Christof Steinbach

Ionization potentials of large sodium doped ammonia clusters.

In a continuous neat supersonic expansion ammonia clusters are generated and doped with sodium atoms in a pickup cell. Thus clusters of the form Na(NH(3))(n) are produced that are photoionized by a tunable dye laser system. The ions are mass analyzed in a reflectron time-of-flight mass spectrometer, and the wavelength dependent ion signals serve for the determination of the ionization potentials (IP) of the different clusters in the size range 10< or =n< or =1500. Aside from a plateau for 10< or =n< or =17 and smaller steps at n=24, 35, and 59 on the average a continuous decrease of the IP with cluster size is observed. The IPs in this size range are linear with (n+1)(-13) and extrapolate to IP(n=infinity)=1.66+/-0.01 eV. The slope is consistent with a dielectric continuum model of the solvated electron and the dielectric constant of the solid. The extrapolated IPs are compared with results obtained for negative ammonia cluster ions and metallic solutions in liquid ammonia. Differences are explained by the presence of counterions and their various distances from the solvated electron.

Detection of the book isomer from the OH-stretch spectroscopy of size selected water hexamers

The vibrational OH-stretch spectrum of size selected water hexamer clusters has been measured for cluster temperatures between 40 and 60 K. By comparison with temperature dependent calculations the spectra were identified to be those of the book isomer. This result is in good agreement with recent predictions of the equilibrium isomer distributions in this temperature range.

Infrared spectroscopy of large ammonia clusters as a function of size

We have measured the vibrational spectra of large ammonia (NH3)n clusters by photofragment spectroscopy in the spectral range from 3150 to 3450 cm(-1) for the average sizes n = 29, 80, 212, 447, and 989 and by depletion spectroscopy for n=8. The spectra are dominated by peaks around 3385 cm(-1) which are attributed to the asymmetric nu3 NH-stretch mode. Two further peaks between 3200 and 3260 cm(-1) have about equal intensity for n = 8 and 29, but only about 0.40 of the intensity of the nu3 peak for the larger sizes. The spectra for the smallest and largest size agree with those obtained by Fourier transform infrared spectroscopy in slit jet expansion and collision cells, respectively. By accompanying calculation we demonstrate that the energetic order of the spectral features originating from the bending overtone 2nu4 and the symmetric NH-stretch nu1 in the range from 3150 to 3450 cm(-1) is changed between n = 10 and 100, while the asymmetric NH-stretch nu3 only exhibits a moderate redshift. The reason is the coupling of the ground state modes to the overtones.

Reaction and solvation of sodium in hydrogen bonded solvent clusters

The reactive scattering of water clusters (H2O)n, n < or = 160 and ammonia clusters (NH3)n, n < or = 250 with 1 to 60 of sodium atoms is investigated. The water and ammonia cluster beams, respectively, are created in a pure supersonic expansion and cross an atmosphere of sodium vapor in a pickup cell. The reaction products are detected by photoionisation close to the threshold and mass analysed in a reflectron time-of-flight mass spectrometer. At low pickup pressures, sodium doped water and ammonia clusters are detected which reflect the correct size distribution of the pure cluster beams. At larger pickup pressures many sodium atoms are captured. In the case of ammonia thereby finally pure sodium clusters are produced, and the initial ammonia cluster is completely evaporated by the heat of formation of the formed sodium clusters. The water clusters, in contrast, react with the captured sodium until pure, even numbered sodium hydroxide clusters Na(NaOH)m doped with one sodium atom and hydrogen molecules are formed. In this way the unique reaction mechanism observed and calculated for small systems is confirmed. The additional Na atom serves together with the solvent water molecules as catalyst for creating the reaction intermediate NaH.

Elastic and rotationally inelastic differential cross sections for He+H2O collisions

Elastic and rotationally inelastic cross sections have been measured for He+H2O scattering at two collision energies, 66.3 and 99.0 meV, using the crossed molecular beam technique. The inelastic events are detected by time-of-flight analysis of the scattered He atoms. The data are converted to elastic differential cross sections and inelastic angular-dependent energy loss spectra in the center-of-mass system. They are compared with averaged, full close-coupling calculations of state-to-state cross sections for rotational excitation based on a newly calculated ab initio potential using symmetry-adapted perturbation theory. The agreement with the elastic differential cross sections is excellent. The energy loss spectra are reproduced satisfactorily and among the largest differential cross sections that contributed to the measurements are excitations around all three possible axes for ΔJ=1 but a preference of the excitation around the in-plane C axis for ΔJ=2 transitions.

Size-selective vibrational spectroscopy of methyl glycolate clusters: comparison with ragout-jet FTIR spectroscopy

A comprehensive experimental study of OH-stretching vibrations of size selected methyl glycolate clusters is presented. A depletion spectroscopy experiment in a crossed molecular beam apparatus was employed to scrutinize the cluster size assignment based on pressure dependence studies in a jet-FTIR experiment. First, the dimer to tetramer size assignments of the FTIR spectrum are confirmed by depletion signal angular dependencies measured at the FTIR absorption maxima. Then, independent depletion spectra of the size selected dimers through tetramers are presented. The depletion spectra exhibit peak broadening and blue-shifts with respect to the FTIR spectrum. These differences are discussed and partially explained by cluster heating through energy transfer in the scattering collisions with Ne atoms.

Total differential cross sections for Ar–CH4 from an ab initio potential

Total differential cross sections for the Ar–CH4 scattering complex at ECM=90.1 meV were obtained from converged close-coupling calculations based on a recent ab initio potential computed by symmetry-adapted perturbation theory (SAPT). Agreement with experiment is good, which demonstrates the accuracy of the SAPT potential.

Vibrational spectroscopy and reactions of water clusters

The condensed phase of water is probably the most investigated substance in Physical Chemistry. On the one hand, there are numerous anomalous properties of the liquid like the heat capacity and the density as well as the many configurations of ice that make water with its tendency to form a network of hydrogen bonds so interesting [1]. On the other hand, water plays a key role as ubiquitous solvent on earth and as promotor of reactions in atmospheric and extraterrestrial chemistry. In spite of all the efforts and the progress made in the last 20 years, a unified description of all the phenomena starting from the basic molecular interaction is still missing. We do not have a consistent description of liquid water nor do we understand completely the spectral and surface properties of the different conformations of ice. One of the reasons is certainly the lack of good, flexible interaction potentials that correctly account for all the many-body effects which play a crucial role in the hydrogen-bonded network of the water interaction [2]. In many of the simulations many-body effects are included by effective two-body interactions and are thus mostly valid in that range of applications to which their parameters are fitted. The other problem is the lack of detailed experimental results that would allow us to derive explicit conclusions about the underlying molecular models. In liquids, for instance, the information is often restricted by averaging processes and only a global picture results.