Question

What are the various contributions to the linewidth of an atomic or molecular transition and explain their origin.

Answer 1

Contribution;

The smallest linewidth values – well below 1 Hz – can be achieved with certain forbidden transitions, which can have a very small natural linewidth, when also suppressing various other contributions to the linewidth. Methods of ultra-precise spectroscopy have been developed for such measurements. Extremely small linewidths are also exploited in optical frequency standards for optical clocks. Here, the emission of a laser is stabilized to a narrow spectral line such that the linewidth of the laser is even far below the width of the spectral line.

The line shape, i.e., the shape of the optical spectrum, is often related to the dominant line broadening mechanism. For example, Lorentzian lines are often observed when lifetime broadening is dominant, while Doppler broadening leads to Gaussian line shapes.

The quite narrowband light from certain spectral lines is often regarded as quasi-monochromatic.

Spectral lines always exhibit a finite linewidth, which can have different origins:

- At high gas pressures, collisions are frequent. These lead to collisional broadening (or pressure broadening) of lines. Essentially, the emitting atoms are frequently disturbed by collisions during their radiation, so that the optical phase cannot evolve continuously over longer times.

-There are Doppler shifts due to the thermal movement of the radiating particles. This leads to the so-called Doppler broadening, the magnitude of which depends on the temperature. There are methods of Doppler-free spectroscopy, which largely eliminate the effect of Doppler broadening.

- Even without any movement, there is a natural linewidth, which is limited by the lifetime of the upper state (lifetime broadening).

- As mentioned above, emitting or absorbing atoms or ions in solids often exhibit broadened absorption and emission lines due to interactions with the host material. For example, there can be Stark effects caused by electric fields. If different atoms or ions experience different modifications of line features, the resulting broadening is called inhomogeneous broadening.