Evidence for interatomic Coulombic decay in Xe $K$-shell-vacancy decay of XeF${}_2$

Charge production, charge redistribution, and ion fragmentation are explored in the decay of a Xe $K$-shell vacancy in XeF${}_2$. Coincidence measurements of all ionic fragments in XeF${}_2$ provide evidence that an interatomic-Coulombic-decay-like (ICD-like) process plays a role in the cascade decay. The signature of the ICD-like process is an enhancement of the total number of electrons ejected as compared to the case of atomic Xe. The results indicate that the F atoms participate in the decay cascade within the first few femtoseconds after core-hole formation and that fragmentation begins during the decay process.

Figure 1

Schematic of the apparatus in the target area. A Ge x-ray fluorescence spectrometer (LEPS) on one side of the interaction region measures x rays from the target. An ion time-of-flight (ITOF) spectrometer on the opposite side is used to measure the charge-to-mass ratios of the ion fragments produced. The x-ray beam from the Advanced Photon Source was crossed with effusive beams of Xe or XeF${}_2$. The x-ray beam is linearly polarized along the spectrometer axis.

Figure 2

Ion time-of-flight spectra for all ions recorded in coincidence with a $K\alpha$ photon. (a) $K$-shell-ionized Xe. Peaks due to Xe${}^{q+}$ are labeled by the charge state $q$. (1 h accumulation.) (b) XeF${}_2$ excited at the $1s^{-1}7{\sigma }_u$ molecular resonance. Ions of charge $q$ are labeled F${}^{q+}$ and Xe${}^{q+}$. (6 h accumulation). The asymmetric splitting of the F${}^{q+}$ peaks is explained in the text. The XeF${}_2$ data show F${}^{q+}$ charge states from $+$1 to $+$4 and Xe${}^{q+}$ ions from $+$3 to $+$5. Less obvious are contributions from Xe${}^{6+}$ and Xe${}^{7+}$ which overlap the F${}^{1+}$ peak.

Figure 3

Scatter plot of hit 1 vs hit 2 for XeF${}_2$ fragmentation events in coincidence with a $K\alpha$ photon. The incident x-ray energy was set to the 1$s^{-1}7{\sigma }_u$ resonance.

Figure 4

Projections from the hit 1 vs hit 2 scatter plot. (a) Projection of the slow component of the F${}^{2+}$ peak onto the hit-2 axis. (b) Projection of the fast component of the F${}^{2+}$ peak onto the hit-2 axis.

Figure 5

Charge-state distributions for XeF${}_2$ (squares with solid curve) compared with Xe (circles with dashed curve) in coincidence with $K\alpha$ photons. (a) XeF${}_2$ breakup pattern for the subset of events involving two fluorine ions in the final state with charge states F${}^{3+}$ and F${}^{2+}$. We plot the fraction of events (in %) vs the total charge (i.e., the Xe charge state plus 5). (b) The distribution of total charge averaged over all XeF${}_2$ breakup modes.

Figure 6

Time evolution of the total number of holes and the population of 5$p$ holes in the decay of a Xe $K$-shell vacancy.

Figure 7

Fragmentation potentials of a XeF${}_2$ ion complex calculated with the independent-ion model for total charge $8+$. The numbers in parentheses correspond to ($q_1$,$q_2$,$q_3$). Both Xe-F bonds are elongated symmetrically.

Figure 8

Detection efficiency vs the bias on the MCP for selected charge states.