Dephasing in attosecond science and the collective nature of structural quantum fluctuations

There has been a long-standing debate regarding the interpretation of the splitting of the 1b₁ peak in the x-ray emission spectrum of liquid water. One view is that this splitting reflects the presence of two discernible structural motifs in ambient liquid water. Another view is that the splitting is caused by nuclear dynamics during the lifetime of the inner-shell-excited states that must be populated in order to trigger x-ray emission. A recent attosecond-pump/attosecond-probe experiment using an x-ray free-electron laser (XFEL) has brought clarity: By eliminating hydrogen motion on the time scale of the inner-shell hole lifetime of about 4 fs, the 1b₁ splitting could be shown to disappear.

Freezing the hydrogen motion in this all-x-ray attosecond-transient-absorption-spectroscopy (AX-ATAS) experiment relied not exclusively on the attosecond duration and delay of the x-ray pulses employed. By building on the theory developed in the context of one of the first ATAS experiments, we demonstrated that the AX-ATAS response in liquid water is confined to the subfemtosecond time scale. This is a consequence of rapid dephasing of the electronic polarization induced by the probe pulse, which is caused by the large inhomogeneous broadening characteristic of a disordered condensed-phase system such as liquid water. Hence, in this case, the ultrafast decay of electronic coherence was key to the success of the experiment.

In other situations, decoherence mechanisms can be a serious obstacle. For example, initial-state quantum fluctuations of the atomic positions and momenta in polyatomic systems can lead to electronic decoherence on the time scale of 1 fs. Characterizing the properties of these quantum fluctuations is therefore not only of general interest, but is also of immediate relevance to attosecond science. Capturing such collective phenomena requires multi-coincidence methods such as Coulomb Explosion Imaging. Exploiting the high degree of ionization that may be induced using XFEL pulses, which creates optimal conditions for CEI, we recently succeeded in recovering the 33-dimensional final-momentum distribution obtained in XFEL-based CEI of an 11-atom molecule. We showed that the correlated fluctuations present in the final momenta directly reflect the collective initial-state quantum fluctuations.