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.

Infrared molecular beam depletion spectroscopy of ammonia molecules embedded in large He clusters has been studied in the spectral region of the ammonia umbrella mode from 940 to 1060 cm−1. The transitions between the lowest rotational energy levels could be resolved. By fitting the spectrum with the Hamiltonian of a free rotor, we observe a large shift of the vibrational frequency of 17.4 cm−1 to the blue, an effective decrease of the rotational constant B by 24%, and a reduction of the tunneling splitting by 31% in the excited and 75% in the ground state. This implies an unusually large interaction between the host cluster and the umbrella motion of the ammonia molecule, an intermediate influence on the rotation and also a large perturbation of the inversion by the helium environment.

Infrared molecular beam depletion spectroscopy has been used to measure the spectrum of ammonia dimers embedded in large helium clusters after the excitation of the umbrella mode. The spectral region from 940 to 1010 cm−1 was covered by a line-tunable CO2-laser. By measuring the dependence of the depletion signal on the pressure of the pick-up cell, we confirmed that all lines are caused by the dimer. The spectrum exhibits two groups of two lines at 978/983 and 992/997 cm−1 separated by 14 cm−1 with a line splitting of 5 cm−1 each. The infrared transitions are only marginally shifted compared to the free ammonia dimer. Also the inversion tunneling splittings are equal to those of the free dimer. Differences in the amplitudes are attributed to relaxation effects of the different inversion components of the same symmetry at the low temperature of 0.4 K. The interchange tunneling splitting, however, which dominates the line splittings in the free dimer is considerably quenched in the helium environment. In addition, the difference in the excitation of the two non-equivalent monomers is remarkably reduced in support of a more cyclic structure in the helium environment.