Observation of Continuum-Continuum Autler-Townes Splitting

Abstract

Photoelectron energy spectra resulting from two-photon ionization of calcium atoms by 180 fs pulses have been studied as a function of wavelength (380--405 nm) and intensity ( $9\ensuremath{-}900\phantom{\rule{0ex}{0ex}}\mathrm{GW}/{\mathrm{cm}}^{2}$). When the wavelength is tuned near the $4s\ensuremath{\rightarrow}4p$ or $4s\ensuremath{\rightarrow}5s$ ionic core transition, the photoelectron peaks display a characteristic splitting proportional to the field strength and assignable to a two-electron continuum-continuum Autler-Townes effect. Spectra obtained by an essential-state model involving three coupled continua are compared to the experiment.