Spectral line shifts and broadenings

The first measurements of collision properties of Rydberg atoms were the pressure shift measurements of Amaldi and Segre,1 which prompted the description of the shifts in terms of free electron scattering by Fermi.2 

Lineshift and broadening measurements provide information complementary to that obtained from the conventional collision measurements described in the previous chapter. Just as the tunable laser has made possible many of the collision experiments described in the previous chapter, it has allowed much more sensitive line broadening measurements. In this chapter we connect the normal description of lineshapes to the Rydberg atom collision processes, briefly describe the two modern experimental techniques, and describe the results of the experiments. 

If we consider the intensity of a transition from the ground state to a Rydberg state, it has the form3 

We are particularly interested in transitions from the ground state to the Rydberg states, in which case the effects of collisions on the ground state are negligible, and only the effects of collisions on the Rydberg states need be considered. First we consider the broadening cross section ob. It arises from all collisions which disrupt the atomic phase during the emission or absorption of 

The elastic scattering cross section for a perturber in state (i incident on a Rydberg atom in state a with momentum K is obtained from the optical theorem. Explicitly3 

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