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The appearance of ice I in the smallest possible clusters and the nature of its phase coexistence with liquid water could not thus far be unraveled. The experimental and theoretical infrared spectroscopic and free-energy results of this work show the emergence of the characteristic hydrogen-bonding pattern of ice I in clusters containing only around 90 water molecules. The onset of crystallization is accompanied by an increase of surface oscillator intensity with decreasing surface-to-volume ratio, a spectral indicator of nanoscale crystallinity of water. In the size range from 90 to 150 water molecules, we observe mixtures of largely crystalline and purely amorphous clusters. Our analysis suggests that the liquid-ice I transition in clusters loses its sharp 1st-order character at the end of the crystalline-size regime and occurs over a range of temperatures through heterophasic oscillations in time, a process without analog in bulk water.

The author reviews the publications on the structural and dynamical properties of the weakly bound clusters. The topics of interest include size selection, the infrared photodissociation and the different methods of generating clusters which have been absorbed on the surface of a larger cluster.

Research is done on molecular dynamics simulations, which have been employed for the study of photolysis of hydrogen bromide placed inside, or on the surface of Ar12, Ar54, Ar97, and Ar146 clusters, representing one to three icosahedral solvation shells. In conclusion, calculation are compared to results of cluster experiments which have measured the kinetic energy distribution of hydrogens originating from HBr photolysis at 243 nm in or on argon clusters with an average size of 115 atoms, and a near-quantitative agreement is found.

The appearance of ice I in the smallest possible clusters and the nature of its phase coexistence with liquid water could not thus far be unraveled. The experimental and theoretical infrared spectroscopic and free-energy results of this work show the emergence of the characteristic hydrogen-bonding pattern of ice I in clusters containing only around 90 water molecules. The onset of crystallization is accompanied by an increase of surface oscillator intensity with decreasing surface-to-volume ratio, a spectral indicator of nanoscale crystallinity of water. In the size range from 90 to 150 water molecules, we observe mixtures of largely crystalline and purely amorphous clusters. Our analysis suggests that the liquid–ice I transition in clusters loses its sharp 1st-order character at the end of the crystalline-size regime and occurs over a range of temperatures through heterophasic oscillations in time, a process without analog in bulk water. [ABSTRACT FROM AUTHOR]

A computational approach capable of modeling homogeneous condensation in nonequilibrium environments is presented. The approach is based on the direct simulation Monte Carlo (DSMC) method, extended as appropriate to include the most important processes of cluster nucleation and evolution at the microscopic level. The approach uses a recombination-reaction energy-dependent mechanism of the DSMC method for the characterization of dimer formation, and the RRK model for the cluster evaporation. Three-step testing and validation of the model is conducted by (i) comparison of clusterization rates in an equilibrium heat bath with theoretical predictions for argon and water vapor and adjustment of the model parameters, (ii) comparison of the nonequilibrium argon cluster size distributions with experimental data, and (iii) comparison of the nonequilibrium water cluster size distributions with experimental measurements. Reasonable agreement was observed for all three parts of the validation. [ABSTRACT FROM AUTHOR]

A new class of sodium-water clusters with a low lying ionization potential (IP) is characterized by their photoionization spectra in molecular beam experiments. This implies that Na(H2O)n clusters coexist for n≥15 in two forms of significant abundances being distinguished by their IPs of ∼2.8 and ∼3.2 eV. A tentative quantum chemical characterization was achieved by simulating ionization spectra for selected cluster sizes using an ab initio molecular dynamics approach. Experiment and theory suggest that the Na+-e- distance is significantly larger in the clusters with the lower IP. This indicates that the solvated electron in Na(H2O)n clusters very probably forms with the Na+ counterion both a solvent separated and a contact ion pair. [ABSTRACT FROM AUTHOR]

Ab initio molecular dynamics simulations modeling low-energy collisions of a sodium atom with a cluster with more than 30 water molecules are presented. We follow the dynamics of the atom-cluster interaction and the delocalization of the valence electron of sodium together with the changes in the electron binding energy. This electron tends to be shared by the nascent sodium cation and the water cluster. IR spectra of the sodium-water cluster are both computationally and experimentally obtained, with a good agreement between the two approaches. [ABSTRACT FROM AUTHOR]

