Ar and CH₄ van der Waals complexes of 1‐ and 2‐fluoronaphthalene: A perturbed spherical top attached to a surface Available to Purchase

We compare and contrast the low and high resolution S₁←S₀ fluorescence excitation spectra of four van der Waals complexes, Ar–1FN, CH₄–1FN, Ar–2FN, and CH₄–2FN (where 1FN and 2FN are 1‐ and 2‐fluoronaphthalene, respectively) in the gas phase. Whereas the Ar and CH₄ complexes exhibit comparable low resolution spectra, their high resolution spectra are significantly different. The CH₄–1/2FN complexes exhibit origin bands that are each split into three distinct subbands with different intensities and separations of less than 1 cm⁻¹. No such splittings are observed in Ar–1/2FN. The relative intensities of the three subbands in both CH₄ complexes are 1:2:2. These are identical, within experimental error, to the total statistical weights of the J=0, 1, and 2 rotational levels of CH₄. Both Ar and CH₄ are weakly attached to 1/2FN at a distance of ∼3.5 Å above the aromatic plane. This distance decreases slightly (∼0.1 Å) on S₁←S₀ excitation. Thus, the splittings observed in CH₄–1/2FN are attributed to ‘‘surface‐induced’’ perturbations of the normally isotropic rotational motion of methane whose magnitudes depend on the electronic structure of the surface to which it is attached. A model is proposed that accounts for these observations. Comparison of the numerical predictions of this model with the experimental results shows that the rotational motion of the attached CH₄ is nearly the same as that of the free molecule.