Probing the Electronic Manifold of MgCl with Millimeter-Wave Spectroscopy and Theory: (3)Σ and (4)Σ States.

The millimeter/submillimeter spectrum of magnesium chloride (MgCl) has been observed in two new electronic excited states, (3)Σ and (4)Σ, using direct absorption methods. The molecule was synthesized in a mixture of Cl, argon, and magnesium vapor. For the (3)Σ state, multiple rotational transitions were measured in the = 0 level for all six isotopologues (MgCl, MgCl, MgCl, MgCl, MgCl, and MgCl), as well as up to = 13 for MgCl. For the (4)Σ state, less intense spectra were recorded for MgCl ( = 0-2). Equilibrium rotational parameters were determined for both states for MgCl, as well as rotational constants and Mg 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)Σ and (4)Σ states agree well with the experimental values of = 2.536 and 2.361 Å. The computations show that the (3)Σ state has a double-well potential; however, the state behaves as a single well with unperturbed vibrational levels up to = 13 due to nonadiabatic interactions with the (4)Σ state.