U. Buck

Electron bombardment fragmentation of Ar van der Waals clusters by scattering analysis

Arn clusters generated in a supersonic nozzle beam are investigated by exploiting their different kinematical behavior in a scattering experiment. In a crossed molecular beam experiment the Arn clusters are scattered from He atoms at E=69.7 meV. By measuring the angular and velocity distributions of the scattered clusters, the different species can be separated from each other. In this way fragmentation probabilities of small Arn clusters up to n=6 by electron impact ionization are measured. The influence of collisional dissociation of the dimer is accounted for by classical trajectory calculations. The dimer fragmentation is found to be f21=0.40±0.05, while the trimer totally fragments to dimers ( f32=0.7±0.3) and monomers ( f31=0.3). All clusters with n from 4 to 6 appear with a probability of 0.95 on the dimer ion mass.

Solid water clusters in the size range of tens-thousands of H2O: A combined computational/spectroscopic outlook

A joint computational and experimental effort was directed towards the understanding of large solid water clusters. The computations included structure optimizations and calculations of OH stretch spectra for select sizes in the range n = 20–931. The measurements focused predominantly on OH stretch spectroscopy as a function of mean cluster size. FTIR spectra are discussed for the size range of tens to hundreds-of-thousands of molecules. Photofragment spectroscopy in molecular beams is shown to be a sensitive probe of the outer cluster surfaces. The crucial element of the different experimental approaches is the control and the estimation of the mean cluster sizes. The combined experimental and computational results are consistent with the physical picture of quasi-spherical nanocrystals with disordered reconstructed surface layers. The surface reconstruction can be viewed as the outcome of recombination of surface dangling atoms, to increase the number of hydrogen bonds. The hydrogen bonds within the mostly crystalline subsurface layer are stretched by the interaction with the disordered component. The emergence of the (strained) crystal core occurs at a size of a few hundred H2O. Smaller clusters are described as compact-amorphous.

Cluster size determination from diffractive He atom scattering

In a crossed molecular beam arrangement helium atoms are scattered from argon clusters which are produced in an averaged size range of n=6 to n=90 by adiabatic expansion through sonic and conical nozzles. The diffraction oscillations in the total differential cross section are used to derive information on the size distribution of the clusters by comparison with quantum mechanical calculations based on a model potential. In the size range covered by the measurements, the average cluster size is given by n=38.4(Γ*/1000)1.64, where Γ* is the scaling parameter of the source conditions introduced by Hagena [Z. Phys. D 4, 291 (1987)]. The results are in agreement with recent measurements of corrected mass spectra but disagree with the results obtained from electron diffraction. General relations are recommended which connect the scaling parameter with the averaged size.

State resolved rotational excitation in D2+H2 collisions

In a crossed molecular beam experiment time‐of‐flight distributions of ortho D2 molecules scattered from normal H2 (nH2) and para H2 (pH2) have been measured in a center‐of‐mass angular range of 75° to 180°. The collision energies were 84.1 and 87.2 meV, respectively. In all spectra the rotational excitation of D2 from j=0 to j=2 has been resolved. With pH2 as secondary beam the same transition could also be observed for H2. The measurements show that the probability for rotational excitation of D2 depends on whether the scattering partner H2 is rotating (nH2) or not (pH2). In the first case the cross sections are larger by a factor of approximately 2. The reason for this behavior is the presence of an additional interaction term which is at long range distances, identical to the quadrupole–quadrupole interaction and which is absent if H2 is in the j=0 state. The experimentally derived differential cross sections for the rotational excitation of D2 and H2 are compared with theoretical results obtained by ...

Differential cross sections for the j=0→1 rotational excitation in HD–Ne collisions and their relevance to the anisotropic interaction

In a crossed molecular beam experiment, time‐of‐flight distributions of HD‐molecules scattered from Ne at an energy of 31.5 meV have been measured using the pseudorandom chopper method. Each time‐of‐flight spectrum shows a clearly resolved inelastic peak due to rotational excitation of HD from j=0 to j=1. With the aid of these spectra, together with additional measurements of the total differential cross section, the angular dependence of the differential cross section for the excitation j=0→1 is derived over a large angular range (20° to 120°). The sensitivity of the data to the interaction potential is carefully studied and the measured inelastic differential cross sections are compared with calculations based on interaction potentials recently proposed for this system.

