Photoionization suppression by continuum coherence: Experiment and theory

We present experimental and theoretical results of a detailed study of lase r-induced continuum structures (LICS's) in the photoionization continuum of helium out of the metastable state 2s S-1(0). The continuum dressing, with a 1064-nm laser, couples the same region of the continuum to the 4s S-1(0) state. The experimental data, presented for a range of intensities, show p ronounced ionization suppression (by as much as 70% with respect to the fat -from-resonance value) as well as enhancement, in a Beutler-Fano resonance profile. This ionization suppression is a clear indication of population tr apping mediated by coupling to a continuum. We present experimental results demonstrating the effect of pulse delay upon the LICS's, and for the behav ior of LICS's for both weak and strong probe pulses. Simulations based upon numerical solution of the Schrodinger equation model the experimental resu lts. The atomic parameters (Rabi frequencies and Stark shifts) are calculat ed using a simple model-potential method for the computation of the needed wave functions. The simulations of the LICS profiles are in excellent agree ment with experiment. We also present an analytic formulation of pulsed LIC S's. We show that in the case of a probe pulse shorter than the dressing on e, the LICS profile is the convolution of the power spectra of the probe pu lse with the usual Fano profile of stationary LICS's. We discuss some conse quences of deviation from steady-state theory. [S1050-2917(99)06604-4].