Gaussian Broadening
Definition and meaning of Gaussian Broadening in chemistry.
Gaussian broadening is the widening of a spectral line or peak into a symmetric, bell-shaped (Gaussian) profile, most often caused by the Doppler effect from the random thermal motion of gas-phase atoms or molecules.
In more detail
Because particle speeds follow the Maxwell-Boltzmann distribution, the Doppler shifts they produce in absorbed or emitted light are also distributed in a bell curve, giving the observed line a Gaussian shape whose width increases with temperature and decreases with the particle's mass. Instrumental effects, such as a finite slit width or detector resolution, can add a similar Gaussian component. Gaussian broadening is distinguished from Lorentzian broadening, which arises from finite excited-state lifetimes (natural broadening) or collisions (pressure broadening) and produces a peak with more extended wings. When both mechanisms act together, the resulting lineshape is a Voigt profile, a convolution of the Gaussian and Lorentzian components, which spectroscopists fit to separate temperature effects from lifetime and pressure effects.
Key facts
| Main Cause | Doppler shifts from thermal (Maxwell-Boltzmann) velocity distribution |
|---|---|
| Lineshape | Gaussian (normal) distribution, proportional to exp(-x^2) |
| Combines With | Lorentzian broadening to form a Voigt profile |
| Field | Physical Chemistry |
The sodium D line observed in a low-pressure gas discharge shows Doppler (Gaussian) broadening with a full width at half maximum of only a few thousandths of a nanometer at room temperature, growing wider as the sample is heated.
Frequently asked questions
What mainly causes Gaussian broadening of a spectral line?
It is mainly caused by the Doppler effect: gas-phase particles moving toward or away from the observer shift the absorbed or emitted frequency, and since particle speeds follow a Maxwell-Boltzmann distribution, the resulting spread of frequencies is Gaussian in shape.
How is Gaussian broadening different from Lorentzian broadening?
Gaussian broadening comes from a distribution of Doppler-shifted frequencies and falls off quickly in the wings, while Lorentzian broadening comes from finite excited-state lifetimes or collisions and has longer, more extended wings; real spectra often show a mix of both, called a Voigt profile.