Influence of thermal ions on the n d3/2-n f5/2 and n d3/2-n g7/2 microwave transition of sodium

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Abstract
We have studied the dipole‐allowed nd3/2‐nf5/2 and the dipole‐forbidden nd3/2‐ng7/2 transitions of sodium in the presence of thermal ions for n=15–17. Although the cross sections for the relevant ion–Rydberg atom collisons are too small to influence the observed transitions, these microwave resonance measurements indicate that there is a remarkable influence of even very low densities, 105 cm−3, of ions on the atoms. For instance, the d‐g transition can be easily driven with a single photon, and the d‐f and d‐g transitions are shifted and broadened. Furthermore, the thermal sodium ions destroy the interference fringes obtained with the Ramsey method of separated oscillatory fields. Although these results are incompatible with reasonable estimates of ion‐atom collision cross sections, they may be explained quantitatively in terms of the time‐averaged charge of the ions.