Ar HBr Research Articles

Probing the accuracy of the isomerization energy of the 3-D morphed potential of Ar–HBr

Investigations of the lowest Σ-bending vibrations of Ar–DBr ( v = 0) and Ar–HBr ( v = 1) are reported using a co-axially configured submillimeter supersonic jet spectrometer. The v = 1 spectra were obtained using glow discharge excitation. Analyses of hyperfine substructure in these spectra provide accurate molecular parameters for Ar–DBr and Ar–BrD isomeric states, direct determination of their corresponding isomerization energies, and respective parameters in the v = 1 HBr stretching vibration of the Ar–HBr isotopomer. This experimental data provides a sensitive probe of the relative energies of potential minima for previously determined 3-D Ar–HBr morphed potential and compared with results from extrapolations to the CCSD(T)/CBS limit.

Effect of the initial excitation of the intermolecular stretching mode in Ar–HBr photolysis

The effect of the initial excitation of the van der Waals (vdW) stretching mode is explored in Ar–HBr(v = 1) photolysis by a comparison with the cases of no initial vdW excitation and bending excitation. It is found that vdW stretching excitation produces a different photolysis outcome than the other two cluster initial states regarding the yield and state distributions of the products. The differences are related to the different shapes of the initial distributions of intra-cluster geometries produced with or without excitation of the different vdW modes. We conclude that selecting the initial cluster state through vdW mode excitation, prior to photo-dissociation, allows for some control over the photolysis products.

Accurate calculation of core-electron binding energies: multireference perturbation treatment.

Multireference perturbation theory (MRPT) with multiconfigurational self-consistent field (MCSCF) reference functions is applied to the calculations of core-electron binding energies (CEBEs) of atoms and molecules. Orbital relaxations in a core-ionized state and electron correlation are both taken into account in a conventional MCSCF-MRPT procedure. In the MCSCF calculation, the target core ionized state is directly optimized as an excited state and this treatment can completely prevent a variational collapse. Multireference Moller-Plesset perturbation theory and multiconfigurational self-consistent field reference quasidegenerated perturbation theory were used to treat electron correlation. The present method quite accurately reproduced the 1s CEBEs of CH4, NH3, H2O, and FH; the average deviation from the experimental data is 0.11 eV using Ahlrichs' VTZ basis set. The C 1s and O 1s CEBEs of formic acid and acetic acid were calculated and the results are consistent with the bonding characters of the atoms in these molecules. The present procedure can also be applied to CEBEs of higher angular momentum orbitals by including spin-orbit coupling. The calculated CEBEs of Ar 2p, HCl 2p, Kr 3d, and HBr 3d are in reasonable agreement with the available experimental values. In the calculation of the 3d CEBEs, a relativistic correction significantly improves the agreements. The effect of polarization functions is also discussed.

Photodissociation dynamics of the Kr-HBr cluster: the effect of the rare gas atom substitution.

The ultraviolet photolysis dynamics of Kr-HBr(v=0) is investigated by means of wave packet calculations, focusing on the fragmentation pathway Kr-HBr+ variant Planck's over 2pi omega-->H+Kr-Br. Photolysis is simulated by starting from two different cluster initial states, namely the ground van der Waals (vdW) and an excited vdW bending state, associated with the Kr-H-Br and Kr-Br-H isomers, respectively. The results show that, for the two initial states of the cluster, the Kr-Br product yield is lower than that of Ar-Br radicals found in previous studies on Ar-HBr photolysis. Despite this decrease, the Kr-Br yield is found to be still rather high, in particular for the initial excited vdW state of Kr-HBr(v=0). In addition, the Kr-Br product state distributions exhibit a remarkably higher excitation (mainly rotational) than the corresponding Ar-Br distributions. The lower yield and higher excitation of Kr-Br as compared to Ar-Br, are attributed to a larger share of the energy available for the radical going to internal excitation in the case of the Kr-Br product. The different partition of the energy available for Kr-Br also causes significant deviations in the photolysis behavior of Kr-HBr when compared to that of Ar-HBr, in the case of the initial excited vdW state of both clusters. A common feature of the photodissociation of Kr-HBr and Ar-HBr is the manifestation of quantum interference effects in the Kr-Br and Ar-Br rotational state distributions, in the form of pronounced structures of supernumerary rotational rainbows.

