Assignment of the lowest 3Π and 1Π states of CaO, SrO, and BaO

Abstract

New techniques for the assignment of perturbations in electronic spectra are described and applied to perturbations in the A¹Σ‐X¹Σ bands of CaO, SrO, and BaO. These techniques utilize observed perturbation matrix elements to establish electronic and vibrational assignments of perturbing states. Using only data reported by Lagerqvist and co‐workers between 1949 and 1951 it has been possible to assign perturbations of the A¹Σ⁺ state to a ¹Π state (A′¹Π) and to three components of an inverted ³Π state (a³Π_i). Absolute vibrational assignments have been made and the following spectroscopic constants obtained (in cm⁻¹ with one standard deviation uncertainty in the last digit in parentheses): $$\begin{array}{ccccccc}& & \mathrm{CaO}& & \mathrm{SrO}& & \mathrm{BaO}\\ & {\mathrm{A\prime }}^1\Pi & a^3{\Pi }_i& {\mathrm{A\prime }}^1\Pi & a^3{\Pi }_i& {\mathrm{A\prime }}^1\Pi & a^3{\Pi }_i\\ {\nu }_{00}& \text{8564(16)}& \text{8225(25)}& \text{9794(10)}& \text{9055(5)}& \text{17 568(10)}& \text{17 372(10)}\\ {\mathrm{A\; (}}^3\mathrm{\Pi )}& \cdots & \text{−58(3)}& \cdots & \text{−70(2)}& \cdots & \text{−105(10)}\\ {\omega }_e& \text{557.5(86)}& \text{556.2(100)}& \text{460.3(8)}& \text{463.5(8)}& \text{448.3(7)}& \text{448.3(7)}\\ {\omega }_e{x}_e& \text{3.31(20)}& \text{3.30(22)}& \text{1.61(5)}& \text{1.61(5)}& \text{2.39(13)}& \text{2.39(13)}\\ B_e& \text{0.3353(68)}& \text{0.3353(68)}& \text{0.2610(5)}& \text{0.2584(5)}& \text{0.2244(5)}& \text{0.2244(5)}\\ {\alpha }_e& \text{0.0015(7)}& \text{0.0015(7)}& \text{0.0023(4)}& \text{0.0020(2)}& \text{0.0014(2)}& \text{0.0014(2)}\end{array}$$ The A′¹Π‐a³Π energy separations are small, and the a³Π state, the lowest state which dissociates to ground state atoms, lies at surprisingly high energy. The X¹Σ⁺ state is now proven to be the ground state of BeO, MgO, CaO, SrO, and BaO.