Measurements of f values for high Rydberg members

As a result of complexities introduced by the presence of doubly-excited states, in the energy range of Rydberg excitation, the theory of collisions with highly excited atoms is a good deal more complex than predicted by simple theory. Although attempts have been made to include doubly-excited states in calculations of broadenings and shifts [147], there does not seem as yet to be a complete theory for such effects. 

There exist well-established methods for the measurement of / values. These are discussed at length in standard references [149]. More recent experimental developments have been quite comprehensively reviewed [151]. The present section is not intended as a substitute for this general literature, but concentrates specifically on rather sensitive techniques based on differential refractive index determinations, which have been successfully applied to observations of high Rydberg members in the laboratory. 

In general, the most sensitive techniques are those based on measurements of the refractive index rather than of the absorption coefficient. Since the absorption coefficient and the refractive index are connected 

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