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

Fluorescence excitation spectra and wavelength-resolved emission spectra of the C₃-Kr and C₃-Xe van der Waals (vdW) complexes have been recorded near the 2^2-₀, 2²₀, 2^4-₀, and 1¹₀ bands of the Au1-X1g+system of the C₃ molecule. In the excitation spectra, the spectral features of the two complexes are red-shifted relative to those of free C₃ by 21.9-38.2 and 34.3-36.1 cm^-1, respectively. The emission spectra from the A state of the Kr complex consist of progressions in the two C₃-bending vibrations (₂, ₄), the vdW stretching (₃), and bending vibrations (₆), suggesting that the equilibrium geometry in the X̃ state is nonlinear. As in the Ar complex Zhang, J. Chem. Phys. 120, 3189 (2004), the C₃-bending vibrational levels of the Kr complex shift progressively to lower energy with respect to those of free C₃ 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 C₃, consist only of progressions in even quanta of the C₃-bending and vdW modes, implying that the geometry in the higher vibrational levels ( _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 C₃ bending vibration is large (u₄ ≥ 4).