Photoabsorption in Sodium Clusters: First Principles Configuration Interaction Calculations

TL;DR

This study uses high-level configuration interaction calculations to accurately predict the photoabsorption spectra of small sodium clusters, revealing the importance of electron correlation effects and potential plasmonic resonances.

Contribution

First comprehensive correlated-electron calculations of sodium cluster spectra using full and quadruple CI methods, including isomer considerations and convergence testing.

Findings

Electron correlation affects ground state geometries.

Valence electron correlation explains low-lying excitations.

Potential plasmonic resonance in Na6 spectrum.

Abstract

We present systematic and comprehensive correlated-electron calculations of the linear photoabsorption spectra of small neutral closed- and open-shell sodium clusters (Na_{n}, n=2-6), as well as closed-shell cation clusters (Na_{n}^{+}, n=3, 5). We have employed the configuration interaction (CI) methodology at the full CI (FCI) and quadruple CI (QCI) levels to compute the ground, and the low-lying excited states of the clusters. For most clusters, besides the minimum energy structures, we also consider their energetically close isomers. The photoabsorption spectra were computed under the electric-dipole approximation, employing the dipole-matrix elements connecting the ground state with the excited states of each isomer. Our calculations were tested rigorously for convergence with respect to the basis set, as well as with respect to the size of the active orbital space employed in the CI calculations. These calculations reveal that as far as electron-correlation effects are concerned, core excitations play an important role in determining the optimized ground state geometries of various clusters, thereby requiring all-electron correlated calculations. But, when it comes to low-lying optical excitations, only valence electron correlation effects play an important role, and excellent agreement with the experimental results is obtained within the frozen-core approximation. For the case of Na_{6}, the largest cluster studied in this work, we also discuss the possibility of occurrence of plasmonic resonance in the optical absorption spectrum.