We report on the first observation of the organoxenon HXeCCH molecule in the gas phase. This molecule has been prepared in a molecular beam experiment by 193 nm photolysis of an acetylene molecule on Xen clusters (n≈390). Subsequently the molecule has been oriented via the pseudo-first-order Stark effect in a strong electric field of the polarized laser light combined with the weak electrostatic field in the extraction region of a time-of-flight spectrometer. The experimental evidence for the oriented molecule has been provided by measurements of its photodissociation. For comparison, photolysis of C2H2 on Arn clusters (n≈280) has been measured. Here the analogous rare gas molecule HArCCH could not be generated. The interpretation of our experimental findings has been supported by ab initio calculations. In addition, the experiment together with the calculations reveals information on the photochemistry of the HXeCCH molecule. The 193 nm radiation excites the molecule predominantly into the 2 1Σ+ state, which cannot dissociate the Xe–H bond directly, but the system evolves along the Xe–C coordinate to a conical intersection of a slightly nonlinear configuration with the dissociative 1 1Π state, which then dissociates the Xe–H bond. [ABSTRACT FROM AUTHOR]

Photolysis of size selected pyrrole clusters has been investigated and compared to the photolysis of an isolated pyrrole molecule. Experimentally, size distributions of different mean cluster sizes ([formula] and [formula]) have been prepared in supersonic expansions and the clusters were photolyzed at 243 and 193 nm. The kinetic energy distributions of the H photofragments have been measured. The distributions exhibit a bimodal character with fast and slow H-fragment peaks similar to the spectra of the bare molecule. However, with increasing cluster size the slow component gains intensity with respect to the fast one. A similar effect is observed with increasing the excitation energy from 243 to 193 nm. Theoretical calculations at the CASSCF/CASPT2 level have been performed for bare and complexed pyrroles (pyrrole is complexed with an argon atom and with another pyrrole unit). Combination of theoretical and experimental approaches leads to the conclusion that the direct dissociative pathway along the πσ* potential energy surface in the N–H stretch coordinate is closed by the presence of the solvent molecule. This pathway is an important channel leading to the fast H atoms in the dissociation of the bare molecule. The solvent molecule influences significantly the electronic structure in the Rydberg-type πσ* state while it has little influence on the valence states. The slow channel is mostly populated by the out-of-plane deformation mode which is also not influenced by solvation. We have also studied other possible reaction channels in pyrrole clusters (hydrogen transfer, dimerization). The present study shows that more insight into the bulk behavior of biologically relevant molecules can be gained from cluster studies. [ABSTRACT FROM AUTHOR]

Photodissociation of water clusters doped with HX(X=Br,Cl), molecules has been studied in a molecular beam experiment. The HX(H2O)n clusters are dissociated with 193 nm laser pulses, and the H fragments are ionized at 243.07 nm and their time-of-flight distributions are measured. Experiments with deuterated species DBr(H2O)n and HBr(D2O)n suggest that the photodissociation signal originates from the presence of the HX molecule on the water cluster, but does not come directly from a photolysis of the HX molecule. The H fragment is proposed to originate from the hydronium molecule H3O. Possible mechanisms of the H3O production are discussed. Experimental evidence suggests that acidic dissociation takes place in the cluster, but the H3O+ ion remains rather immobile. [ABSTRACT FROM AUTHOR]

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 =29, 80, 212, 447, and 989 and by depletion spectroscopy for =8. The spectra are dominated by peaks around 3385 cm-1 which are attributed to the asymmetric ν3 NH-stretch mode. Two further peaks between 3200 and 3260 cm-1 have about equal intensity for =8 and 29, but only about 0.40 of the intensity of the ν3 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 2ν4 and the symmetric NH-stretch ν1 in the range from 3150 to 3450 cm-1 is changed between n=10 and 100, while the asymmetric NH-stretch ν3 only exhibits a moderate redshift. The reason is the coupling of the ground state modes to the overtones. [ABSTRACT FROM AUTHOR]

