Extreme-ultraviolet pump–probe studies of one-femtosecond-scale electron dynamics

Abstract

Pump–probe measurements are now an essential tool for investigating ultrafast dynamics in atoms and molecules. A lack of sources producing high-intensity attosecond pulses of extreme-ultraviolet (EUV) light has, however, hindered progress. Now, a technique that induces nonlinear processes with EUV light is demonstrated that could circumvent this problem. Ultrafast-dynamics studies and femtosecond-pulse metrology both rely on the nonlinear processes induced solely by an incident light pulse. Extending these approaches to the extreme-ultraviolet (XUV) spectral region would open up a new route to attosecond-scale dynamics. However, this has been hindered by the limited intensities available in coherent XUV continua. In the present work, we realized conditions at which simultaneous ejection of two bound electrons by two-XUV-photon absorption becomes more efficient than their removal one-by-one. In this regime we have succeeded in tracing atomic coherences evolving at the 1-fs scale with simultaneous determination of the average XUV-pulse duration. The rich and dense structure of the autoionizing manifold demonstrates the applicability of the approach to complex systems. This initiates the era of XUV-pump–XUV-probe experiments at the boundary between femto- and attosecond scales.