Influence of the shot-to-shot variation of sub-fs soft X-Ray free electron laser pulses on pump-probe measurements of attosecond charge migration in aromatic molecules

Sub-fs soft X-Ray pulses are currently being produced at the LCLS X-Ray free electron laser (XFEL) with the recent implementation of the X-Ray laser-enhanced attosecond pulse generation method [1]. These pulses are many orders of magnitude more intense than their high harmonic generation (HHG) counterparts, making it possible to conduct nonlinear X-Ray spectroscopies of attosecond electron dynamics. However, most XFELs build up their pulses from noise via self-amplified spontaneous emission introducing a stochastic shot-to-shot variation of the pulses’ properties, which translates into different created superpositions of states and resulting charge dynamics. Measuring transient spectra averaged over an ensemble of pump pulses may thus diminish the dynamical features. While one can exclude this effect by only considering data for a small fraction of similar pump pulses, such an approach requires considerably longer beamtimes, questioning its practical feasibility.

This contribution showcases our first-principles calculation results regarding the impact of the shot-to-shot variation of a hundred sub-fs pump pulses of 260 eV, obtained from start-to-end simulations of the XFEL at the LCLS, onto pump-probe studies of the charge migration dynamics in the prototypical para-aminophenol molecule (NH₂-C₆H₄-OH). Therein, the static-exchange B-spline DFT method is used to explicitly model the molecular continuum states, while the bound states can be described independently with the CASPT2 method.

We will show that the shot-to-shot variation alone reduces the charge fluctuation amplitudes by no more than 35%. We found this to be much weaker than the natural effect of the initial nuclear wave function that has been accounted for explicitly by averaging over an ensemble of geometries sampled from the nuclear Wigner distribution of the molecule. The so-introduced dephasing damps the dynamics considerably beyond 3 fs after the pump pulse. These results already suggest that attosecond soft X-Ray spectroscopies at XFELs can be reliably used to investigate pristine charge dynamics at its early stages. We will demonstrate in addition how the shot-to-shot variation manifests in explicitly simulated pump-probe spectroscopic observables, completing the picture.

References:

[1] Duris J. et al. Nat. Phot. 14, 30 (2020)