The electric deflection of molecular beams of (C6H6)2, produced by adiabatic expansion, has been measured. The benzene dimer is observed to be a polar species. It is likely that the structure of this species is that of two perpendicular planes, as is observed for nearest neighbors in crystal and liquid benzene.

Communication: The formation of helium cluster cations following the ionization of helium nanodroplets: Influence of droplet size and dopant Liquid helium droplets of initial mean cluster size, ͗N͘, ranging from 600 to 8000 atoms are doped with argon using the pick-up technique. The doped clusters are ionized by electron impact, and the resulting fragment ions are monitored as a function of argon pressure in the pick-up volume. Analysis of the pressure dependent ion signals is used to determine ͑1͒ the probability for charge transfer from He ϩ to the Ar atoms within the droplet, and ͑2͒ the probability for fragmentation of the Ar k subclusters upon ionization. The measured charge transfer probability from He ϩ to Ar ranges from 0.05Ϯ0.02 for clusters of mean original size ͗N͘ϭ8000 to 0.26Ϯ0.05 for ͗N͘ϭ600. Charge transfer to the Ar k constituent results in the following qualitative trends; a single Ar atom yields He n Ar ϩ ions; Ar 2 mainly yields Ar 2 ϩ , and Ar 3 mainly fragments to yield Ar 2 ϩ .Simulations of the results are performed to extract information on how the charge transfer and fragmentation processes within the ionized droplet dependent on the size of the helium droplet and the number of argon atoms captured. We use the positive-hole resonant-hopping mechanism to determine that the He ϩ hops 3-4 times prior to localization with either the Ar dopant or another He atom to form He 2 ϩ . This corresponds to a time scale for He 2 ϩ formation of 60-80 fs.

A new mechanism for the thermal desorption of molecular hydrogen from the monohydride phase on Si(100) has been identified. The unusual first-order desorption kinetics that are observed are due to the irreversible excitation of a hydrogen adatom into a delocalized, two-dimensional band state on the surface with an activation energy of 47 kcal/mol. The desorption reaction occurs between this excited hydrogen adatom and a second, localized hydrogen adatom.

The kinetics of the thermal recombinative desorption of hydrogen from the monohydride phase on the Si(100) surface has been studied by laser-induced thermal desorption (LITD). A rate law that is first order in the atomic hydrogen coverage with an activation energy of 45 kcal/mol gives an accurate fit to the data over a temperature range of 685–790 K and a coverage range of 0.006 to 1.0 monolayer. A new mechanism is proposed to explain these surprising results, namely, that the rate limiting step of the reaction is the promotion of a hydrogen atom from a localized bonding site to a delocalized band state. The delocalized atom then reacts with a localized atom to produce molecular hydrogen which desorbs. Evidence to support these conclusions comes from isotopic mixing experiments. Studies of recombinative desorption from other surfaces of silicon, which had been assumed to obey second-order kinetics, are discussed in the light of these results.

Anharmonic vibrational frequencies and intensities are calculated for 1:1 and 2:2 (HCl) n (NH3) n and (HCl) n (H2O) n complexes, employing the correlation-corrected vibrational self-consistent field method with ab initio potential surfaces at the MP2/TZP computational level. In this method, the anharmonic coupling between all vibrational modes is included, which is found to be important for the systems studied. For the 4:4 (HCl) n (H2O) n complex, the vibrational spectra are calculated at the harmonic level, and anharmonic effects are estimated. Just as the (HCl) n (NH3) n structure switches from hydrogen-bonded to ionic for n = 2, the (HCl) n (H2O) n switches to ionic structure for n = 4. For (HCl)2(H2O)2, the lowest energy structure corresponds to the hydrogen-bonded form. However, configurations of the ionic form are separated from this minimum by a barrier of less than an O−H stretching quantum. This suggests the possibility of experiments on ionization dynamics using infrared excitation of the hydrogen-bonded form. The strong cooperative effects on the hydrogen bonding, and concomitant transition to ionic bonding, makes an accurate estimate of the large anharmonicity crucial for understanding the infrared spectra of these systems. The anharmonicity is typically of the order of several hundred wavenumbers for the proton stretching motions involved in hydrogen or ionic bonding, and can also be quite large for the intramolecular modes. In addition, the large cooperative effects in the 2:2 and higher order (HCl) n (H2O) n complexes may have interesting implications for solvation of hydrogen halides at ice surfaces.

Pure liquid helium droplets of mean size 〈N〉=100–15 000 atoms ionized by electron impact show surprisingly similar ion fragment distributions. For all cluster sizes He2+ is the most probable cluster ion fragment, accounting for 30%–70% of the total ion yield. The high relative intensity of He2+ for the larger clusters shows that the droplets dissipate the ionization energy through an impulsive process, which ejects He2+ from the cluster, rather than by thermal evaporation. The other helium ion fragments that have been the focus of previous studies are most likely formed by a similar mechanism.

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An investigation of the electron impact ionization and fragmentation of helium clusters that contain Ne atoms and Nek subclusters has been performed. The charge transfer probability from He+ to Ne and the branching ratios for fragmentation of the Nek subclusters were found by analyzing the dependence of the ion signal intensities on the Ne pressure in the “pickup” region. The measured charge transfer probability from He+ to Ne ranges from 0.06±0.01 for clusters of mean original size 〈N〉=3300 to 0.43±0.02 for 〈N〉=1100. Charge transfer to a single Ne atom within the helium clusters never yields bare Ne+ ions. Instead, fragments of the type NeHen+ are produced. The charge transfer from He+ to Ne2 subclusters yields mainly Ne2+ for smaller initial cluster sizes, but NeHen+ or Ne2Hen+ fragments are more probable for larger clusters. This shows that He droplets of a few thousand atoms are able to cage Ne2 subclusters by dissipating the entire energy released by charge transfer and formation and vibrational relaxation of the Ne2+ ion. Interestingly, it was found that in these relatively small helium clusters the Ne3 and Ne4 subclusters never survive the charge transfer from He+. Fragments such as Ne2+ and Ne2Hen+ are more likely to survive than are Ne3+ and Ne4+. In general, the results presented here are qualitatively similar to those for a recent study of the ionization of Ar in helium droplets. In both cases fragmentation to the bare ion is rare, while fragmentation to the dimer ion dominates. However, the helium cluster caging effect is more efficient for Ne subclusters than for Ar subclusters. Also, there is no evidence for shell structures in the NeHen+ ion fragment distributions.