Electronic and vibrational spectroscopies of aromatic clusters with He in a supersonic jet: The case of neutral and cationic phenol–He_n (n = 1 and 2).

This article focuses on the first spectroscopic characterization of phenol–helium clusters (PhOH–Heₙ, n ≤ 2) using ultraviolet (UV) and infrared (IR) spectroscopy to investigate intermolecular interactions in weakly bound van der Waals systems. The study finds small, additive shifts in the S₁ ← S₀ electronic origin and ionization energies consistent with π-bound structures, supported by rotational contour analyses, while the OH stretching vibrations remain essentially unperturbed by helium tagging in both neutral and cationic states. Complementary quantum chemical calculations confirm that dispersion and induction forces dominate the intermolecular potentials and that helium acts as an ideal spectroscopic tag with minimal matrix shifts. Additionally, the work discusses ionization-induced structural migration from π-bound to hydrogen-bound configurations in the cationic clusters, providing fundamental insights into weak intermolecular forces in neutral and ionic aromatic systems.