A Fully Size-Resolved Perspective on the Crystallization of Water Clusters

Minimal Ice Water clusters comprising fewer than 100 molecules have long been studied in gas phase to model the more complex structures of ice and liquid water. At some stage, as clusters grow larger, they effectively become tiny crystals of ice, but it has been hard to pinpoint precisely where in the range between 100 and 1000 molecules the formal transition takes place. Pradzynski et al. (p. 1529) used vibrational spectroscopy to show that the onset of an icelike structure, indicated by a characteristically distinct absorption band in the infrared, occurs at a cluster size of approximately 275 molecules. Infrared spectroscopy of water clusters of increasing size indicates the onset of an ice-like structure at around 275 molecules. The number of water molecules needed to form the smallest ice crystals has proven challenging to pinpoint experimentally. This information would help to better understand the hydrogen-bonding interactions that account for the macroscopic properties of water. Here, we report infrared (IR) spectra of precisely size-selected (H2O)n clusters, with n ranging from 85 to 475; sodium doping and associated IR excitation–modulated photoionization spectroscopy allowed the study of this previously intractable size domain. Spectral features indicating the onset of crystallization are first observed for n = 275 ± 25; for n = 475 ± 25, the well-known band of crystalline ice around 3200 cm−1 dominates the OH-stretching region. The applied method has the potential to push size-resolved IR spectroscopy of neutral clusters more broadly to the 100- to 1000-molecule range, in which many solvents start to manifest condensed phase properties.

The anisotropic interaction potential of D2Ne from state‐to‐state differential cross sections for rotational excitation

Differential cross sections for the rotational excitation from j=0 to j=2 of D2 scattered by Ne have been measured at an energy of E=84.9 meV. The experiments have been performed in a crossed nozzle beam apparatus with time‐of‐flight analysis of the scattered particles using the pseudorandom chopper method. A detailed analysis of the experimental data which are peaked in the backward direction showed that they are mainly sensitive to the repulsive part of the pure anisotropic potential. From a combined analysis of the state‐to‐state differential cross sections of the j=0 to j=0 and the j=0 to j=2 transition of D2+Ne and the j=0 to j=1 transition of HD+Ne previously measured, the complete potential energy surface for the hydrogen–neon system is obtained using the coupled states method. The anisotropic contribution varies from 37% of the isotropic part in the repulsive region (2.4 A) to 12% in the attractive region (3.5 A). The results differ from the other potential models derived for this system from calc...

Structure and vibrational spectra of methanol clusters from a new potential model

The structures and vibrational spectra of small methanol clusters from dimer to decamer have been calculated using a newly developed intermolecular potential which is essentially based on monomer wave functions. Special care has been taken for the description of the electrostatic interaction using a distributed multipole representation and including a penetration term. In addition, the potential model consists of repulsion, dispersion, and induction terms. Based on this potential model cluster structures have been calculated. The lowest energy dimer configuration is linear, while from trimer to decamer for the most stable structures ring configurations were found. Tetramer, hexamer, and octamer have S4-, S6-, and S8-symmetry, respectively. Vibrational spectra of the CO stretch and the OH stretch mode have been determined in the harmonic and in the anharmonic approximation using perturbation theory and variational calculations. Up to the tetramer the experimental spectra of the CO stretch mode are well rep...

THE ASYMMETRIC CAGE STRUCTURE OF (H2O)7 FROM A COMBINED SPECTROSCOPIC AND COMPUTATIONAL STUDY

The vibrational OH stretch spectra have been measured for the size-selected pure water clusters (H2O)7. In contrast to (H2O)n, n=8–10 clusters, which exhibit three distinct bands corresponding to three distinct types of OH bonds, the heptamer spectrum displays seven bands spanning the range from 2935 to 3720 cm−1. Calculations suggest that the spectra originate from two isomers, derived from the S4 octamer cube by removal of either one double donor or one double acceptor water molecule.

Determination of Intermolecular Potentials by the Inversion of Molecular Beam Scattering Data. I. The Inversion Procedure

A practical solution of the inversion problem within the realms of the WKB approximation is reported. The basic input information is only the measured relative differential cross section, from which the WKB phase function or the classical deflection function is constructed. The absolute potential curve is then determined by the application of a modified Firsov method. The problem of which part of the differential cross section is readily measurable and also suitable for the inversion procedure is studied. A practical procedure is proposed and tested by using a simulated example.