Question

Discuss the nature of the three main types of atomic line broadening and include the relative magnitude of broadening for each. Also, discuss what can be done, if anything, to reduce the magnitude of each type in specific experiments.

Answer 1

Intrinsic/ Natural Broadening :

Natural broadening is directly related to the principle of Heisenberg. This is one of the basic principles of quantum mechanics, and is characteristic of particle waves behavior. The principle, in its energy-time version, states that in a physical phenomenon the product of the uncertainties ΔE and Δt assumes the minimum value:

ΔEΔt ~ h/2π

Now we know that the lifetime of excited atomic states is finite (of the order of microseconds-nanoseconds), so also the uncertainty Δt has this value. From Heisenberg’s relation it can be deduced that ΔE ≠ 0 (would be null in the case of infinite lifetime) and therefore also the frequency of the photon emitted has an intrinsic uncertainty that is :

Δν ~ ΔE/h ~ 1/2πΔt

the value of this uncertainty is really low : 10-5 nm and normally it is absolutely negligible.

No one can escape natural broadening.

Thermal Doppler Broadening :

Thermal broadening is due to the doppler effect that occurs when the radiating atoms have a movement relative to the observer. The random atomic movement of the atoms is directly related to the temperature, which is why this broadening mechanism is called thermal.
As we know from the Maxwell-Boltzmann statistics the speed of gas atoms follow a gaussian profile and the mean energy is on the order of kT, where k is the Boltzmann constant and T is the temperature. Mathematically it can be shown that the spectral line broadening profile is also Gaussian, with a value given by:

The equation shows that broadening increases with temperature and is higher for light atoms. By doing the calculations you see that for hydrogen at the temperature of 6000°K, the width of the Hα line is 0.036 nm.

This broadening can be reduced upon lowering the temperature as we can see there is direct temperature dependence.

Collision Broadening :

The last phenomenon that causes broadening of spectral lines is the collisions between excited atoms which can become predominant in high-density gas plasma emissions (high pressures). In this case, the spectral line profile is not Gaussian but Lorentzian, characterized by a narrower spike and longer wings. In the context of high pressure and high temperatures, the phenomenon of resonant self-absorption can also occur. This phenomenon consists in the auto-absorption of the emitted radiation by colder external gas layers.