Optical singlet and triplet bands of homonuclear alkali diatomics attached to cold helium clusters

Large helium clusters consisting of several thousand atoms are produced in a supersonic expansion and doped with alkali atoms in a low pressure pick-up scattering cell. Laser-induced fluorescence spectroscopy is used to probe the influence of the cold He environment (internal temperature of 0.4K) on the formation and spectra of alkali molecules. Here, the authors will report measurements on the homonuclear diatomics Li2, Na2, and K2. As published earlier, weakly bound states can be formed due to the low temperature. The authors have recorded both laser excitation spectra and spectrally resolved emission for the 13Πg - 13Σu+ transition of Li2 and K2 as well as for the 13Σg+ - 13Σu+ transition of Na2. The triplet transitions in Li2 and in K2 had not been directly observed so far. In the case of Li2, the upper state potential is weakly repulsive and, for K2, the 13Πg state has a shallow well with a barrier towards dissociation. After excitation of this state, the spectrally resolved fluorescence reveals the atomic K (4p-4s) transition is the strongest decay channel, while also the K2 B1Πu-X1Σg+ and 13Πg-l3Πu+ emission are observed. Singlet A1Σu+-X1Πg+ transitions have been recorded for Na2 and K2. The vibrational bands consist of distinct, narrow zero-phonon lines, a narrow gap, and a wide phonon wing towards the blue. The dynamics of the alkali dimer-He cluster interaction will be discussed