High resolution laser spectroscopy of free radical-inert gas complexes: C5H5ṡHe, C5H5ṡHe2, C5H5ṡNe, and CH3-C5H4ṡHe2

Several inert-gas complexes of the free radical cyclopentadienyl and its methyl derivative (C₅H₅ṡHe, C₅H₅ṡNe, C₅H₅ṡHe₂, and CH₃-C₅H₄ṡHe₂, and CH₃-C₅H₄ṡHe) have been studied at low temperature via their laser induced fluorescence spectra in a supersonic free jet expansion. The rotational structures of the near-uv electronic transitions of the radicals have been resolved and analyzed. These analyses lead to precisely defined geometries for the radicals. In contrast to inert-gas, closed-shell aromatic complexes, the inert-gas radical ring bond lengths are found to be strongly dependent upon electronic state and methyl substitution. Additional molecular parameters were measured in the complexes including the electronic angular momentum, the (2,2) interaction parameter for the C₅H₅ complexes, and for the methyl derivative, the barrier to methyl rotation. These are compared to their values in the uncomplexed radicals.