Wave packet study of the Ar–HBr photolysis: Stereodynamical effects

The ultraviolet photolysis of Ar–HBr(v=1) is studied through wave packet dynamics simulations, focusing on the fragmentation pathway Ar–HBr +ℏω→H+Ar–Br. Photolysis starts from two initial states of Ar–HBr(v=1) with a different angular shape, namely the ground and the first excited van der Waals (vdW) states, corresponding to the Ar–H–Br and Ar–Br–H isomers, respectively. It is found that the yield of Ar–Br radical products is substantially higher for the initial excited vdW state of the cluster, where H dissociation is less hindered. In addition, the yield of radical formation is much higher in the Ar–HBr(v=1) photolysis than that previously found in the Ar–HCl(v=0) case, even for the ground vdW state, where the initial angular distribution of both clusters is similar. Another unexpected difference is that Ar–HCl(v=0) photolysis exhibits strong manifestations of quantum interference, while these effects are much weaker in Ar–HBr(v=1). A lower probability of the first collision between the recoiling hydrogen and the Ar atom in the case of Ar–HBr(v=1), due to geometrical differences between its initial state and that of Ar–HCl(v=0), is suggested to explain the different photolysis behavior of both clusters. The implications of the present findings in the photolysis of other related precursor clusters are discussed.

Photodissociation of the ArHBr complex investigated with the multiconfiguration time-dependent Hartree approach

We apply the multiconfiguration time-dependent Hartree method to the study of the photodissociation of the Ar–HBr van der Waals complex. The dynamics is studied in both Jacobi and valence coordinates. The evolution of the probability density in the different modes shows that the direct or nearly direct mechanism, where the hydrogen atom interacts only very weakly with the Ar and Br atoms, dominates the dissociation process. By projecting the quantum flux onto the vibrational levels of ArBr, we find at the maximum of the absorption profile, i.e., around E=3 eV, that 65% of the dissociation gives rise to partial fragmentation into hot H fragments and bound ArBr molecules.

Stereodynamics and control effects in the ultraviolet photolysis of Ar–HBr

The ultraviolet photolysis of the Ar–HBr cluster is studied by means of quasiclassical trajectory simulations. The goal is to investigate the effect of the initial distribution of intracluster orientations on the photolysis process. Cluster photolysis is simulated starting from two different initial states of Ar–HBr(v=1), namely the ground and the first excited van der Waals states corresponding to the Ar–H–Br and Ar–Br–H isomers, respectively. These two states represent different initial situations for hydrogen dissociation, which in the first case is hindered to a large extent, and in the second case is not. The probability for the fragmentation pathway into H and Ar–Br radical fragments is found to be substantially higher (and therefore the efficiency of the total fragmentation pathway, into H+Ar+Br, is lower) for the cluster initial state where H dissociation is less hindered. The effect of the angular distributions shape of the initial state is also clearly manifested in the state distributions of the different photofragments. It is concluded that selecting the initial state of the cluster on the basis of the shape of its angular distribution provides an efficient means to control the yield of the different fragmentation pathways, and the final states of their corresponding photolysis products.

Differentiation of the ground vibrational and global minimum structures in the Ar:HBr intermolecular complex

A fully three-dimensional morphed potential energy surface is reported for Ar:HBr. The morphed potential was obtained from parametrized scaling and shifting transformations of an ab initio potential. The optimum parameters of the morphed potential were determined by a regularized nonlinear least-squares fit to available experimental data. The rovibrational dynamics of the complex were computed using an adiabatic separation of the H–Br intramolecular stretching mode from the intermolecular modes of the system. The ground rovibrational state of the morphed potential was found to have the hydrogen-bound structure Ar–HBr. This state was 10.99 cm−1 more stable than the corresponding state having the van der Waals structure, Ar–BrH, in agreement with experimental data. However, the global minimum of the morphed potential was found to have the van der Waals structure, Ar–BrH. This structure was 20.9 cm−1 lower in energy than the local minimum having the hydrogen-bound structure, Ar–HBr.

