Subfemtosecond Control of Molecular Fragmentation by Hard X-Ray Photons

Tuning hard x-ray excitation energy along Cl $1s\to {\sigma }^{*}$ resonance in gaseous HCl allows manipulating molecular fragmentation in the course of the induced multistep ultrafast dissociation. The observations are supported by theoretical modeling, which shows a strong interplay between the topology of the potential energy curves, involved in the Auger cascades, and the so-called core-hole clock, which determines the time spent by the system in the very first step. The asymmetric profile of the fragmentation ratios reflects different dynamics of nuclear wave packets dependent on the photon energy.

Figure 1

Effect of photon energy detuning on $LMM1$ Auger decay spectra recorded for gaseous HCl on top of the Cl $1s\to {\sigma }^{*}$ resonance (red) and with detuning $\mathrm{\Omega }=-2\mathrm{eV}$ (blue). The sharp lines correspond to the atomic contributions after MUST UFD in the Cl $2p^{-2}{\sigma }^{*}$ double-core-hole states of HCl. The difference spectrum (gray) shows molecular contributions to the $LMM1$ Auger decay and is obtained by subtraction of the $\mathrm{\Omega }=0\mathrm{eV}$ spectrum from the $\mathrm{\Omega }=-2\mathrm{eV}$ spectrum in a way that atomic $A_1$ and $A_2$ lines disappear.

Figure 2

Middle (in blue): Experimental atomic-versus-molecular intensity ratio for MUST UFD in the Cl $2p^{-2}{\sigma }^{*}$ double-core-hole state of HCl following Cl $1s\to {\sigma }^{*}$ excitation using photon energies across the Cl $1s^{-1}{\sigma }^{*}$ resonance. Top (in red): Theoretical atomic-versus-molecular ratio [including both ${\tau }_{\mathrm{eff}}$ and $dE(r)/dr$, full line; only ${\tau }_{\mathrm{eff}}$, dotted line]. Error bars are indicated by shaded areas. Bottom: Cl $1s\to {\sigma }^{*}$ partial electron yield spectrum (equivalent to x-ray absorption).

Figure 3

Diagram describing the effect of the different energy gradients (dotted straight lines) on the wave packet dynamics at the first two steps of the $KLL$ Auger decay cascade path for positive (blue) and negative (red) detunings. The calculated initial (bottom), initially excited wave packets (top), and their distributions in the Cl $2p^{-2}{\sigma }^{*}$ state after 2, 4, 6, 8 fs (middle).