PRECISION-MEASUREMENTS ON LITHIUM ATOMS IN AN ELECTRIC-FIELD COMPAREDWITH R-MATRIX AND OTHER STARK THEORIES

We have made careful measurements of Stark resonances in Li-7 Rydberg states above and below the classical saddle point to test various theo ries and to obtain a precise calibration of the electric field. Rydber g states were populated by two-step diode laser excitation to the 3 S- 2 State followed by He-Ne or diode laser excitation to Stark sublevels near the n = 15 manifold energy. Calibration was performed by compari ng measured resonance positions with theoretical results. For zero-fie ld energies, the theoretical calculations were made using quantum defe ct parameters obtained by fitting available spectral data on Li, and f rom recent polarization model results of Drachman and Bhatia [Phys. Re v. A51, 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 res ources but is not reliable beyond a precision of about 500 ppm in an e lectric field; (3) a recently developed R-matrix method. The last of t hese was most accurate and, like the second, could be used both below acid above the saddle point. From the measured resonance positions and an optimum set of Li quantum defect parameters, the R-matrix calculat ions 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.