Experimental investigation of the interatomic Coulombic decay in NeAr dimers

The kinetic-energy distribution of interatomic and -molecular Coulombic decay (ICD) electrons emitted following photoionization of $2s$ electrons from Ne atoms in NeAr dimers has been measured in a synchrotron radiation experiment with a velocity-map-imaging photoelectron-photoion-coincidence spectrometer. The position of the peak of the experimental distribution agrees well with calculations. The broadening of the distribution to high energies with respect to calculations of the ICD spectrum of the NeAr $v=0$ ground state is explained by the contribution to the spectrum due to the population in the excited vibrational states $v=1$ and $v=2$ of the electron ground state. On the other hand, the broadening on the low-energy side of the measured distribution may be explained by the dimer contracting in the intermediate state before ICD occurs, electrons coming from the ICD to spin-orbit split excited ion states, or a combination of these effects.

Figure 1

Schematic illustration of the electron velocity-map-imaging photoelectron-photoion-coincidence spectrometer used in the measurements.

Figure 2

(a) The 2D histogram of the ion-ion coincidence arrival times in the region of the ${\mathrm{Ar}}^{+}-{\mathrm{Ne}}^{+}$ ions. (b) The 2D histogram of electron arrival positions on the VMI detector detected in coincidence with the ions within the red ellipses on the ion-ion map shown in (a). (c) The angularly averaged VMI image (see text for details).

Figure 3

(a) Kinetic-energy distribution of ICD electrons detected in coincidence with two ${\mathrm{Ne}}^{+}$ ions: the present results are shown by the red solid line with squares, the experimental results of Jahnke et al. [5] are shown by blue crosses, and the ab initio calculations of the kinetic-energy distribution of ICD electrons from ${\mathrm{Ne}}_2,v=0$, from Cederbaum and workers [27, 28, 29] are shown by the black solid line. (b) The kinetic-energy distribution of the low-energy electrons detected in coincidence with one ${\mathrm{Ne}}^{+}$ ion and one ${\mathrm{Ar}}^{+}$ ion (red solid line with squares) together with the ab initio calculations of the KEDs of ICD electrons produced following photoionization of the Ne $2s$ orbital of the electronic ground state of NeAr in its $v=0$ (thin black line), $v=1$ (thin dotted line), and $v=2$ (thin dashed line) vibrational states. The relative contributions of $v=0,v=1$, and $v=2$ are normalized according to a vibrational temperature of 20 K, are summed (thick dashed line), and are then convolved with the experimental resolution (thick solid line); see the text for details.

Figure 4

Potential-energy curves of NeAr taking part in the ICD process outlined in Eq. (1). ${\mathrm{ICD}}_{\mathrm{ND}}$ stands for ICD after nuclear dynamics. ${\mathrm{ICD}}_{\mathrm{v}>0}$ represents the ICD from excited vibrational states of NeAr, showing how the ICD electrons from these states have higher KE (see the text for details). The potential-energy curves have been digitized from Ref. [17].