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IR Spectroscopy: Molecular Vibration Overview

IR Spectroscopy: Molecular Vibration Overview

When Infrared (IR) radiation passes through a covalently bonded molecule, the bonds transition from lower to higher vibrational levels. The fundamental vibrational motions that result in infrared absorption can be classified as stretching or bending vibrations.
Stretching vibrations are vibrational motions that occur along the bond line, changing the bond length or distance between two bonded atoms. They are further distinguished as symmetric or asymmetric. In symmetric stretching, the...

Infrared (IR) Spectroscopy: Overview

Infrared (IR) Spectroscopy: Overview

When electromagnetic radiation passes through a material, atoms or molecules transition from a lower to a higher energy state by absorbing radiation corresponding to the energy difference between the two states. The absorption of infrared (IR) radiation causes transitions between vibrational energy levels in a molecule. Therefore, IR spectroscopy is a useful analytical tool for determining the molecular structure of molecules.
Different compounds display unique properties due to their...

IR Absorption Frequency: Hybridization

IR Absorption Frequency: Hybridization

Hydrocarbons such as alkanes, alkenes, and alkynes show characteristic C–H stretching absorption bands. These IR stretching frequencies depend on the hybridization of the involved carbon atom and can be explained in terms of the s character of each hybridized atomic orbital.
Among the sp, sp2, and sp3 hybridized orbitals, sp orbitals have the maximum s character (50%). Consequently, the electrons are held more closely to the nucleus, resulting in stronger and shorter C–H bonds that stretch at a...

IR Spectroscopy: Hooke's Law Approximation of Molecular Vibration

IR Spectroscopy: Hooke's Law Approximation of Molecular Vibration

A covalently bonded heteronuclear diatomic molecule can be modeled as two vibrating masses connected by a spring. The vibrational frequency of the bond can be expressed using an equation derived from Hooke's law, which describes how the force applied to stretch or compress a spring is proportional to the displacement of the spring. In this case, the atoms behave like masses, and the bond acts like a spring.
According to Hooke's law, the vibrational frequency is directly proportional to the...

IR Absorption Frequency: Delocalization

IR Absorption Frequency: Delocalization

Electron delocalization refers to the distribution of electrons across multiple atoms within a molecule rather than being confined to a single atom or bond. This phenomenon is common in systems with conjugated bonds—structures where alternating single and double bonds allow π-electrons to move freely across the network. The movement of electrons stabilizes the molecule and can affect various chemical properties, including vibrational frequencies observed in IR spectroscopy.
In IR spectroscopy,...

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Related Experiment Video

Updated: May 16, 2026

Characterizing Far-infrared Laser Emissions and the Measurement of Their Frequencies

Characterizing Far-infrared Laser Emissions and the Measurement of Their Frequencies

Published on: December 18, 2015

Subkilohertz linewidth room-temperature mid-infrared quantum cascade laser using a molecular sub-Doppler reference.

F Cappelli¹, I Galli, S Borri

¹CNR-INO-Istituto Nazionale di Ottica, Firenze 50125, Italy. francesco.cappelli@ino.it

|December 4, 2012

Summary

We narrowed a room-temperature mid-infrared quantum cascade laser

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Area of Science:

Quantum optics
Laser physics
Spectroscopy

Background:

Room-temperature mid-infrared quantum cascade lasers (QCLs) are crucial for various applications.
Achieving narrow linewidths and frequency stabilization in these lasers remains a challenge.

Purpose of the Study:

To narrow the linewidth and stabilize the frequency of a room-temperature mid-IR quantum cascade laser.
To demonstrate frequency locking to a CO2 sub-Doppler transition.

Main Methods:

Utilized polarization spectroscopy to obtain a CO2 sub-Doppler transition.
Employed frequency locking techniques to stabilize the QCL.
Achieved a locking bandwidth of 250 kHz.

Main Results:

Narrowed the laser linewidth by over two orders of magnitude, achieving sub-kHz linewidth.
Stabilized the absolute frequency of the laser to sub-kHz precision.
Demonstrated effective frequency locking to the CO2 transition.

Conclusions:

Frequency locking to a CO2 sub-Doppler transition is an effective method for narrowing and stabilizing mid-IR QCLs.
The achieved sub-kHz linewidth and frequency stability open possibilities for high-precision spectroscopy and sensing.