Photodissociation experiments were carried out at 193 nm for single HCl molecules which are adsorbed on the surface of large Arn clusters and small (HCl)m complexes which are embedded in the interior of these clusters. For the surface case the size dependence is measured for the average sizes =140–1000. No cage exit events are observed in agreement with the substitutional position of the molecule deeply buried in the outermost shell. This result is confirmed by a molecular dynamics simulation of the pickup process under realistic conditions concerning the experiment and the interaction potentials. The calculations of the dissociation process employ the surface hopping model. For the embedded case the average sizes covered are =3 and 6 and =8–248. The kinetic energy of the H atom fragments is measured exhibiting peaks at zero and around 2.0 eV which mark completely caged and unperturbed fragments, respectively. The ratio of theses peaks strongly depends on the cluster size and agrees well with theoretical predictions for one and two closed icosahedral shells, in which the nonadiabatic coupling of all states was accounted for. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A set of photodissociation experiments and simulations of hydrogen iodide (HI) on Ar[sub n] clusters, with an average size =139, has been carried out for different laser polarizations. The doped clusters are prepared by a pick-up process. The HI molecule is then photodissociated by a UV laser pulse and the outgoing H fragment is ionized by resonance enhanced multiphoton ionization in a (2+1) excitation scheme within the same laser pulse at the wavelength of 243 nm. The measured time-of-flight spectra are transformed into hydrogen kinetic energy distributions. They exhibit a strong fraction of caged H atoms at zero-kinetic energy and peaks at the unperturbed cage exit for both spin–orbit channels nearly independent of the polarization. At this dissociation wavelength, the bare HI molecule exhibits a strict state separation, with a parallel transition to the spin–orbit excited state and perpendicular transitions to the ground state. The experimental results have been reproduced using molecular simulation techniques. Classical molecular dynamics was used to estimate the HI dopant distribution after the pick-up procedure. Subsequently, quasi-classical molecular dynamics (Wigner trajectories approach) has been applied for the photodissociation dynamics. The following main results have been obtained: (i) The HI dopant lands on the surface of the argon cluster during the pick-up process, (ii) zero-point energy plays a dominant role for the hydrogen orientation in the ground state of HI–Ar[sub n] surface clusters, qualitatively changing the result of the photodissociation experiment upon increasing the number of argon atoms, and, finally, (iii) the scattering of hydrogen atoms from the cage which originate from different dissociation states seriously affects the experimentally measured kinetic energy distributions. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report the production in the gas phase of ionically bound HXeI molecules. The molecules are generated by the photodissociation of HI molecules in large Xe[SUBn] clusters and are identified from the asymmetry of the detected H atom fragments arising from the dissociation of oriented HXeI. The orientation, resulting from a synergistic action of a pulsed laser field with a weak electrostatic field, is quite pronounced, due to a large ratio of the polarizability anisotropy to the rotational constant of HXeI. [ABSTRACT FROM AUTHOR]

Elastic and rotationally inelastic cross sections have been measured for He+H[sub 2]O 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. [ABSTRACT FROM AUTHOR]

The excitation of the low-energy intermolecular modes of ammonia clusters by helium atom scattering has been calculated using classical trajectories. The energy transfer is investigated as a function of scattering angle (from 10° to 90°), collision energy (94.8 and 50.5 meV), cluster size (n = 18, 100, 1000), and cluster temperature (T[sub c]= 1 K, 30-50 K, and 105 K). It is observed that predominantly the mode at 7 meV and to a lesser extent also the one at 12 meV are excited. These are surface modes that mainly originate from the angular motion of three adjacent N atoms. The excitation is nearly independent of the cluster size and the probability for multiphonon excitation steadily increases with increasing deflection angle. This trend is even strengthened by increasing the collision energy. The role of the cluster temperature is to broaden the energy transfer distribution with increasing values. The calculations are compared with previous and new measurements presented here of the double-differential cross sections (dσ/dω)[sub ΔE] of ammonia clusters of average size 〈n〉 = 92 at two collision energies and 〈n〉 = 1040 at one energy. While the general trends in the angular and energy dependence could be well reproduced, the correct cluster temperature was crucial in getting good agreement at the lower collision energy for n = 100. At the higher collision energy, the large energy transfer is not reproduced, probably a shortcoming of the potential models to account correctly for the anharmonicity of the strong multiquantum excitations. [ABSTRACT FROM AUTHOR]

We report the first production of the molecule HXeI, which is bound by ionic forces, in the gas phase. The molecule is generated by the photodissociation of HI molecules on a large Xe[sub n] cluster and is identified by detecting the asymmetric distribution of the H atom fragments of the oriented HXeI. The orientation is achieved in a combined pulsed laser and weak electrostatic field making use of the large anisotropy in the polarizability and the large dipole moment of this molecule. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ab initio calculations complemented by a photodissociation experiment are used for analyzing the nature of hydrogen halide (HX) molecules bound on free ice nanoparticles. The acidic dissociation of HX has led to an increase by about 4 orders of magnitude of the HCl photolysis rate, which is relevant for the halogen budget in the atmosphere.

Molecular beam scattering experiments and ab initio calculations are using for studying the structure and dynamics of size-related charged pyrrole clusters. The ionized pyrrole clusters are formed with a dimeric core that is solvated by neutral pyrrole molecules and the dissociation energies of the Py-Ar and [(Py-Ar).sup.+] complexes have shown that Ar atoms have rapidly evaporated after ionization.

