Observation of the pure long-range1ustate of an alkali-metal dimer by photoassociative spectroscopy

The pure long-range $1_u$ state of an alkali-metal dimer is observed in ${}^{39}{\mathrm{K}}_2$ using photoassociative spectroscopy of ultracold atoms. High-resolution spectra for vibrational levels $v=\overrightarrow{0}7$ are recorded by resonance-enhanced two-photon ionization. Strong rotation-hyperfine couplings are observed. Major molecular constants and a hyperfine structure analysis are presented. This $1_u$ state is useful as an ungerade Franck-Condon “window” at large internuclear distance for studies of highly excited long-range interactions.