Theoretical study of double-resonance processes in the helium continuum

The ionization spectrum of the He atom under double-resonance irradiation is studied theoretically. A previously developed theory is extended and adapted to treat this process. The first source (synchrotron) is tuned around the transition from the ground state to the 23sp– ${}_{}{}^1{}_{}{}^{}P_{}^o{}_{}{}^{}$ autoionizing state, and the second source (laser) is put in resonance with the transition from 23sp– ${}_{}{}^1{}_{}{}^{}P_{}^o{}_{}{}^{}$ to the higher-lying quasibound state 2p3p– ${}_{}{}^1{}_{}{}^{}P_{}^e{}_{}{}^{}$ (or 2p4p, 2p5p). Accurate ab initio calculations have been performed to obtain all the necessary parameters. It is found that the single-field ionization spectrum is strongly modified, exhibiting coherent trapping and ac Stark splitting. These results are in consonance with previous model calculations as well as with other ab initio calculations on a different zone of the He continuum. The results show that for the resonances considered these strong effects can be observed at megawatt laser intensities.