Knowledge Bank

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 temparature of 0.4K) on the formation and spectra of alkali molecules. Here, we will report our measurements on the homonuclear diatomics $Li2$, $Na2$, and $K2$. As published earlier $\mathrm{<mrow\; data-mjx-texclass="ORD">1,2</mrow>}$, weakly bound states can be formed due to the low temperature. We have recorded both laser excitation spectra and spectrally resolved emission for the $13\Pi g-13\Sigma u+$ transition of $Li2$ and $K2$ as well as for the $13\Pi g-13\Sigma u+$ 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 $1g\Pi$ 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, which also the $K2B1\Pi u-X1\Sigma g+$ and $13\Pi g-13\Sigma u+$ emission are observed. Singlet $A1\Sigma u+-X1\Sigma g+$ 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.