Experimental and theoretical investigations of HeNeI₂ trimer

We report on the results of spectroscopic studies of the HeNeI₂ van der Waals trimer using the two-step two-color HeNeI₂(E0_g ⁺, v_E = 0-3 ←hν₂B0_u ⁺, v_B = 19 ←hν₁X0_g ⁺, v_X = 0) excitation scheme. The excitation spectra of the HeNeI₂(B, E) decay product luminescence and the luminescence spectra of I₂ ion-pair states formed after HeNeI₂(E) decay have been recorded and analyzed. The HeNeI₂(X, B, E) binding energies have been estimated to be less than 111.8 cm^-1, 101.6 cm^-1, and 117.9 cm^-1, respectively. The HeNeI₂(B, v_B) state decay has been found to be sequential with the formation of the HeI₂(B, v_B - 1) and NeI₂(B, v_B - 1) complexes without intermolecular excitation at the first step. An analysis of the HeNeI₂(E) decay process based on the relative probabilities of the I₂(D0_u ⁺) and I₂(β1_g) formation after decay, as well as vibrational populations of these states obtained from luminescence spectra, has also been performed. Calculations of the HeNeI₂(X, B) vibrational energies using potential energy surfaces (PESs) of HeNeI₂(X, B) constructed as a sum of the HeNe, HeI₂, and NeI₂ potentials have been carried out. The values obtained agree well with the experimental estimations, and the observed transitions can be ascribed to the HeNeI₂ trimer of a tetrahedral geometry. Besides, a comparison of the PES constructed as a sum of the coupled-cluster single double triple [CCSD(T)] potentials with the PES based on the "direct" CCSD(T) calculations has been performed for the HeNeI₂(X) trimer to verify the applicability of this representation to the trimer under study.