Widths and Profiles of Spectral Lines

Spectral lines in discrete absorption or emission spectra are never strictly monochromatic. Even with the very high resolution of interferometers one observes a spectral distribution l(u) of the absorbed or emitted intensity around the central frequency Uq = (Ei-Ej )/h corresponding to a molecular transition with the energy difference AE = E—Ej between upper and lower levels. The function l(u) in the vicinity of Uq is called the line profile (Fig. 3.1). The frequency interval Sv = between the two frequencies 

The half width is sometimes written in terms of the angular frequency u) = 2m with 8u) = I kSv or in terms of the wavelength A (in units of nm or A) with 5A = A -A2. From A = c/2/, it follows that 

The relative halfwidths, however, are the same in all three schemes  

The spectral region within the half width is called the kernel of the line, the regions outside (2/ and u 1 2) are the line wings. 

In the following sections we discuss various origins of the finite line-width. Several examples illustrate the order of magnitude of different line- 

Demtroder, Laser Spectroscopy 1, DOI 10.1007/978-3-642-53859-9 3, Springer-Verlag Berlin Heidelberg 2014 

In the following sections we discuss various causes of the finite line-width. Several examples illustrate the order of magnitude of the various effects in different spectral regions and their importance for high-resolution spectroscopy [3.1-3]. Following the usual convention we shall often use the angular frequency w to avoid factors of 2ir in the equations. 

---