Spectral broadening is the spreading out of frequencies in light or signals from a narrower range to a wider range. This leads to the transformation of sharp, discrete spectral lines corresponding to transitions between energy states of atoms, molecules, or ions into broader and less defined features. The broadening can occur due to several reasons, including the natural linewidth of atoms or molecules, the Doppler effect, collisions, and external perturbations such as temperature and pressure. 

Homogeneous and inhomogeneous broadening are two types of spectral broadening phenomena that arise due to different physical mechanisms.

Figure 1: Spectral Broadening

Homogeneous Broadening

Homogeneous broadening is the broadening of a spectral line (representing an atomic transition) where all atoms or particles emit or absorb radiation with the same frequency. This uniformity of frequencies results in symmetrically broadened spectral lines, where all the particles experience identical effects, such as temperature, pressure, or an applied magnetic field.

In homogeneous broadening, all the atoms or particles experience the same external conditions, such as temperature, pressure, or an applied magnetic field. These uniform external perturbations cause the energy levels of the particles to be affected in a consistent manner, leading to identical transition energies.

Consequently, during their energy level changes, particles emit or absorb radiation at the same frequency. This uniformity in emitted or absorbed frequencies causes the spectral lines to broaden by the same amount, resulting in a symmetric line shape.

The key to homogeneous broadening is the coherence among the particles, where they all respond similarly to the external influences, leading to collective, synchronized behavior of their spectral responses. Homogeneous broadening is commonly observed in situations where particles interact frequently or are subject to similar environmental conditions, such as in dense gases, liquids, and certain solid-state systems at moderate temperatures.

Figure 2: Homogeneous Broadening

The broadening phenomenon can be described by a Lorentzian line shape, which exhibits a symmetric, bell-shaped curve that extends infinitely along the frequency axis. This line shape is characterized by a parameter known as the half-width at half-maximum (HWHM), representing the width of the curve at half of its maximum intensity as depicted in Figure 3. The HWHM represents the width of the spectral line at half of its maximum intensity and is proportional to the collision rate between the atoms or molecules.

Figure 3: HWHM parameter

The line shape of a spectral line that undergoes homogeneous broadening can be described by a Lorentzian distribution, which is given by the following equation:

where I(ν) is the intensity of the spectral line at frequency ν, I₀ is the maximum intensity of the line, ν₀ is the center frequency of the line, and γ is the linewidth parameter known as the half-width at half-maximum (HWHM).

It is often observed in dense gases, liquids, and solids, where there are frequent collisions. In such media, the collisions randomize the energy states, resulting in a broadening of the spectral lines. 

Applications of Homogeneous Broadening

Homogeneous broadening has significant implications for various areas of science and technology, including laser physics, spectroscopy, and communication systems. In laser physics, the broadening limits the linewidth of laser emission, and the HWHM parameter determines the minimum achievable linewidth. In spectroscopy, it can be used to measure collision rates between atoms or molecules and their environment. In communication systems, this broadening can limit the bandwidth of optical fibers.

Inhomogeneous Broadening

Inhomogeneous broadening refers to the broadening of a spectral line (representing an atomic transition) caused by different atoms or particles emitting or absorbing radiation at different frequencies. The difference in frequencies cause the spectral lines to broaden unevenly, leading to an irregular or asymmetric shape.

In this type of broadening, various atoms or particles experience diverse external conditions or interactions, leading to variations in their transition energies.

Due to these differing energy levels, as these particles emit or absorb radiation during energy level transitions, it occurs at different frequencies. This results in the spectral lines broadening unevenly, as each particle contributes to the line shape with its unique emission or absorption frequency.

Inhomogeneous broadening is common in scenarios where the sample contains a distribution of energy levels, such as in gases, liquids, and solids at different temperatures or pressures. It can also arise due to random fluctuations in the local environments of the particles.

The key to inhomogeneous broadening is the lack of coherence among the particles, where they respond differently to external influences or have varied interactions with their surroundings. This diversity in their spectral responses leads to an irregular or asymmetric shape of the spectral lines. Inhomogeneous broadening is often observed in complex systems, where the heterogeneity of energy levels and local conditions among the particles is a significant factor.

The broadening can be described by a Gaussian line shape that is characterized by a standard deviation parameter. The Gaussian line shape, also known as the Gaussian profile or Gaussian curve is a mathematical function used to represent the spectral intensity distribution of the emission or absorption of light from a source. The curve is characterized by its symmetrical bell-shaped appearance, where the highest intensity (peak) is at the center, and the intensity decreases gradually as you move away from the center.

The equation of the Gaussian line shape is given by:

Where x is a subsidiary variable, maximum value of 1 at x =0 and 1/2 at x = ±1

This parameter represents the width of the spectral line and is related to the Doppler effect, which is the change in frequency or wavelength of a wave caused by relative motion between the wave source and the observer.

where I(ν) is the intensity of the spectral line as a function of frequency ν, I_o is the maximum intensity of the spectral line, ν_o is the central frequency of the line, and σ is the standard deviation of the distribution. The standard deviation σ is related to the velocity of the atoms or molecules through the following equation:

where c is the speed of light, k is the Boltzmann constant, T is the temperature of the medium, and m is the mass of the atom or molecule. This equation shows that the width of the spectral line due to inhomogeneous broadening depends on the temperature and the mass of the atom/molecule. 

Inhomogeneous broadening is often observed in dilute gases, where the atoms or molecules are well separated and do not experience frequent collisions. In such media, the Doppler effect causes the spectral lines to broaden. The Doppler effect arises due to the random motion, causing them to emit or absorb radiation at slightly different frequencies due to the relative motion between the atom or molecule and the observer. 

Applications of Inhomogeneous Broadening 

Inhomogeneous broadening has significant implications for various areas of science and technology, including astrophysics, atmospheric physics, and remote sensing. In astrophysics, the broadening can be used to study the motion of stars in galaxies. In atmospheric physics, it can be used to measure the temperature and pressure profiles of the atmosphere. In remote sensing, this broadening can be used to detect the presence of pollutants in the atmosphere.