Electronic spectroscopy on van der Waals-bound alkali trimers on He nanodroplets

Full text: Helium nanodroplets (Nbar = 104) are produced by supersonic free jet expansion and provide a cold environment (T = 0.4 K) for dopants. Helium clusters can dissipate energy very efficiently by evaporating their own atoms (binding energy per He atom: 5 cm-1). Alkali atoms are deposited on the helium surface by passing the droplet beam through one or more heated pick-up cells containing alkali vapor. Capture of multiple atoms per cluster leads to molecular formation. Due to the amount of binding energy released into the cluster, strongly bound low-spin molecules will be expelled from the droplet beam, while weakly bound high-spin van der Waals-molecules will remain on the droplets. In the wavelength range 10500 - 17500 cm-1 we have found a variety of electronic spectra of the homo- and heteronuclear trimers K3, Rb3, K2Rb and KRb2 in the highspin quartet states. Various schemes of beam depletion spectroscopy, such as two-laser excitation and mass selective depletion were applied to assign the species and separate overlapping spectral features. Quantum chemistry calculations are presently being performed in our group to support assignment of the observed spectra. In this talk, our experimental work will be presented and compared to the most recent theoretical results. (author)