We measured laser-induced-fluorescence (LIF) and beam-depletion (BD) spectra of rubidium atoms (5S-5P transition) on the surface of superfluid helium nanodroplets (M-He_{N} with M=Rb). It is known that when M is a lighter alkali atom electronic excitation always leads to detachment of the excited atom (M;{*}). The dissociation energy, few tens cm;{-1}, comes either as photon excess energy or from the barrierless formation of a M;{*}-He exciplex. We observe that this picture does not hold when M=Rb and the photon excess energy is small: we are able to excite atoms without detaching them from the droplet, thanks to a barrier preventing formation of the exciplex. This system is ideally suited for optical spin pumping in a He nanodroplet, whose achievement we explicitly demonstrate in a pump-probe magnetic circular dichroism experiment.

The electronic excitation spectra of all possible homo- and heteronuclear high-spin (quartet) trimers of K and Rb (KxRb(3-x), x=0...3) assembled on the surface of superfluid helium droplets, are measured in the spectral range from 10,600 to 17,400 cm(-1). A regular series of corresponding bands is observed, reflecting the similar electronic structure of all these trimers. For the assignment and separation of overlapping bands, we determine x directly, with mass-selected beam depletion, and indirectly with a V-type double-resonance scheme. The assignment is confirmed by high-level ab initio calculations of the electronic structure of the bare trimers. The level structure is rationalized in terms of harmonic-oscillator states of the three valence electrons in a quantum-dot-like confining potential. We predict that three should be a magic number for high-spin alkali clusters.

KRb and Rb2 dimers are formed on cold helium nanodroplets. Laser excitation and emission spectra from the transition (1)3Πg–3Σu+ of Rb2 and the transition (2)3Π–3Σ+ of KRb have been measured. A thorough analysis of the emission, which takes place after desorption of the dimers from the surface of the helium cluster, has been performed. It allows us to determine the redistribution of vibrational population on the droplet surface. A comparison of the experimental spectra with simulations, obtained with the potential energy curves and dipole moments calculated by Aymar and Dulieu, allows a test of the accuracy of the ab initio potentials.

In this paper, we discuss the electronic structure of alkali dimer molecules in 3Pig states on the surface of a helium droplet. The perturbation due to the droplet will in general not satisfy rotational symmetry around the internuclear axis of the diatom and thus, in addition to a broadening and blue shift, will cause a splitting of electronic levels that are degenerate in the free molecules. We propose a model based on general symmetry arguments and on a small number of physically reasonable parameters. We demonstrate that such a model accounts for the essential features of laser-induced fluorescence (LIF) and magnetic circular dichroism (MCD) spectra of the (1)3Pig-a3Sigma+ transition of Rb2 and K2. Furthermore the MCD spectra, analyzed according to the approach of Langford and Williamson [J. Phys. Chem. A 1998, 102, 2415], allow a determination of the populations of Zeeman sublevels in the ground state and thus a measurement of the surface temperature of the droplet. The latter agrees with the accepted temperature, 0.37 K, measured in the interior of a droplet.

Electronic excitation spectra of homo- (K(3),Rb(3)) and heteronuclear (K(2)Rb,KRb(2)) alkali trimers in the high-spin quartet state have been investigated in a broad spectral range (10,600-17,400 cm(-1)). Ten new bands showing laser induced fluorescence (LIF) were measured. Due to the pickup statistics, overlapping spectra of all possible oligomers are present at once, complicating the unraveling and assignment of individual spectra. To circumvent the problem, two variations of beam depletion spectroscopy were employed in addition to the conventional analysis of the relation between signal and pickup pressure: A two-laser V-type double resonance scheme combining beam depletion with LIF, and a mass selective beam depletion scheme. In principle, these allow accurate separation of an arbitrary number of overlapping spectra. The benefits and drawbacks of each method are discussed. Assignment to electronic states is achieved by comparison with ab initio complete active space self-consistent field calculations of the excited electronic level structure of the molecules.