Knowledge Bank

Tunable, coherent, and monochromatic radiation, generated by four-wave sum-mixing in Hg $vapor1$ was used to investigate spectra of $Ar2$ in the region 104-108 nm. Argon dimers were formed by pulsed supersonic expansion, and rovibronic transitions were detected by fluorescence $emission2$ and by photo-ionization. A comprehensive study of high-resolution spectra, corresponding to transitions from the van der Waals ground state to the three lowest excimer states was carried out, resulting in new information on spectroscopic constants. The observation of isotopic spectra of $\mathrm{<mrow\; data-mjx-texclass="ORD">40</mrow>}Ar2$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">36</mrow>}Ar\mathrm{<mrow\; data-mjx-texclass="ORD">40</mrow>}Ar$ has led to unambiguous vibrational numbering and to vibrational constants for the A and B states for the first time. Analyses of rovibronic structures have yielded new and $improved3$ rotational constants and internuclear distances. These data have been used to calculate potential energy curves for the excited states of $Ar2$. At a resolving power $>5\times 105$, three branches were observed in the band system A-X, revealing $\Omega$-type doubling of rotational levels of the A state. This has firmly established Hund's coupling case (c) and the symmetry $Iu$ for the A state. A brief description of the experimental technique will be presented along with the spectroscopic results.