The photoionization of neutral liquid helium droplets (mean particle number 〈N〉=102–107) was studied using synchrotron radiation at photon energies ranging from 15 to 30 eV. Mass spectra as well as total and mass selective ion yields were measured as a function of the photon energy for different droplet sizes. The experiments indicate that ionization occurs not only by a direct process at photon energies above the atomic ionization potential but also at energies below the threshold by an autoionization process. The latter ionization mechanism proceeds via the electronically excited states of the neutral droplet, which show a strong neutral droplet size dependence. For large neutral droplets HeN(〈N〉≳104) retarding field measurements established that a predominant part of the total ion yield results from larger cluster ions He+k(k≳103). These measurements also show that a decay by fluorescence emission is much more probable than one by ionization following the photoexcitation process. In droplets with embedded SF6 molecules these are ionized indirectly by Penning ionization via excitons which leads to a large ion signal on the mass of the embedded species. No evidence for direct photoionization of the impurities was found.

Infrared molecular beam depletion spectroscopy has been employed to study the vibrational spectroscopy of water molecules and small water polymers [(H2O)n, n=2,3,4] embedded in large liquid helium clusters (HeN, 100≤〈N〉≤5000). The spectral region between 3500 and 3800 cm−1 was covered with an injection-seeded optical parametric oscillator. The following vibrational bands could be located and investigated: for the monomer the ν3 asymmetric stretch, for the dimer the asymmetric stretch of the proton acceptor molecule and the free and bonded O–H stretches of the donor molecule, for the trimer both free and bonded O–H stretches, and for the tetramer the free O–H stretch. By comparison with the data on free gas phase water complexes, it was found that the helium host clusters induce only minor perturbations in form of small frequency redshifts and that they constitute an ideal nano-matrix. Furthermore, it was found that both the water molecule and the dimer rotate almost freely in the host cluster and that the internal-rotationlike tunneling motion of the water dimer is not quenched. Due to the extremely low internal temperatures, a splitting of the trimer band associated with the O–H ring vibration could be resolved for the first time. This splitting indicates that the trimer structure is asymmetric, as has been predicted by theoretical calculations.

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Structure and hydration of the C4H4 •+ ion formed by electron impact ionization of acetylene clustersInfrared molecular beam depletion spectroscopy of small methanol and acetonitrile clusters embedded in large helium clusters has been studied in the spectral region of the CO stretch and the CH 3 rock mode from 1023 to 1059 cm Ϫ1 . The results are compared with the experimental spectra of the corresponding free clusters generated in adiabatic expansions and calculations based on density functional theory or empirical potential models. For methanol clusters, the two types of experimental results are the same for the dimer and trimer structure. Different isomers are found in cold helium for the tetramer and pentamer, namely a monomer and dimer attached to a cyclic trimer. For acetonitrile clusters in helium, aside from the dimer, different structures are observed. The spectra from the trimer to the hexamer are dominated by structures which contain the antiparallel dimer as building block with D 2d symmetry for the tetramer. They do not correspond to the minimum configurations observed for the free clusters. The fragmentation of the two cluster groups in helium droplets by electron impact ionization is discussed.

The optical absorption of size-selected helium cluster ions was studied via photofragmentation spectroscopy. Absorption cross sections were measured for He n ϩ ͑nϭ3, 4, 10, 21, and 30͒. A broad absorption peak was found, which for He 3 ϩ is centered at Ϸ5.3 eV, and which with increasing cluster size shifts slightly to the red. In addition, the kinetic energy release to the ionized and neutral photofragments was measured by a time-of-flight technique for cluster sizes between nϭ3 and 10. From the energy balance the total binding energy of the He 3 ϩ trimer ion was determined to be 2.6Ϯ0.15 eV. The results further indicate that a charged linear trimer acts as a core molecule for the sizes nϭ4-7. For nϾ7, an additional isomer with a tetramer core is identified. The results are compared with recent ab initio calculations.