Analysis and control of doped helium droplets

Coherent control of molecular processes in adaptive feedback loops using tailored laser pulses has developed into a very active research field and a variety of successful results have been achieved. Also, spectroscopy of doped superfluid helium nanodroplets has attained considerable interest in recent years since these cold nanosystems exhibit unique properties. The aim of the proposed research project is to combine these two fields by irradiation of doped helium droplets with optimally shaped laser pulses. A joint effort of the German and Austrian groups will be essential for in-depth investigations. This opens interesting perspectives since it provides access to steer the photoinduced dynamics of van der Waals systems in the finite superfluid helium environment. The influence of the helium matrix on the molecular dynamics will be examined, particularly regarding its superfluid properties, bubble dynamics, cage effects, relaxation processes, and decoherence properties. Massive exchange of equipment and know-how between the two partners in Innsbruck and Berlin are essential for the proposed studies. Laser pulse control has been successful for dissociative processes but it still remains difficult for associative binary reactions. The main problem in photoassociative processes lies in the initial states, which in most cases constitute an incoherent continuum, meaning that there is no correlation between the colliding particles and the driving laser field. Thus, close proximity and pre-correlation of the reactants is required for effective photoassociation. Here, van der Waals complexes in helium droplets provide an exceptional starting point for performing further association. Shaped laser pulses will then be employed to optimally steer the system to a covalently bound target state. This dynamics will be investigated in Berlin for helium-alkali atom exciplex formation with several attached helium atoms by using polarization shaped laser pulses where the building process will be selectively optimized. The predominantly formed weakly bound high-spin alkali dimers and trimers on helium droplets will be excited in order to convert them by intersystem crossing to covalently bound low spin states. This can be performed vibrationally state selective via different electronically excited states. In both laboratories van der Waals complexes inside helium droplets will be examined regarding photoassociation. These complexes may be present in metastable configurations inside helium droplets, which permit to initiate the photoinduced dynamics from these metastable states. Moreover, possible peptide bond formation will be investigated and controlled starting from amino acid complexes. Another major goal will be to examine the molecular dynamics of ionic complexes in helium droplets. Thereto, we first ionize the dopant via electron collisions and then photoexcite and possibly further ionize the emerging ions with laser pulses. Particularly, molecules with structural motifs similar to fullerenes like triphenylenes and coronenes will be investigated due to their astrophysical relevance. Clusters of these molecules embedded in helium droplets will be converted by electron collisions and photoexcitation to covalently bound polycyclic aromatic hydrocarbon systems.

One of the most exciting results from the present project is the discovery of an intense signal of negatively charged helium atoms upon electron irradiation of large helium droplets. This anion has been known since 1939 when it was observed mass spectrometrically upon passing keV He+ ions through Na or nitrogen vapor. The lifetime of this anion is in the microsecond range and thus scientists considered this exotic anion to be of little relevance for chemical reactions. The present results, however, come to a completely different view and demonstrate that anions of helium can drive a variety of chemical reactions, some of them unique to this peculiar negatively charged anion. Helium minus in fact is an electronically excited He atom which weakly binds another electron. The electronic energy of this exotic anion is almost 20 eV which is sufficiently high to ionize upon low-energy collisions every atom or molecule except for neon. Besides driving some typical chemical reactions such as salt formation between alkali atoms and halogen containing molecules, He* can also transfer both loosely bound electrons to an appropriate reaction partner having a sufficiently high electron affinity, such as fullerenes. This double-electron-transfer is a novel and highly efficient mechanism for the formation of dianions. In collaboration with the German partner, PD Dr. Albrecht Lindinger from the FU-Berlin, we investigated weakly bound van der Waals complexes of C60+ with He atoms. Upon absorption of an infrared photon that is in resonance with an electronic transition of such complexes we determined absorption lines as a function of the number of the attached He atoms n from n=2 up to n=150. This action spectroscopy enabled us to probe spectroscopically the formation and melting of a solid layer of He atoms attached to a fullerene cation. These phase transitions from solid to liquid and even superfluid could be examined in detail and the results provide important insight and benchmarks for theoretical models and calculations. The present data also confirm independently the absorption lines of C60+ published recently in Nature by the group of John Maier from Basel. In the last two years several experimental groups in Europe and the USA developed instruments to perform action spectroscopy with He-tagged molecular ions. The weak matrix effect of He with the ions enables spectroscopy of almost unperturbed ions. The method developed in Innsbruck is suitable for all molecules and clusters and provides spectroscopic information from a single He atom attached to the completely solvated ion, atomically resolved.