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

Abstract

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.