Spectroscopy of the XeC₂ molecule in xenon, argon, and krypton matrices

A self-igniting DC-electric discharge of C₂H₂ in Xe (matrix gas) or C₂H₂ and Xe in Ar or Kr (matrix gas) is used to produce and study the XeC₂ molecule in these various rare gases at 12 K. Unlike in Ar and Kr, the well-known electronic spectra of C ₂ is completely absent in a Xe matrix. This together with annealing experiments in Ar matrices indicate that ground state Xe and C₂ react uniquely and without a barrier to form the XeC₂ molecule. The IR-active C-C stretch of this compound is found to be close to the C-C stretching frequency of the C₂ anion, in excellent agreement with our density functional theoretical (DFT) calculations, which yield a XeCC singlet species bent by 148.6° and with substantial charge separation approaching Xe⁺C₂^- and a notably short (2.107 Å) Xe-C bond. The spectra of the Xe-¹³C-¹²C, Xe- ¹²C-¹³C, and Xe-¹³C-¹³C species are also obtained and the isotopic shifts are in excellent agreement with the DFT predictions, although not sufficient to distinguish a bent from a linear structure. Numerous broad absorptions centered near 423 nm (in Xe) are observed, which are clearly due to the XeC₂ molecule. Laser-induced fluorescence studies reveal a near-IR emission likely due to XeC₂ but not yet understood. Infrared spectra in the Xe matrix reveal also formation of the HXeCCH molecule.