Experimental and theoretical investigations of HeNeI trimer.

We report on the results of spectroscopic studies of the HeNeI van der Waals trimer using the two-step two-color HeNeI(E0 , v = 0-3 ←hνB0 , v = 19 ←hνX0 , v = 0) excitation scheme. The excitation spectra of the HeNeI(B, E) decay product luminescence and the luminescence spectra of I ion-pair states formed after HeNeI(E) decay have been recorded and analyzed. The HeNeI(X, B, E) binding energies have been estimated to be less than 111.8 cm, 101.6 cm, and 117.9 cm, respectively. The HeNeI(B, v) state decay has been found to be sequential with the formation of the HeI(B, v - 1) and NeI(B, v - 1) complexes without intermolecular excitation at the first step. An analysis of the HeNeI(E) decay process based on the relative probabilities of the I(D0 ) and I(β1) formation after decay, as well as vibrational populations of these states obtained from luminescence spectra, has also been performed. Calculations of the HeNeI(X, B) vibrational energies using potential energy surfaces (PESs) of HeNeI(X, B) constructed as a sum of the HeNe, HeI, and NeI potentials have been carried out. The values obtained agree well with the experimental estimations, and the observed transitions can be ascribed to the HeNeI 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 HeNeI(X) trimer to verify the applicability of this representation to the trimer under study.