F. Stienkemeier

Spectroscopy and dynamics in helium nanodroplets

This article provides a review of recent work in the field of helium nanodroplet spectroscopy with an emphasis on the dynamical aspects of the interactions between molecules in helium as well as their interaction with this unique quantum solvent. Emphasis is placed on experimental methods and studies introducing recent new approaches, in particular including time-resolved techniques. Corresponding theoretical results on the energetics and dynamics of helium droplets are also discussed.

Electronic spectroscopy in He droplets

Recent experiments on the electronic spectroscopy of atoms, clusters, and organic molecules embedded in helium nanodroplets are reviewed. Electronic transitions imply a larger degree of distortion of the helium environment as compared to vibrational and rotational excitations. Thus new phenomena arise such as the appearance of side bands in the spectra, which are due to the excitation of helium collective vibrations, large changes of the effective molecular rotational constants and even the expulsion of an atom (or molecule) from the cluster upon excitation. These features make it possible to probe the helium environment and its interactions with molecular chromophores on the atomic scale. Real-time studies of the manifestations of superfluidity and of chemical processes in the droplets via femtosecond excitation techniques, provide a new perspective to this field. The considerable amount of data available so far shows the large potential of helium droplets for isolation and spectroscopy of large molecule...

Photoinduced Chemical Dynamics of High-Spin Alkali Trimers

Nanometer-sized helium droplets, each containing about 104 helium atoms, were used as an inert substrate on which to form previously unobserved, spin-3/2 (quartet state) alkali trimers. Dispersed fluorescence measurements reveal that, upon electronic excitation, the quartet trimers undergo intersystem crossing to the doublet manifold, followed by dissociation of the doublet trimer into an atom and a covalently bound singlet dimer. As shown by this work, aggregates of spin-polarized alkali metals represent ideal species for the optical study of fundamental chemical dynamics processes including nonadiabatic spin conversion, change of bonding nature, and unimolecular dissociation.

On the use of liquid helium cluster beams for the preparation and spectroscopy of the triplet states of alkali dimers and other weakly bound complexes

Sodium dimers in their lowest triplet state are prepared by pick‐up of sodium atoms by a beam of large (n≂104) liquid helium clusters when these cross a scattering box containing sodium vapor. Using laser induced fluorescence detection the vibrationally resolved laser excitation spectrum of the 1 3Σ+g←1 3Σ+u transitions of Na2 is obtained. The spectral shifts with respect to the corresponding gas phase values are surprisingly small. These data show the feasibility of using helium clusters as an almost ideal matrix for the synthesis and spectroscopic analysis of weakly bound complexes.

Steric effects in the state specific reaction Li+HF (v=1, j=1, m=0)→LiF+H

Results of a crossed beam study on the state specific reaction Li+HF (v=1, j=1, m=0)→LiF+H at a translational collision energy of Etr=0.42 eV are reported. Angular distributions of LiF have been measured for three different distributions of the internuclear axis of HF, namely an isotropic one, one where the axis is aligned with the relative velocity of reagents V and one where the axis is aligned perpendicular to both V and the scattering plane. We find a marked influence of these collision geometries (steric effects) on (i) the angular distributions; (ii) the partition of available energy; and (iii) the integral reaction cross sections. The ratio of the latter for preferred side‐on and end‐on collisions with HF amounts to 1.76. From the angular distributions of products, double‐differential cross sections in the center‐of‐mass frame are determined which exhibit in all three cases preferred backward scattering of LiF. They provide three out of four accessible moments of the orientation‐dependent double‐di...

Formation and properties of metal clusters isolated in helium droplets

The unique conditions forming atomic and molecular complexes and clusters using superfluid helium nanodroplets have opened up an innovative route for studying the physical and chemical properties of matter on the nanoscale. This review summarizes the specific characteristics of the formation of atomic clusters partly generated far from equilibrium in the helium environment. Special emphasis is on the optical response, electronic properties as well as dynamical processes which are mostly affected by the surrounding quantum matrix. Experiments include the optical induced response of isolated cluster systems in helium under quite different excitation conditions ranging from the linear regime up to the violent interaction with a strong laser field leading to Coulomb explosion and the generation of highly charged atomic fragments. The variety of results on the outstanding properties in the quantum size regime highlights the peculiar capabilities of helium nanodroplet isolation spectroscopy.

Coherent control with a short-wavelength free-electron laser

Researchers demonstrate correlation of two colours (63.0 and 31.5 nm wavelengths) in a free-electron laser and control photoelectron angular distribution by adjusting phase with 3 attosecond resolution.

Alkaline earth metals (Ca, Sr) attached to liquid helium droplets: Inside or out?

Superfluid helium droplets HeN at 0.4 K are doped with single alkaline earth atoms (Ca, Sr). The absorption spectra for excitation of the lowest singlet transitions display broad, blue-shifted peaks if compared to the atomic lines. The shift increases with droplet size increasing from N≈1000 up to N≈10000 but converges to only about one third of the bulk value.

Spectroscopy of Cs attached to helium nanodroplets.

Cesium oligomers are formed on helium nanodroplets which are doped with one or a few Cs atoms. The monomer absorption of the first electronic p<--s transition upon laser excitation is probed. Spectra employing laser-induced fluorescence, beam depletion, and resonant photoionization are compared. In particular, mass-resolved photoionization allows us to specifically probe excitation induced processes such as, e.g., the formation of cesium-helium exciplexes. Absorption spectra of Cs dimers and trimers are recorded in the spectral region accessible by a Ti:sapphire laser. Assignment of dimer spectra is achieved by comparison with model calculations based on ab initio potentials. Electronic absorption lines of Cs trimers are attributed to transitions in the quartet manifold.

Quantum Interference Spectroscopy of Rubidium-Helium Exciplexes Formed on Helium Nanodroplets

Femtosecond multiphoton pump-probe photoionization is applied to helium nanodroplets doped with rubidium (Rb). The yield of Rb+ ions features pronounced quantum interference (QI) fringes demonstrating the coherence of a superposition of electronic states on a time scale of tens of picoseconds. Furthermore, we observe QI in the yield of formed RbHe exciplex molecules. The quantum interferogram allows us to determine the vibrational structure of these unstable molecules. From a sliced Fourier analysis one cannot only extract the population dynamics of vibrational states but also follow their energetic evolution during the RbHe formation.