Manifestation of van der Waals interactions in the vibrational spectra of alkali metal dimers.
Year: 2026
Type: preprint
Abstract: This article is devoted to the analysis of the vibrational frequencies of alkali metal dimers from Li2 to Cs2 in the ground electronic state X 1Σ+ g, with the aim of identifying manifestation of van der Waals interactions at high vibrational excitation. Based on literature data on the energies of the vibrational levels G(v), vibrational frequency functions have been constructed: the first difference G1(v), representing the sequence of vibrational quanta (G1(v) > 0), and the second difference G2(v), representing the rate of decrease in their energy (G2(v) < 0). For five dimers, these functions have a similar form. The function G1(v) decreases monotonically with acceleration, and concavity appears near the asymptote. The function G2(v) increases in magnitude with acceleration up to an inflection point, where it undergoes a rapid decrease, forming the so-called tail. It is shown that, for the tail levels, the turning points r2 correspond to the van der Waals pair state. Since the excited molecule spends the maximum amount of time in the vicinity of the outer classical inflection point r2, at these energy levels the interatomic distances of ~3.5 Å (Li2)–~3.1 Å (Cs2) exceeds twice the van der Waals radius of the atom. It can be assumed that the valence contribution is small in these states. The main conclusion of this work is that a qualitative change in G2(v) for alkali metal dimers occurs in the same region of the potential well U(r), including the tail. The transition to van der Waals interaction occurs at a constant distance from the asymptote of approximately 120 cm-1, or, possibly, varying from 120 to 90 cm-1 in the studied series of molecules.
Source: ChemRxiv
Author: Gleb S. Denisov
Institution: St Petersburg University
Language: English
Subfield: Spectroscopy
Field: Chemistry
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7212108411