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Molecular Spectroscopy

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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:

  1. Pure rotational spectra

  2. Vibrational rotational spectra

  3. 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

Q1: List the Types of Spectroscopy.

Ans: The types of spectroscopy are:

Acoustic resonance



X-ray photoelectron

Circular Dichroism

IR Spectroscopy (Infrared spectroscopy)

Raman spectroscopy

Q2: What is Raman Spectroscopy?

Ans: Raman Spectroscopy is named after our renowned scientist Sir C.V. Raman. 

Raman Spectroscopy is a technique that we use to analyze/evaluate the vibrational, rotational, and other low-frequency modes in a system. 

We use this technique commonly in the branch of chemistry to provide a fingerprint by which we can identify the molecules. 

This phenomenon relies on the inelastic scattering of monochromatic light a.k.a Raman scattering. 

The energy of the laser photons moves up and down because of the interaction of the light with the molecules or phonons of an object. This up-down movement of laser photons results in the vibrational modes of an object/system.

However, other advanced types of Raman spectroscopy are:

Surface-enhanced Raman

Resonance Raman

Tip-enhanced Raman

Polarized Raman

Stimulated Raman (similar to stimulated emission)

transmission Raman

Spatially offset Raman

Hyper Raman

Q3: What is the Relationship Between the Energy (e) and Frequency (v) of Electromagnetic Radiation?

Ans: The fundamental discoveries of Max Planck explain the emission of light by a blackbody radiator.

Further, Albert Einstein explains the observations in the photoelectric effect. His observation led to the realization that the energy of electromagnetic radiation varies proportionally with the frequency. 

The proportionality expression gets converted to equality via the use of Planck’s constant, which is given as;

                         E = hv