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The millimeter/submillimeter spectrum of magnesium chloride (MgCl) has been observed in two new electronic excited states, (3)2Σ+ and (4)2Σ+, using direct absorption methods. The molecule was synthesized in a mixture of Cl2, argon and magnesium vapor. For the (3)2Σ+ state, multiple rotational transitions were measured in the v = 0 level for all six isotopologues (24Mg35Cl, 24Mg37Cl, 25Mg35Cl, 25Mg37Cl, 26Mg35Cl, 26Mg37Cl), as well as up to v = 13 for 24Mg35Cl. For the (4)2Σ+ state, less intense spectra were recorded for 24Mg35Cl (v = 0-2). Equilibrium rotational parameters were determined for both states for 24Mg35Cl, as well as rotational constants and 25Mg hyperfine parameters for the other isotopologues. A perturbation was observed between rotational levels of the two states due to an avoided crossing. Computations were also carried out at the CASPT2 and MRCISD+Q levels, and the resulting bond lengths for (3)2Σ+ and (4)2Σ+ states agree well with the experimental values of re = 2.536 Å and 2.361 Å. The computations show that the (3)2Σ+ state has a double-well potential; however, the state behaves as a single well with unperturbed vibrational levels up to v = 13 due to non-adiabatic coupling with the (4)2Σ+ state. 

Abstract Bonding in the ground state of C $${}_{2}$$ 2 is still a matter of controversy, as reasonable arguments may be made for a dicarbon bond order of $$2$$ 2 , $$3$$ 3 , or $$4$$ 4 . Here we report on photoelectron spectra of the C $${}_{2}^{-}$$ 2 − anion, measured at a range of wavelengths using a high-resolution photoelectron imaging spectrometer, which reveal both the ground $${X}^{1}{\Sigma}_{\mathrm{g}}^{+}$$ X 1 Σ g + and first-excited $${a}^{3}{\Pi}_{{\mathrm{u}}}$$ a 3 Π u electronic states. These measurements yield electron angular anisotropies that identify the character of two orbitals: the diffuse detachment orbital of the anion and the highest occupied molecular orbital of the neutral. This work indicates that electron detachment occurs from predominantly $$s$$ s -like ( $$3{\sigma}_{\mathrm{g}}$$ 3 σ g ) and $$p$$ p -like ( $$1{\pi }_{{\mathrm{u}}}$$ 1 π u ) orbitals, respectively, which is inconsistent with the predictions required for the high bond-order models of strongly $$sp$$ s p -mixed orbitals. This result suggests that the dominant contribution to the dicarbon bonding involves a double-bonded configuration, with 2 $$\pi$$ π bonds and no accompanying $$\sigma$$ σ bond.