Probing the Electronic Manifold of MgCl with Millimeter-Wave Spectroscopy and Theory: (3)2Σ+ and (4)2Σ+ States.

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, and 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 r_e = 2.536 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 nonadiabatic interactions with the (4)²Σ⁺ state.