Recently, high-accuracy single-electron ionization rates of helium at optical and UV wavelengths have been calculated. Analysis of the rates has revealed simple scaling law that describe the intensity- and wavelength-scaling behaviour of the ionization rates as intensity varies over the range 2.0 x 1013 W/cm2 to 3.0 x 1015 W/cm2, and as wavelength varies from 195 nm to 780 nm.

High-accuracy single-electron ionization rate calculations have also been performed using static electric fields. We confirm, to within a few percent, the static-field ionization rates obtained using time-independent methods. We derive the functional form of the static-field ionization rates as electric field strength varies in the range from 0 to 0.4 atomic units, and show that the rate undergoes an abrupt change in functional form in the region where above-barrier ionization is predicted to take place.

Most recently, calculations of double-ionization energy spectra and momentum distributions for the helium atom driven by few-cycle laser pulses of 195 nm wavelength have been performed. This work considers the problem for few-cycle pulses, in which a limited number of electron-ion recollisions take place. Through analysis of the time-evolution of the two-electron energy spectra, we show that the origins of above-threshold ionization resonances arise from interference between doubly-ionizing wavepackets located in a common spatial hemisphere.

Host: James Colgan, T-1, 5-0291, jcolgan@lanl.gov