Structure of excitation and fluorescence spectra recorded at the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow/><mml:mrow><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:mrow><mml:mrow><mml:msubsup><mml:mrow><mml:mn>0</mml:mn></mml:mrow><mml:mrow><mml:mi>u</mml:mi></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msubsup></mml:mrow><mml:mo>(</mml:mo><mml:mn>5</mml:mn><mml:mn/><mml:mrow><mml:msup><mml:mrow><mml:mi/></mml:mrow><mml:mrow><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi>P</mml:mi></mml:mrow><mml:mrow><mml:mn>1</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mo>)</mml:mo><mml:mo>–</mml:mo><mml:mi>X</mml:mi><mml:mrow><mml:msup><mml:mrow><mml:mi/></mml:mrow><mml:mrow><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:mrow><mml:mrow><mml:msubsup><mml:mrow><mml:mn>0</mml:mn></mml:mrow><mml:mrow><mml:mi>g</mml:mi></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msubsup></mml:mrow></mml:math>transition of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Cd</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

Excitation and fluorescence ultraviolet spectra of ${\mathrm{Cd}}_{2}$ recorded at the ${}^{1}{0}_{u}^{+}(5{}^{1}{P}_{1})--X{}^{1}{0}_{g}^{+}$ transition are reported. The ${\mathrm{Cd}}_{2}$ molecules (seeded in Ar) produced in a continuous free-jet supersonic beam were excited in a vacuum chamber with a pulsed dye-laser beam. A well-resolved vibrational structure of the ${}^{1}{0}_{u}^{+}\ensuremath{\leftarrow}X{}^{1}{0}_{g}^{+}$ excitation spectrum as well as the isotopic structure of the vibrational components were recorded. Analysis of the spectrum yielded vibrational constants for the ${}^{1}{0}_{u}^{+}$ state: ${\ensuremath{\omega}}_{e}^{\ensuremath{'}}=100.50\ifmmode\pm\else\textpm\fi{}0.25{\mathrm{cm}}^{\ensuremath{-}1},$ ${\ensuremath{\omega}}_{e}^{\ensuremath{'}}{x}_{e}^{\ensuremath{'}}=0.325\ifmmode\pm\else\textpm\fi{}0.003{\mathrm{cm}}^{\ensuremath{-}1},$ ${D}_{0}^{\ensuremath{'}}=8638\ifmmode\pm\else\textpm\fi{}15{\mathrm{cm}}^{\ensuremath{-}1},$ ${D}_{e}^{\ensuremath{'}}=8688\ifmmode\pm\else\textpm\fi{}15{\mathrm{cm}}^{\ensuremath{-}1},$ and $\ensuremath{\Delta}{R}_{e}{=R}_{e}^{\ensuremath{''}}\ensuremath{-}{R}_{e}^{\ensuremath{'}}=1.04\ifmmode\pm\else\textpm\fi{}0.01\AA{}$ derived for the ${}^{226}{\mathrm{Cd}}_{2}$ isotopomer. The ${}^{1}{0}_{u}^{+}$ state potential-energy (PE) curve was obtained numerically using an inverse perturbation approach (IPA) procedure. Condon internal diffraction (CID) patterns in the ${}^{1}{0}_{u}^{+}\ensuremath{\rightarrow}X{}^{1}{0}_{g}^{+}$ fluorescence band, emitted upon the selective excitation of the ${v}^{\ensuremath{'}}=38$ and ${v}^{\ensuremath{'}}=39$ vibrational components of the ${}^{226}{\mathrm{Cd}}_{2}$ isotopomer, were observed and improved the ${v}^{\ensuremath{'}}$ assignments derived from the analysis of the isotopic structure. Analysis of the fluorescence spectrum yielded information on the repulsive part of the ground-state interatomic potential. The result confirms a relatively soft repulsion between two Cd atoms in the short-range (2.53--4.05 \AA{}) region and makes allowance for a covalent admixture to the ground-state van der Waals bonding. Quasirelativistic valence ab initio calculations on the PE curves for the investigated states have been performed at the complete-active-space multiconfiguration self-consistent-field (CASSCF/CAS) multireference second-order perturbation theory (CASPT2) level with the total of 40 correlated electrons. In the calculations, the Cd atom is considered as a 20-valence electron system whereas the ${\mathrm{Cd}}^{20+}$ core is replaced by an energy-consistent pseudopotential which also accounts for scalar-relativistic effects and spin-orbit interaction within the valence shell. A comparison with results obtained from other experiments and ab initio calculations is presented.

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