Slow Interatomic Coulombic Decay of Multiply Excited Neon Clusters

Ne clusters ($\sim 5000\mathrm{atoms}$) were resonantly excited ($2p\to 3s$) by intense free electron laser (FEL) radiation at FERMI. Such multiply excited clusters can decay nonradiatively via energy exchange between at least two neighboring excited atoms. Benefiting from the precise tunability and narrow bandwidth of seeded FEL radiation, specific sites of the Ne clusters were probed. We found that the relaxation of cluster surface atoms proceeds via a sequence of interatomic or intermolecular Coulombic decay (ICD) processes while ICD of bulk atoms is additionally affected by the surrounding excited medium via inelastic electron scattering. For both cases, cluster excitations relax to atomic states prior to ICD, showing that this kind of ICD is rather slow (picosecond range). Controlling the average number of excitations per cluster via the FEL intensity allows a coarse tuning of the ICD rate.

Figure 1

Schematic representation of interatomic Coulombic decay scenarios (a) when only few atoms are excited in the cluster and (b) in the multiply excited cluster.

Figure 2

FEL intensity normalized electron emission spectra of Ne clusters ($N=5000$) measured for (a) surface and (b) bulk $2p\to 3s$ excitations at $h\nu =17.12$ and 17.65 eV, respectively. Spectra were recorded at different FEL intensities as indicated. The vertical dashed lines indicate the electron kinetic energy ($2\times h\nu -\mathrm{IP}$) when uncondensed Ne atoms or clusters are ionized by FEL 2nd harmonic radiation. Labels on top of the spectra indicate ICD electron energy after the excited cluster atoms relaxed to atomic $3s$ states as well as ICD electron energy losses via inelastic scattering on other $3s$ excited atoms, promoting them to higher Rydberg states (knock-up) as well as to the continuum (knock-out) (Ne energy levels from NIST ASD [42]).