, in this case, both rotational and vibrational transition takes place. This is how we obtain vibrational rotational spectra.
The vibrational spectra are observable in the Near-Infrared Spectral region. We call the vibrational rotational spectra the Infrared spectra.
Electronic Band Spectra
When the exciting energy of the radiation is large enough to cause the transition of a molecule from one electronic level to another electronic level. This transition is accompanied by both rotational and vibrational level changes.
Additionally, for each vibrational transition, a set of closely spaced lines appear.
Because of the corresponding rotational level changes, these closely spaced lines are known as bands. Hence, we call it the Electronic band spectra.
Molecular Spectroscopy Stages
The certainty of the molecular spectra resides on the availability of nuclear degrees of freedom within the target, which will be excited by the projectile and participate within the decay process. The final possible results of a collision between a molecule (M) and a photon are:
M + ħω
→ Mn + ne- (Ionization)
→ M* + ħω’ (resonance fluorescence)
→ A+ + B (neutral dissociation)
→ A+ + B- (ion-pair formation)
FAQs on Molecular Spectroscopy
1. What is molecular spectroscopy?
Molecular spectroscopy is a scientific technique used to study the interaction between electromagnetic radiation and matter at a molecular level. By analysing the spectrum of light that a molecule absorbs, emits, or scatters, scientists can determine its chemical structure, composition, and physical properties. It essentially provides a unique 'fingerprint' for each type of molecule.
2. What is the fundamental principle behind molecular spectroscopy?
The fundamental principle of molecular spectroscopy is based on the concept of quantized energy levels. Molecules can only exist in discrete energy states (rotational, vibrational, and electronic). When a molecule absorbs a photon of light, it gets excited to a higher energy state. The energy of the absorbed photon must exactly match the energy difference between the two states. This selective absorption of specific wavelengths creates a unique spectrum that can be analysed.
3. What are the main types of molecular spectra?
The main types of molecular spectra are classified based on the energy transitions involved:
- Pure Rotational Spectra: Caused by transitions between rotational energy levels within the same vibrational state. These are observed in the far-infrared or microwave region.
- Vibrational-Rotational Spectra: Caused by transitions between vibrational levels, which are also accompanied by changes in rotational levels. These are observed in the infrared region.
- Electronic Band Spectra: Caused by transitions between electronic energy levels. These high-energy transitions are accompanied by both vibrational and rotational changes and are seen in the visible and ultraviolet (UV) regions.
4. What is the difference between atomic and molecular spectroscopy?
The primary difference lies in the complexity of the spectra produced. Atomic spectroscopy involves individual atoms and shows sharp, discrete spectral lines because it only deals with electronic transitions. In contrast, molecular spectroscopy produces broad bands of closely spaced lines. This is because molecules have additional vibrational and rotational energy levels, leading to a much larger number of possible transitions for each electronic jump.
5. How do rotational, vibrational, and electronic transitions differ in terms of energy?
The energy required for each type of transition follows a clear hierarchy:
- Rotational transitions require the least amount of energy and involve changes in the molecule's rotational motion.
- Vibrational transitions require more energy and involve changes in the bond vibrations within the molecule.
- Electronic transitions require the most energy, as they involve exciting an electron from one orbital to another. An electronic transition is typically accompanied by simultaneous vibrational and rotational transitions.
6. Why are molecular spectra observed as bands instead of discrete lines like atomic spectra?
Molecular spectra appear as bands because each electronic energy level in a molecule has a set of associated vibrational energy levels, and each of these vibrational levels has its own set of rotational energy levels. When a molecule undergoes an electronic transition (a big jump), it can land in any of the numerous vibrational and rotational levels of the higher electronic state. This results in a multitude of transitions with very slightly different energies, which appear as a broad band rather than a single, sharp line.
7. What are some real-world applications of molecular spectroscopy?
Molecular spectroscopy is a versatile tool with numerous applications, including:
- Chemical Analysis: Identifying unknown substances and determining the concentration of components in a mixture, such as in quality control for pharmaceuticals.
- Environmental Monitoring: Detecting and measuring pollutants in the air, water, and soil.
- Astrophysics: Determining the chemical composition of distant stars, galaxies, and planetary atmospheres.
- Medical Diagnostics: Analysing biological samples like blood and tissue to detect diseases or monitor metabolic processes.
8. What is Raman Spectroscopy and how is it different from Infrared (IR) Spectroscopy?
Raman Spectroscopy is a technique that analyses the inelastic scattering of monochromatic light (usually a laser) by a molecule to study its vibrational modes. It is different from Infrared (IR) Spectroscopy in its core mechanism:
- Mechanism: IR spectroscopy measures the direct absorption of light that matches the molecule's vibrational frequency. In contrast, Raman spectroscopy measures the light that is scattered with a change in energy.
- Complementary Information: The selection rules for the two techniques are different. Some vibrations are 'IR active' while others are 'Raman active'. Therefore, using both methods provides a more complete picture of a molecule's vibrational structure.
