Abstract: | Rydberg $\gets$ Rydberg transitions of Ar$\sb2$ and Ne$\sb2$ have been observed for the first time by laser-excitation spectroscopy. Molecular Rydbergs converging to the $A\sp2\Sigma\sbsp{1/2\rm u}{+}$ ion core having principal quantum numbers as large as 26 for Ar$\sb2$ and 17 for Ne$\sb2$ have been observed despite the predissociative nature of the high-lying Ar$\sb2$ and Ne$\sb2$ Rydbergs. The most prominent Rydberg series have been identified as the np$\pi\sb{\rm u}$ $\gets$ ms$\sigma\sb{\rm g}$ (Ar: m = 4, 5 $\leq$ n $\leq$ 26; Ne: m = 3, 4 $\leq$ n $\leq$ 17). The adiabatic ionization limits, relative to the lowest metastable state (4s$\sigma\sb{\rm g}$(1$\sb{\rm u}$, 0$\sbsp{\rm u}{-}$) for Ar$\sb2$, and 3s$\sigma\sb{\rm g}\sp3\Sigma\sbsp{\rm u}{+}$ for Ne$\sb2$), were also determined to be 29373 $\pm$ 2 cm$\sp{-1}$ for Ar$\sb2$ and 34399 $\pm$ 3 cm$\sp{-1}$ for Ne$\sb2$. The associated np$\pi\sb{\rm u}$ $\gets$ ms$\sigma\sb{\rm g}$ (v$\sp\prime$, v$\sp{\prime\prime}$) = (0, 1) Rydberg series has also been observed (Ar: m = 4, 5 $\leq$ n $\leq$ 10; Ne: m = 3, 4 $\leq$ n $\leq$ 7) with ionization limits of 29079 $\pm$ 5 cm$\sp{-1}$ for Ar$\sb2$ and 33870 $\pm$ 20 cm$\sp{-1}$ for Ne$\sb2$. Two more Rydberg series have been observed in Ar$\sb2$ and assigned to be the np$\sigma\sb{\rm u}$ and the nf$\pi\sb{\rm u}$ with the same series limit as the np$\pi\sb{\rm u}$ (29373 cm$\sp{-1}$). Also, several of the Rydberg $\gets$ Rydberg transitions in Ar$\sb2$ and Ne$\sb2$ have been rotationally resolved. This, to the knowledge of the author, is the first time any Rydberg $\gets$ Rydberg transitions have been rotationally resolved in the rare gas dimers heavier than He$\sb2$. Analysis of two of the bands indicate that B$\sb0$ $\simeq$ 0.17 cm$\sp{-1}$ for Ar$\sb2$4s$\sigma\sb{\rm g}$(1$\sb{\rm u}$, 0$\sbsp{\rm u}{-}$) and B$\sb0$ = 0.564 $\pm$ 0.007 cm$\sp{-1}$ for Ne$\sb2$3s$\sigma\sb{\rm g}\sp3\Sigma\sbsp{\rm u}{+}$. The photoionization cross section, as a function of wavelength, for the lowest metastable states have also been calculated from the data presented in this work. |