The laser-induced fluorescence spectra of single tetracene (C 18 H 12 ) and pentacene (C 24 H 14 ) molecules embedded in liquid He N droplets (N ˜= 10 4 ) show sharp zero phonon lines (ZPL) (δν e 0.2 cm -1 ), accompanied by weaker phonon wings (PW) on the blue side. The ZPL of tetracene is anomalously split into a doublet with a separation of 1.1 cm -1 , whereas for pentacene, the ZPL is not split. Hole-burning measurements with two pulsed dye lasers and lifetime measurements indicate that inside He droplets the ground and excited states of tetracene are each split into two levels. The splitting is attributed either to the occupation of two nearly equivalent sites by localized helium atoms or to a tunneling of one or two localized helium atoms through the barrier in the double-well potential on the surface of the tetracene molecule.

Helium has a unique phase diagram and below 25 bar it does not form a solid even at the lowest temperatures. Electrostriction leads to the formation of a solid layer of helium around charged impurities at much lower pressures in liquid and superfluid helium. These so-called ‘Atkins snowballs' have been investigated for several simple ions. Here we form HenC60+ complexes with n exceeding 100 via electron ionization of helium nanodroplets doped with C60. Photofragmentation of these complexes is measured by merging a tunable narrow-bandwidth laser beam with the ions. A switch from red- to blueshift of the absorption frequency of HenC60+ on addition of He atoms at n=32 is associated with a phase transition in the attached helium layer from solid to partly liquid (melting of the Atkins snowball). Elaborate molecular dynamics simulations using a realistic force field and including quantum effects support this interpretation.

Site-specific interaction between hydrocarbon cations and inert ligands: IR spectra of isomeric C 3 H 3 + -L dimers (L ¼ Ne,Ar,O 2 ,N 2 ,CO 2 ) Doris Roth and Otto Dopfer 4866 Formation of hydrogen-bonded bridges in jet-cooled complexes of a chiral chromophore as studied by IR/UV double resonance spectroscopy. (AE)2-Naphthyl-1-ethanol/(methanol) n ¼ 1,2 complexes K. Le Barbu-Debus, N. Seurre, F. Lahmani and A. Zehnacker-Rentien 4877 Tautomers and electronic states of jet-cooled adenine investigated by double resonance spectroscopy Chr. Plu ¨tzer and K. Kleinermanns 4883 L Microwave rotational spectroscopic investigation of the Ar 2 -NH 3 van der Waals trimer Jennifer van Wijngaarden and Wolfgang Ja ¨ger 4890 L Refined, scaled and canonical force fields for the cis-and trans-3-fluorostyrene conformers. An interplay between theoretical calculations, IR/Raman and INS data J. M. Granadino-Rolda ´n, M. Ferna ´ndez-Go ´mez, A. Navarro, L. M. Camus and U. A. Jayasooriya 4902 L Intermolecular interactions of the CX 3 OCHO dimers, and complexes CX 3 OCHO-n(H 2 O), CX 3 OCHOn(HO 2 ) (X ¼ H,F; n ¼ 1,2

The photoionization and photoelectron spectroscopy of He nanodroplets (10(4) atoms) has been studied by photoelectron imaging with photon energies from 22.5-24.5 eV. Total electron yield measurements reveal broad features, whose onset is approximately 1.5 eV below the ionization potential of atomic He. The photoelectron spectra are dominated by very low energy electrons, with less than 0.6 meV. These results are attributed to the formation and autoionization of highly vibrationally excited He(*)(n) Rydberg states within the cluster, followed by strong final state interactions between the photoelectron and the droplet.

High resolution vibronic absorption spectra are reported for the amino acids tryptophan and tyrosine embedded in large (N≃103–104) helium droplets. The spectra exhibit a large number of sharp (δν≈0.5 cm−1) 000 band origins which are attributed to different conformers. The significant differences to previous spectra of the isolated molecules from seeded beam expansions are interpreted in terms of the lower temperatures and the suppression of certain conformers in the liquid helium environment.