First quantum investigation of the photodissociation of the Ar–HBr complex: three-dimensional time-dependent approach

The ultraviolet photodissociation of the Ar–HBr van der Waals complex is investigated using an exact three-dimensional time-dependent wave packet approach with zero-total angular momentum. Due to the large size of the required grid, calculations were carried out up to 60 fs. This time was sufficient for calculating the absorption spectrum as well as the final kinetic energy distribution of the H atom. Both were found to be completely structureless. Thus, the dynamics is entirely dominated by the fast direct dissociation of the hydrogen atom producing hot H fragments and bound Ar–Br molecules.

Structural and electronic effects of argon sputtering and reactive ion etching on In0.53Ga0.47As and In0.52Al0.48As studied by inelastic light scattering

The effects of Ar sputtering and HBr reactive ion etching on In0.53Ga0.47As and In0.52Al0.48As epitaxial layers were investigated by Raman spectroscopy. First-order phonon scattering was used as a probe of structural modification, while coupled phonon–plasmon mode scattering was used to investigate electrical modification. Second-order phonon scattering proved insensitive to sputter- and etch-induced structural modification. For both In0.53Ga0.47As and In0.52Al0.48As, Ar sputtering leads to more structural modification than does HBr etching, the difference being greater for In0.53Ga0.47As. In0.52Al0.48As exhibits a greater resistance to sputter-induced structural damage than does In0.53Ga0.47As, and nominally undoped material displays less susceptibility to being disordered structurally than does more highly doped material for both In0.53Ga0.47As and In0.52Al0.48As. HBr etching results in no electrical modification of In0.53Ga0.47As, whereas a decrease in carrier density and/or an increase in plasmon damping is observed in In0.52Al0.48As. Annealing In0.52Al0.48As under conditions known to restore electrical activity of dopants in GaAs and Al0.3Ga0.7As results in no change in the coupled mode spectra, indicating that electrical modification is not due to hydrogen passivation of dopants.

Evidence for a cage effect in the UV photolysis of HBr in ArHBr. Theoretical and experimental results

Evidence for a cage effect in the 193 nm photodissociation of HBr in the ArHBr cluster is found. This effect manifests itself as a tail extending toward lower energies in the hydrogen photofragment kinetic energy distribution (KED). This is a consequence of energy transfer in collisions between the light and the heavy atoms. There is good agreement between the experimental and theoretical KEDs.

Photoinitiated processes in complexes: subpicosecond studies of CO2—Hl and stereospecificity in Ar—HX

Build-up times for the OH product of photoinduced reactions in CO2—HI complexes were measured for the photolysis wavelength range 235–263 nm by using the subpicosecond resolution pump–probe method pioneered by Zewail and co-workers. There is reasonable accord with theory and recent results from crossed-molecular-beams experiments. To illustrate the squeezed-atom effect, complementary measurements of hydrogen atom translational-energy distributions were carried out by using the high resolution high-n Rydberg time-of-flight (HRTOF) method of Welge and co-workers. Prototypical cases of Ar—HBr and Ar—HI complexes are reported. The former illustrates the squeezed-atom effect or caging of the exiting hydrogen. The latter is dominated by the formation of radical–molecule complexes. Extensions to other radical–molecule complexes are discussed.

Anisotropic intermolecular potentials. III. Rare-gas–hydrogen bromide systems

Intermolecular potentials for Ar and Kr interacting with HBr are obtained by least-squares fitting of potential parameters to data obtained from the molecular-beam microwave spectra of the Ar–HBr and Kr–HBr van der Waals complexes. The equilibrium geometry is linear Rg–H–Br in each case, but there are substantial secondary minima at the linear Rg–Br–H geometries; for Ar–HBr, the secondary minimum is only about 5 cm−1 shallower than the primary minimum. This potential feature is found to explain the anomalous H/D isotope effects in centrifugal distortion constants that have been observed for the Rg–HBr complexes. It is predicted that Ar–HBr will have a very low-energy bending state, only 11 cm−1 above the ground state, arising from the secondary minimum.