Line shapes of ionizing Stark resonances in helium.

Abstract

High-resolution laser spectroscopy is used to study the Stark effect in helium. The energies, widths, intensities, and shapes of resonances above the classical ionization threshold are measured for a wide range of electric field strengths. The high resolution of the lasers combined with careful design of the electric field plates allow accurate determination of the line-shape parameters, including linewidths and asymmetries accurate in the best cases to 6%. The WKB quantum-defect method is used to obtain detailed theoretical predictions for these Stark resonances with no adjustable parameters. Our study concentrates on the diabatic states of greatest slope arising from the zero-field n=32 and 40 manifolds in regions where these levels are fairly isolated and fit well to Fano profiles. In all cases, we obtain excellent agreement between experimental and theoretical line-shape parameters. A simpler autoionization model also reproduces the results, although less accurately.