Metastable a 3Π 13CO: Molecular-Beam Electric-Resonance Measurements of the Fine Structure, Hyperfine Structure, and Dipole Moment

The radio‐frequency spectrum of metastable $a^3{\Pi }^{13}\mathrm{CO}$ has been measured in molecular‐beam electric resonance in the range 0–2000 MHz. Electric‐dipole transitions with $\text{ΔJ\hspace{0.17em}=\hspace{0.17em}0}\mathrm{and}\text{ΔF\hspace{0.17em}=\hspace{0.17em}0, ±\hspace{0.17em}1}$ have been observed in states $\text{|\hspace{0.17em}υ, Ω, J, F〉\hspace{0.17em}=\hspace{0.17em}|\hspace{0.17em}0–3, 1–2, 1–7, J\hspace{0.17em}±\hspace{0.17em}}\frac{1}{2}〉$ at zero field. The set of transitions in each vibrational state has been analyzed to give three parameters describing the lowest‐order magnetic hyperfine interactions of the ¹³C nuclear spin $({I}\mathrm{\hspace{0.17em}=\hspace{0.17em}}\frac{1}{2})$ with the spin (S) and orbital (L) angular momenta of the two unpaired electrons in the $\text{5σ2π}$ open‐shell configuration. The vibrational dependence of the measured hyperfine parameters is fitted to a polynomial in $\mathrm{(\upsilon \hspace{0.17em}+\hspace{0.17em}}\frac{1}{2}{)}^n$ to give: $$\begin{array}{cccc}\mathrm{Origin}& \mathrm{Parameter}& & \mathrm{Value\; at}{R}_e{\mathrm{(a}}^3\mathrm{\pi )\; (}\mathrm{in\; MHz})\\ \mathbf{I}·\mathbf{S}\mathrm{Fermi\; contact}& K& =& \text{1935.1\hspace{0.17em}±\hspace{0.17em}0.5}\\ \mathbf{I}·\mathbf{S}\mathrm{spin–spin\; dipole}& D^{\delta }& =& \text{106.5\hspace{0.17em}±\hspace{0.17em}0.1}\\ & D& =& \text{7.9\hspace{0.17em}±\hspace{0.17em}0.1}\\ \mathbf{I}·\mathbf{L}\mathrm{spin–orbital}& \mathrm{G}& =& \text{162.5\hspace{0.17em}±\hspace{0.17em}0.1}\end{array}$$ Restricted Hartree–Fock calculations of these hyperfine terms are in good agreement with the measured values. The Stark effect has been measured in states $\text{|\hspace{0.17em}υ, Ω, J, F〉\hspace{0.17em}=\hspace{0.17em}|\hspace{0.17em}0–3, 2, 2–3, J\hspace{0.17em}+\hspace{0.17em}}\frac{1}{2}〉$ of $a^3{\Pi }^{13}\mathrm{CO}$ to give dipole moments which display a slight nonvibrational isotopic shift (1 in 10⁴) from the moments measured in the corresponding states $\mathrm{|\hspace{0.17em}\upsilon ,\; \Omega ,\; J〉}$ of $a^3{\Pi }^{12}\mathrm{CO}$ .