In classical spectroscopy, one excited the atoms in an electrical discharge, and sorted the wavelengths with a monochromator or spectroscope. Especially in the case of molecules, it could be difficult to be sure of which species a given spectral line came from. In the case of ions, one can accelerate them and form a beam in vacuum. With all the molecules traveling at the same speed, in the same direction, in a good vacuum, there will be no collisions to broaden the spectral line width. This is important because the most accurate determination of the wavelength requires the narrowest possible spectral line.
Atoms colliding with each other at atmospheric pressure have enormously broad spectral lines.
In the absence of collisions, the width is then primarily due to the Doppler effect of atoms moving in all directions with a distribution of speeds that depends on temperature. In an ion beam, they all have very nearly the same speed and direction as a result of acceleration by a large potential, leading to very narrow lines. The relatively small amount of random energy the ions had before acceleration is still the limiting factor in the line width, but an effect called kinematic compression makes it very advantageous to accelerate as much as possible. Because the Doppler effect depends on the velocity , which in turn depends on the square root of the ion's energy , a small spread of energy leads to a large spread in velocity when the ion has very little energy, but leads to a small spread of velocity when the ion has been accelerated to high energy. The line width is less than 200 MHz, whereas it would be several GHz in the spectrum of a discharge light source.