Ab Initio Calculations on the Electronically Excited States of Small Helium Clusters

Abstract

The vertical excitation energies of small helium clusters, He(7) and He(25), have been calculated using configuration interaction singles, and the character of the excited states was determined using attachment/detachment density analysis. It was found that in the n = 2 manifold the excitations could be interpreted as superpositions of atomic states, with excitations on the surface of the clusters being lower in energy than those in the bulk. For the n = 2 excited states with significant density on the interior of the cluster, mixing with the atomic n = 3 states resulted in lower excitation energies. For the n = 3 states the spatial extent of the excited-state density can be much larger than the size of the cluster, making analysis of the states more difficult and highly dependent on the internuclear distance. Introducing disorder into the clusters results in some localization of the excited states, although highly delocalized states are always observed in these small clusters. In addition, experimental results for small clusters are interpreted in terms of these findings.

Femtosecond extreme ultraviolet ion imaging of ultrafast dynamics in electronically excited helium nanodroplets

“…The different dynamic trends are explained in a model that distinguishes between bulk and surface excitations. The model is supported by recent ab initio calculations of Closser and Head-Gordon [26]. An intraband relaxation mechanism in the lower electronic band of the droplets at ∼21.6 eV is proposed that corresponds to the transfer of electronic excitation in the Rydberg n = 2 manifold from bulk to surface states on a time scale of ∼10-20 ps.…”

“…In the low-density surface region, the shift is expected to be much less. This qualitative picture has recently been confirmed for excited states of 25-atom helium clusters by the ab initio calculations of Closser and Head-Gordon [26]. Their analysis of cluster states that emerge from the atomic Rydberg n = 2 manifold indicates that the ratio of bulk-state absorption to surface-state absorption increases with increasing energy across the absorption band centered at ∼21.6 eV.…”

“…We note that, in particular, recent calculations by Closser and Head-Gordon indicate that this is an oversimplification [26]. Nevertheless, concentrating on excited states of helium atoms and dimers provides a good starting point for a qualitative description of the physics that underlie the results presented here.…”

“…Based on the simple model of density-dependent blue shifting of atomic states and supported by the calculations of Closser and Head-Gorden [26], the ∼10-20 ps intraband relaxation in the lower electronic band is interpreted as a transfer of electronic excitation from bulk to surface states, i.e., as a "surfacing" process of bulk excitations that emerge from the n = 2 Rydberg manifold. The physics of this excitation migration process is currently under investigation.…”

Electron-Transfer-Induced Dissociation of H2on Gold Nanoparticles: Excited-State Potential Energy SurfacesviaEmbedded Correlated Wavefunction Theory

“…Smaller clusters for which MRSDCI would be tractable tend to overestimate reaction barriers as charge lost to the adsorbed molecule is not sufficiently screened [35]. A potential alternative approach is CI Singles (CIS, i.e., treating all possible single excitations), which has been shown to treat excited states well, as long as they do not involve any double excitations [36]. An embedded CIS calculation for the Au 12 cluster is numerically challenging, but feasible.…”