The ionization energies and photoelectron spectra of van der Waals (vdW) Helium nano clusters (Hen) were studied in this work. The clusters with different number of helium atoms, in the range from n=2 to n=100, and belong to different symmetry point groups were selected.Density functional theory (DFT) employing CAM-B3LYP functional and 6-31+G(d) basis set were used for optimizing the structures. Symmetry adapted cluster-configuration interaction (SAC-CI) methodology and 6-31+G(3df) basis set were also used to calculate the ionization energies and their intensities. The calculated ionization energies and their intensities were used to simulate the photoelectron spectrum of each structure. It was found that the clustering of He atoms decreases the first ionization energy of the cluster compared to the isolated helium atom in the gas phase. The variation of the first ionization energies of the nanostructures were plotted versus their sizes and fitted into a mathematical equation. The simulated photoelectron spectra of the nano clusters were compared with each other comprehensively to investigate the change in the shape of spectrum with the size of cluster. The calculated ionization bands of each cluster were assigned using natural bonding orbital (NBO) analysis and their changes with the size were studied. Also, it was studied that how much the ionization processes is governed by the electron correlation and effect of the size on the electron correlation was examined.

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