The vibrational OH-stretch spectra of size-selected Na[([H.sub.2]O).sub.n] clusters for n = 8, 10, 16 and 20 are compared with first-principle calculations, which has described the interaction of the sodium cation, the electron and the water molecules with the hydrogen-bonded network. The electron is sitting at the surface of the clusters for all the sizes.

The ionization and fragmentation of neutral krypton clusters up to [Kr.sub.7] are studied by electron impact method. The study reveals results, which are quite different from those, previously obtained for [Ar.sub.n-] and also from the predictions of the latest calculations in that monomer [Kr.sup.+] ion is always by far the most probable fragment.

The vibrational OH stretch spectra is measured for sodium-doped water clusters in the size range from n = 8 to 60. It is demonstrated that specific transitions are observed at 3640 [cm.sup.-1] and in the range of single donor bonds between 3000 and 3200 [cm.sup.-1].

Focuses on the ultraviolet photolysis of hydrogen bromide molecules and clusters at three different wavelengths. Measurement of the kinetic energy distribution of the hydrogen photofragment using a time-of-flight mass spectrometer; Emerging groups which can be classified according to their kinetic energy.

Studies the photodissociation of hydrogen chloride adsorbed on the surface of an argon cluster by semiclassical molecular dynamics simulations, using a surface-hopping approach for the nonadiabatic transitions. System, interactions and simulation method; Electronic transitions and recombination; Kinetic energy distribution of hydrogen atoms.

Investigates a molecule with the elemental formula of silicon carbide, a closed-shell singlet electronic ground states, and a planar rhomboidal geometry that consists of atoms in the shape of a distorted four-membered ring with a transannular carbon-carbon bond. Identification of the identity and precise determination of the length of rhomboidal silicon carbon through isotopic substitution.

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. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The anisotropic potential energy surface of He–C2H2 is determined by analyzing total differential cross sections measured at E=71.3 meV and differential energy loss spectra obtained at two energies, E=102.9 and 62.0 meV. Calculations are carried out using infinite-order-sudden, coupled states, and close coupling methods. The data evaluation clearly demonstrates that the experimental results are much better reproduced by a spindle-shaped contour in the repulsive potential than by the more usual hard ellipsoidal shape. This change also explains the unusually small damping of the diffraction oscillations in the total differential cross section. [ABSTRACT FROM AUTHOR]

A method for predicting splittings and shifts of bands in infrared spectra of small clusters of polyatomic molecules is presented. Based on an approach of early publications of Buckingham, the influence of the intermolecular forces on the vibrational energy levels of the constituent molecules is calculated using perturbation theory to second order. In order to describe the interaction of identical molecules, this ansatz is extended to also cover degenerate systems. In first order, a coupling of the vibrational modes of the interacting molecules occurs which leads to delocalized vibrations of all the molecules in the cluster. The second order correction of the vibrational excitation frequencies are found to be dominated by the intramolecular couplings of the normal modes due to the cubic anharmonicity of the force field. The procedures developed here are applied for the interpretation of vibrational photodissociation spectra of small methanol clusters in the region of the fundamental excitation frequency of the OH stretching mode (ν1, 3681.5 cm-1), the CH3 rocking mode (ν7, 1074.5 cm-1), and the CO stretching mode (ν8, 1033.5 cm-1). Using semiempirical models for the intermolecular potential functions, splittings and positions of the experimental bands can well be explained. The nonequivalent positions of the two molecules in the linear dimer structure give rise to two different absorption frequencies for each of the three modes of the donor and the acceptor molecule, respectively. The trimer and tetramer spectrum with only one absorption band are in agreement with the existence of symmetric planar ring structures (C3h and C4h) for these species. The pentamer spectrum which also consists of one band is explained by the occurrence of three closely spaced frequencies of an asymmetric ring. The double peak structure in the hexamer spectra can be attributed to a distorted ring structure of S6 symmetry, while the occurrence of other energetically... [ABSTRACT FROM AUTHOR]

Infrared photodissociation spectra of (CH3NH2)n clusters were measured from n=2 to n=6 near the absorption of the C–N stretching mode of the monomer at 1044 cm-1 using a line tunable cw CO2 laser. The clusters are size selected by scattering them from a helium beam. The dimer spectrum shows a double-peak structure with a red- (1038 cm-1) and a blue- (1048 cm-1) shifted peak which is attributed to the nonequivalent position of the C–N in the open dimer structure. The larger clusters exhibit only one peak between 1045.4 and 1046.0 cm-1 which is caused by the equivalent position of the C–N in the cyclic structures of the larger clusters. Calculations of the minimum-energy configurations confirm these results. The linewidth increases by more than a factor of 3 for cold and internally excited dimers. [ABSTRACT FROM AUTHOR]