Precision measurements on lithium atoms in an electric field compared with R-matrix and other Stark theories

We have made careful measurements of Stark resonances in [Formula Presented] Rydberg states above and below the classical saddle point to test various theories and to obtain a precise calibration of the electric field. Rydberg states were populated by two-step diode laser excitation to the 3[Formula Presented]S state followed by He-Ne or diode laser excitation to Stark sublevels near the n=15 manifold energy. Calibration was performed by comparing measured resonance positions with theoretical results. For zero-field energies, the theoretical calculations were made using quantum defect parameters obtained by fitting available spectral data on Li, and from recent polarization model results of Drachman and Bhatia [Phys. Rev. A 51, 2926 (1995)]. Three theoretical methods were used: (1) matrix diagonalization over a basis of spherical coordinate states, for which the precision declines as one approaches the saddle point; (2) frame transformation theory, which makes very economical use of computer resources but is not reliable beyond a precision of about 500 ppm in an electric field; (3) a recently developed R-matrix method. The last of these was most accurate and, like the second, could be used both below and above the saddle point. From the measured resonance positions and an optimum set of Li quantum defect parameters, the R-matrix calculations provided a calibration of the electric field to about ± 2 ppm ±4 mV/cm. We briefly discuss certain refinements and shortcomings of the other two theoretical methods, and the special procedures used to obtain high accuracy with the R-matrix method.