We report non‐preturbative results on multiphoton transiton and up‐conversion enhancement by dressing photoionisation continua using radiative interaction analogous to configuration interaction in autoionisation.
The author predicts that bound atomic or molecular states can be dressed by an applied field in a fashion analogous to the pseudo-autoionising structure induced in a photoionising continuum. Just as the continuum structure can enhance the photoionisation rate, the induced virtual structure between real states can be used to enhance multiphoton processes by tuning one of the intermediate fields near this virtual resonance. The author concentrates on stimulated Raman scattering, but the connection to multiphoton absorption is pointed out.
Two-laser multiphoton ionisation under the autoionising-like resonance condition is studied theoretically within an extended atomic model dictated by the experiment of Feldmann et al (1986). Ordinary coupling between a selected pair of bound states via the continuum only is supplemented in the author's model by Raman-type coupling involving the atomic spectrum below the ionisation threshold. In the case considered this additional Raman coupling enhances the ionisation rate to a much greater extent than the coupling via the continuum can do. The Raman coupling modifies the autoionising-like shape parameter and is decisive as far as the resonance peak profile is concerned. Exploiting this Raman coupling within the atomic model with four effective bound states, the author gives explanations for all the details of the ionisation spectrum detected by Feldmann et al, i.e. the number of resonance peaks, their strength, fine structure, shape, relative heights and sensitivity to the laser field.
We report on the observation of the modification of the two spin-orbit ionization continua of Xe through laser-induced continuum structure (LICS). The radiative decay of the Xe ground state into the two electronic continua that correspond to the two fine-structure levels of the ground state of the Xe+ ion can be varied by electromagnetically embedding a bound state of the atom into the two continua. The dressed 2P1/2 continuum, probed from the atomic ground state through three-photon absorption, exhibits a clear induced structure, while the 2P3/2 continuum possesses no observable structure under the present experimental conditions, as manifested through high-resolution photoelectron spectra. The modified ionization ratio is controlled by the wavelength of the dressing field.