The C3-bending vibrational levels of the C3–Kr and C3–Xe van der Waals complexes studied by their Ã−X̃ electronic transitions and by ab initio calculations

Abstract

Fluorescence excitation spectra and wavelength-resolved emission spectra of the C(3)-Kr and C(3)-Xe van der Waals (vdW) complexes have been recorded near the 2(2-)(0), 2(2+)(0), 2(4-)(0), and 1(1)(0) bands of the Ã(1)Π(u)-X̃(1)Σ(g)(+) system of the C(3) molecule. In the excitation spectra, the spectral features of the two complexes are red-shifted relative to those of free C(3) by 21.9-38.2 and 34.3-36.1 cm(-1), respectively. The emission spectra from the à state of the Kr complex consist of progressions in the two C(3)-bending vibrations (ν(2), ν(4)), the vdW stretching (ν(3)), and bending vibrations (ν(6)), suggesting that the equilibrium geometry in the X̃ state is nonlinear. As in the Ar complex [Zhang et al., J. Chem. Phys. 120, 3189 (2004)], the C(3)-bending vibrational levels of the Kr complex shift progressively to lower energy with respect to those of free C(3) as the bending quantum number increases. Their vibrational structures could be modeled as perturbed harmonic oscillators, with the dipole-induced dipole terms of the Ar and Kr complexes scaled roughly by the polarizabilities of the Ar and Kr atoms. Emission spectra of the Xe complex, excited near the Ã, 2(2-) level of free C(3), consist only of progressions in even quanta of the C(3)-bending and vdW modes, implying that the geometry in the higher vibrational levels (υ(bend) ≥ 4, E(vib) ≥ 328 cm(-1)) of the X̃ state is (vibrationally averaged) linear. In this structure the Xe atom bonds to one of the terminal carbons nearly along the inertial a-axis of bent C(3). Our ab initio calculations of the Xe complex at the level of CCSD(T)∕aug-cc-pVTZ (C) and aug-cc-pVTZ-PP (Xe) predict that its equilibrium geometry is T-shaped (as in the Ar and Kr complexes), and also support the assignment of a stable linear isomer when the amplitude of the C(3) bending vibration is large (υ(4) ≥ 4).