Photodissociation and size analysis of (HBr)_n clusters

Abstract

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.

Mass spectrometry of hydrogen bonded clusters of heterocyclic molecules: Electron ionization vs. photoionization

“…The experimental apparatus was built and used previously in the Max-Planck Institute in Göttingen [19][20][21][22], and moved to the J. Heyrovský Institute in Prague. The schematic picture of the apparatus is shown in Fig.…”

Cluster cross sections from pickup measurements: Are the established methods consistent?

“…Our complex cluster beam apparatus was described in detail elsewhere, 42,43 therefore only the parts of the machine relevant for the present experiment are described here and shown in Fig. 1.…”

Photodissociation of HBr adsorbed on the surface and embedded in large Arn clusters

“…31,25 Here, only a short summary of the major components is given, including the implemented experimental improvements. 24 The cluster beams are produced by a supersonic expansion of the neat argon gas or a mixture of HBr in argon into the source chamber through a nozzle of conical shape with a diameter of dϭ60 m, an opening angle of 30°and a length of 2 mm.…”

“…If the fraction of HBr is larger than 5%, the beam consists of pure ͑HBr͒ n clusters. 25 The actual beam data used in the present experiments are given in Table I.…”

“…In fact, in a 5% mixture of HBr in Ar, only ͑HBr͒ n clusters were detected. 25 In that experiment we used the scattering analysis in order to avoid the fragmentation problem in the detection process. In the case of the dilute mixtures the complete mass spectrum is used for the indication of its content.…”