Probing strong-field two-photon transitions through dynamic interference

We demonstrate how strong-field multiphoton transitions between dynamically shifted atomic levels can be traced in the energy spectra of emitted photoelectrons. Applying an ultrafast and intense laser pulse, two-photon Rabi oscillations are induced between two bound states of an atom. A third photon from the same pulse directly ionizes the atom, thus the emitted photoelectrons coherently probe the underlying dynamics. As the instantaneous energy of photoelectrons follows the pulse intensity envelope, modulated by the ac Stark shifts, electrons emitted with the same energy but at different times-at the rising and falling edge of the pulse-will interfere leading to pronounced dynamic interference pattern in the spectra. We investigate this phenomenon both numerically and analytically by developing a minimal three-state model that incorporates two-photon coupling and dynamically shifted atomic levels. On the example of atomic lithium (2s -> -> 4s -> continuum) we show how the individual ac Stark shifts and the two-photon Rabi frequency are reflected through the asymmetry, shifting and splitting of the interference structure of the computed photopeaks.

Automated summary

This study demonstrates how strong-field multiphoton transitions between dynamically shifted atomic levels can be traced in the energy spectra of emitted photoelectrons. By inducing two-photon Rabi oscillations between bound states of an atom with an ultrafast and intense laser pulse, researchers are able to observe pronounced dynamic interference patterns in the spectra. Numerical and analytical investigations revealed how the asymmetry, shifting, and splitting of interference structures in the computed photopeaks reflect the individual ac Stark shifts and two-photon Rabi frequency in the system.