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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 Spectrum

IR Spectrum

When infrared (IR) radiation passes through a molecule, the bonds stretch or bend by absorbing the radiation. This absorption creates the molecule's absorption spectrum, which is the plot of its percentage transmittance versus wavenumber.
Transmittance is defined as the ratio of the radiant power passing through a sample to that from the radiation's source. Multiplying the transmittance by 100 gives the percent transmittance (%T), which varies between 100% (no absorption) and 0% (complete...

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 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...

Molecular Spectroscopy: Absorption and Emission

Molecular Spectroscopy: Absorption and Emission

Molecules possess discrete energy levels called quantum states. Unlike atoms, which have simpler energy levels, molecules possess additional rotational and vibrational energy levels. Each energy level is separated by an energy gap, with the gaps between adjacent electronic, vibrational, and rotational levels varying significantly. The three types of energy levels in a diatomic molecule are shown in Figure 1.

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...

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Related Experiment Video

Updated: Jun 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

On interpretation of infrared absorption lines.

|February 2, 2010

Summary

This study presents a universal contour method for interpreting infrared absorption spectra under low pressure and long path conditions. It offers a way to correct for wing absorption and validate the Lorentzian line shape assumption.

Area of Science:

Spectroscopy
Physical Chemistry
Atmospheric Science

Background:

Infrared (IR) absorption spectra are crucial for analyzing molecular composition.
Accurate spectral interpretation requires robust methods, especially under specific conditions like low pressure and long path.
The Lorentzian line shape is a common assumption, but its validity needs verification.

Purpose of the Study:

To develop and present interpretation procedures for IR absorption spectra.
To introduce a universal absorption contour method for spectral analysis.
To provide a means for correcting wing absorption and validating spectral line shape assumptions.

Main Methods:

Utilizing a universal absorption contour for spectral interpretation.
Measuring central dip and slopes of observed line contours.

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Developing an expression to correct for wing absorption.

Employing the spectral line itself as a chopper to test the Lorentzian line shape assumption.

Main Results:

A method for interpreting low-pressure, long-path IR spectra is established.
Quantification of spectral line features (central dip, slopes) is used for analysis.
An expression for correcting wing absorption is derived.
A novel technique for validating the Lorentzian line shape assumption is described.

Conclusions:

The universal absorption contour provides a framework for accurate IR spectral interpretation.
The developed methods enhance the reliability of spectral data analysis.
The study offers practical tools for researchers working with IR spectroscopy.