Spectrum and Structure of the He2 Molecule. V. Characterization of the Triplet States Associated with the UAO's 6− 17pπ and 7− 12pσ

Abstract

The electronic transitions ${\text{6\hspace{0.17em}−\hspace{0.17em}17pπ}}^3{\Pi }_g{\text{→2s a}}^3{\Sigma }_u^{+}$ and ${\text{7\hspace{0.17em}−\hspace{0.17em}12pσ}}^3{\Sigma }_g^{+}{\text{→2s a}}^3{\Sigma }_u^{+}$ of He₂ are described and the previously unreported ${\mathrm{np\sigma }}^3{\Sigma }_g^{+}$ and ${\text{11\hspace{0.17em}−\hspace{0.17em}17pπ}}^3{\Pi }_g$ states characterized. Existing data for the triplet states associated with the UAO's $\mathrm{np\pi }$ and $\mathrm{np\pi }$ are summarized and discussed. The $\text{N\hspace{0.17em}=\hspace{0.17em}0, v\hspace{0.17em}=\hspace{0.17em}0}$ level of the ${\text{2s a}}^3{\Sigma }_u^{+}$ state (the lowest stable electronic state for He₂) is found to lie $\text{34,302.2\hspace{0.17em}±\hspace{0.17em}0.1}{\mathrm{cm}}^{\text{−1}}$ below the $\text{v\hspace{0.17em}=\hspace{0.17em}0}$ level of the $X^2{\Sigma }_u^{+}$ state of He₂. From extrapolation of the molecular constants for the ${\mathrm{np\pi }}^3{\Pi }_g^{-}$ series, approximate values for the ground state of He₂⁺ are ${\omega }_{\mathrm{e}}{\text{\hspace{0.17em}≅\hspace{0.17em}1698.}}_5{\mathrm{cm}}^{\text{−1}}{\mathrm{,\; x\omega }}_{\mathrm{e}}{\text{\hspace{0.17em}≅\hspace{0.17em}35.}}_{25}{\mathrm{cm}}^{\text{−1}}{\mathrm{,\; B}}_e\text{\hspace{0.17em}≅\hspace{0.17em}7.211}{\mathrm{cm}}^{\text{−1}}$ , and $r_e{\text{\hspace{0.17em}≅\hspace{0.17em}1.080}}_6\AA$ . “Accidental pure pecession” between the state pairs ${\mathrm{np\Pi }}^3{\Pi }_g^{+}$ and ${\text{n\hspace{0.17em}+\hspace{0.17em}1 pσ}}^3{\Sigma }_g^{+}$ is identified as the major (although not exclusive) perturbation systematically affecting the rotational levels of the npλ components with plus systmmery.