Time-Resolved Measurement of Interatomic Coulombic Decay in ${\mathrm{Ne}}_2$

The lifetime of interatomic Coulombic decay (ICD) [L. S. Cederbaum et al., Phys. Rev. Lett. 79, 4778 (1997)] in ${\mathrm{Ne}}_2$ is determined via an extreme ultraviolet pump-probe experiment at the Free-Electron Laser in Hamburg. The pump pulse creates a $2s$ inner-shell vacancy in one of the two Ne atoms, whereupon the ionized dimer undergoes ICD resulting in a repulsive ${\mathrm{Ne}}^{+}(2p^{-1})-{\mathrm{Ne}}^{+}(2p^{-1})$ state, which is probed with a second pulse, removing a further electron. The yield of coincident ${\mathrm{Ne}}^{+}-{\mathrm{Ne}}^{2+}$ pairs is recorded as a function of the pump-probe delay, allowing us to deduce the ICD lifetime of the ${\mathrm{Ne}}_2^{+}(2s^{-1})$ state to be $(150\pm 50)\mathrm{fs}$, in agreement with quantum calculations.

Figure 1

Relevant potential energy curves of ${\mathrm{Ne}}_2$ [22] with possible pathways resulting in coincident ${\mathrm{Ne}}^{+}-{\mathrm{Ne}}^{2+}$ fragments, assuming single-photon absorption in both the pump and the probe pulse.

Figure 2

(a) KER of coincident ${\mathrm{Ne}}^{+}-{\mathrm{Ne}}^{2+}$ fragments as a function of pump-probe delay $t_D$. (b) Projection of all KERs between 3 and 11 eV onto the delay axis. The ICD lifetime is determined via an exponential fit (red line). For comparison, we show quantum mechanical calculations (black line) including nuclear dynamics for the wave packet evolving on the $2^2{\Sigma }_u^{+}$ (long-dashed lines) or $2^2{\Sigma }_g^{+}$ (short-dashed lines) state of ${\mathrm{Ne}}_2^{+}(2s^{-1})$. The theoretical curves are scaled according to the experimental statistics. (c) Projection onto the KER axis for $t_D$ between 400 and 600 fs.

Figure 3

(a) Simulation of KER versus time delay for the primary ICD pathway into the fragmentation channel ${\mathrm{Ne}}^{2+}-{\mathrm{Ne}}^{+}$ with an input lifetime of 100 fs. Propagation on the $2^2{\Sigma }_g^{+}$ state is drawn in green while the purple color denotes the $2^2{\Sigma }_u^{+}$ state. (b) Projections of the simulated KER versus time delay spectra on the delay axis for the primary ICD (blue line) and the competing ${\mathrm{ICD}}_2$ (orange line). The sum of both is shown in red. Solid lines are exponential fits to the data.