Photodissociation of hydrogen iodide on the surface of large argon clusters: The orientation of the librational wave function and the scattering from the cluster cage Ultraviolet ͑UV͒ photodissociation experiments are carried out for Ar n (HBr) clusters in which the HBr is adsorbed on the surface of the Ar n , and also on isomers of these systems in which HBr is embedded within the rare-gas cluster. The mean size of the cluster distribution in the experiments is around n ϭ130. The kinetic energy distribution ͑KED͒ of the hydrogen atoms that left the clusters is measured. Molecular dynamics ͑MD͒ simulations of the photodissociation of the chemically similar clusters Ar n (HCl) are used to provide a qualitative interpretation of the experimental results. The clusters with embedded HBr give a very cold H-atom KED. The clusters with the surface-adsorbed HBr give a KED with two peaks, one corresponding to very low energy H atoms and the other pertaining to high energies, of the order of 1.35 eV. The theoretical simulations show that already for nϭ54, there is a strong cage effect for the ''embedded'' molecule case, resulting in slow H atoms. The surface-adsorbed case is interpreted as due to two types of possible adsorption sites of HX on Ar 55 : for a locally smooth adsorption site, the cage effect is relatively weak, and hot H atoms emerge. Sites where the HBr is adsorbed at a vacancy of Ar n lead to ''encapsulation'' of the H atom produced, with a strong cage effect. A weak tail of H atoms with energies well above the HBr monomer excess energy is observed for the embedded case. Simulations support that this is due to a second photon absorption by recombined, but still vibrationally hot, HBr. The results throw light on the differences between the cage effect inside bulk structure and at surfaces.

The ultraviolet photolysis of HBr molecules and (HBr)n clusters with average size around n̄=9 is studied at three different wavelengths of 243, 205, and 193 nm. Applying polarized laser light, the kinetic energy distribution of the hydrogen photofragment is measured with a time-of-flight mass spectrometer with low extraction fields. In the case of HBr monomers and at 243.1 nm, an almost pure perpendicular character (β=−0.96±0.05) of the transitions is observed leading to the spin–orbit state Br(2P3/2). The dissociation channel associated with the excited state Br*(2P1/2) is populated by a parallel transition (β*=1.96±0.05) with a branching ratio of R=0.20±0.03. At the wavelength of 193 nm, about the same value of R=0.18±0.03 is found, but both channels show a mainly perpendicular character with β=−0.90±0.10 for Br and β*=0.00±0.10 for Br*. The results for 205 nm are in between these two cases. For the clusters at 243 nm, essentially three different groups appear which can be classified according to their kinetic energy: (i) A fast one with a very similar behavior as the monomers, (ii) a faster one which is caused by vibrationally and rotationally excited HBr molecules within the cluster, and (iii) a slower one with a shoulder close to the fast peak which gradually decreases and ends with a peak at zero velocity. The zero energy fragments are attributed to completely caged H atoms. The angular dependence of the group (iii) is isotropic, while that of the other two is anisotropic similar to the monomers. At 193 nm only the fast and the slow part is observed without the peak at zero energy. Apparently the kinetic energy is too large to be completely dissipated in the cluster.

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 Xen 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.

Hydrogen bromide clusters that are generated in an adiabatic expansion with Ar are size-selected by scattering from a He or a Ne beam and a subsequent separation by a velocity selector. The complete fragmentation pattern for electron impact ionization of clusters n = 3,5,7,10, and 15 is obtained for electron energies of 70 eV. Up to cluster sizes of n = 15 strong fragmentation is observed. Based on these results the cluster sizes are measured as a function of different source conditions. For the same experimental conditions the (HBr), clusters are photoionized at 243.1 nm. A time-of-flight mass spectrometer with low extraction fields is used to obtain the translational energy distributions of hydrogen atoms. Several distinct features are observed which sensitively depend on the cluster size. At small cluster sizes a tail appears towards lower energies in the fast hydrogen kinetic energy distribution which is caused by few collisions with the cluster molecules. Peaks at higher energies are attributed to internally excited HBr molecules which gain their energy in collisions with photofragmented H atoms. Small clusters exhibit pure rotational excitation with averaged J = 7, while the larger ones show vibrational excitations to u = 1 with J = 8 and u = 2. Finally, we observe a narrow H component peaking near zero velocity and exhibiting an isotropic angular distribution. This effect appears only at larger cluster sizes and is attributed to a complete caging of the photofragments.

Molecular dynamics simulations have been employed for the study of photolysis of hydrogen bromide placed inside or on the surface of Ar 12 , Ar 54 , Ar 97 , and Ar 146 clusters, representing one to three icosahedral solvation shells. A large set of classical Wigner trajectories, which take into account the initial quantum rotational or librational delocalization of HBr, is generated and analyzed in terms of transient hydrogen populations inside the cluster and final kinetic energy distributions. The key result is that for fully solvated HBr the size effect on caging is dramatic in the studied range of cluster sizes, while it is only moderate for surface isomers. Simulations also demonstrate that caging can be efficiently turned off by a librational preexcitation of HBr on argon clusters. Calculations 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.