Isotropic Angular Distribution Research Articles

Photodissociation dynamics of OClO

Photofragment translational energy spectroscopy was used to study the dissociation dynamics of a range of electronically excited OClO(A 2A2) vibrational states. For all levels studied, corresponding to OClO(A 2A2←X 2B1) excitation wavelengths between 350 and 475 nm, the dominant product (≳96%) was ClO(2Π)+O(3P). We also observed production of Cl+O2 with a quantum yield of up to 3.9±0.8% near 404 nm, decreasing at longer and shorter wavelengths. The branching ratios between the two channels were dependent on the OClO(A 2A2) excited state vibrational mode. The Cl+O2 yield was enhanced slightly by exciting A 2A2 levels having symmetric stretching+bending, but diminished by as much as a factor of 10 for neighboring peaks associated with symmetric stretching+asymmetric stretching. Mode specificity was also observed in the vibrationally state resolved translational energy distributions for the dominant ClO(2Π)+O(3P) channel. The photochemical dynamics of OClO possesses two energy regimes with distinctly different dynamics observed for excitation energies above and below ∼3.1 eV (λ∼400 nm). At excitation energies below 3.1 eV (λ≳400 nm), nearly all energetically accessible ClO vibrational energy levels were populated, and the minor Cl+O2 channel was observed. Although at least 20% of the O2 product is formed in the ground (X 3Σ−g) state, most O2 is electronically excited (a 1Δg). At E<3.1 eV, both dissociation channels occur by an indirect mechanism involving two nearby excited states, 2A1 and 2B2. Long dissociation time scales and significant parent bending before dissociation led to nearly isotropic polarization angular distributions (β∼0). At excitation energies above 3.1 eV (λ<400 nm), the Cl+O2 yield began to decrease sharply, with this channel becoming negligible at λ<370 nm. At these higher excitation energies, the ClO product was formed with relatively little vibrational energy and a large fraction of the excess energy was channeled into ClO+O translational energy. The photofragment anisotropy parameter (β) also increased, implying shorter dissociation time scales. The sharp change in the disposal of excess energy into the ClO products, the decrease of Cl+O2 production, and more anisotropic product angular distributions at E≳3.1 eV signify the opening of a new ClO+O channel. From our experimental results and recent ab initio calculations, dissociation at wavelengths shorter than 380 nm to ClO+O proceeds via a direct mechanism on the optically prepared A 2A2 surface over a large potential energy barrier. From the ClO(2Π)+O(3P) translational energy distributions, D0(O–ClO) was found to be less than or equal to 59.0±0.2 kcal/mol.

Role of Intersystem Crossing in the Dynamics of the O(3P) + C2H5I Reaction

Reactive scattering of O(3P) atoms with C2H5I molecules has been studied at initial translational energies E ∼ 51 and 16 kJ mol-1 using a supersonic beam of O atoms seeded in He and Ne buffer gas generated from a microwave discharge source. A mildly forward- and backward-peaked angular distribution of IO product observed at lower initial translational energy becomes backward-scattered at higher initial translational energy. An isotropic angular distribution is observed for HOI product at lower initial translational energy which becomes forward- and backward-peaked, slightly favoring the backward direction at higher initial translational energy. Extended phase space calculations indicate that reaction leading to IO product proceeds via intersystem crossing from the lowest triplet 3A‘‘ potential energy surface to a bound OIC2H5 intermediate on the singlet 1A‘ potential energy surface in small impact parameter collisions at low initial translational energy. However, the emergence of backward-scattered IO product indicates the onset of direct reaction over the triplet 3A‘‘ potential energy surface at higher initial translational energy. The formation of HOI reaction product arises from the singlet OIC2H5 complex via a five-membered ring transition state where the H atom is abstracted from the terminal CH3 group at the top of a barrier leading to vibrational excitation of HOI and repulsion between the reaction products in this strongly exoergic exit valley of the potential energy surface.

Dynamics of SiO desorption in reactive scattering of O2 with a silicon surface

The reactive scattering of O2, from a pulsed molecular beam, with a hot Si(100) (about 1250 K) has been studied with resonance enhanced multiphoton ionization (REMPI) mass spectroscopy in the wavelength region between 282 and 295 nm (F 1Σ+←X 1Σ+ and H 1Σ+←X 1Σ+). The vibrational and rotational state distributions of scattered SiO species are Boltzmann with temperature near that of the Si(100) surface. An isotropic angular distribution of the SiO desorption is observed and the internal state distribution is independent of the desorbing angle. The results suggest that the SiO desorption goes through a state in which the SiO rotates almost freely.

Electronic collimation for precise measurement of resolution, with a gridded ionization chamber

An ionization chamber with a Frich grid is used to determine both the energy (E) of the charged particles emitted from the sample positioned coplanar with the cathodes and the cosine of the emission angle (θ) with respect to the cathode. Using the combined information of cos (θ) and E, problems in particle counting due to sample absorption and scattering effects can be circumvented. An isotropic angular distribution for alpha particles is obtained. Investigations on affected areas using backscattering and self-absorption, and on unaffected areas were carried out and a resolution difference between the two areas was measured. The difference is greater than 0.6%.

Displacement Dynamics of Fluorine Atoms Reacting with Bromobenzene and Iodobenzene Molecules

Reactive scattering of F atoms with C6H5Br and C6H5I molecules leading to both hydrogen and halogen atom displacement has been studied at an initial translational energy E ∼ 38 kJ mol-1 using a supersonic beam of F atoms seeded in He buffer gas. The center-of-mass angular distributions of C6H5F reactive scattering show symmetrical forward and backward peaking, which is consistent with a long-lived collison complex, but the product translational energy distributions peak at a fraction f‘pk ∼ 0.18 of the total available energy and lie well above the predictions of phase space theory. The branching ratio ∼4 favors halogen atom over hydrogen atom displacement. The H atom displacement pathway is impeded by a potential energy barrier and occurs in competition with atom migration around the benzene ring. The halogen atom displacement occurs directly an F or H atom becomes bonded to the carbon atom adjacent to the halogen atom, following F atom or possibly H atom migration from other locations on the ring. The H atom displacement pathway for both FC6H4Br and FC6H4I reaction products shows a nominally isotropic angular distribution and a product translation energy distribution peaking at a fraction f‘pk ∼ 0.2 of the total available energy, in line with the H atom displacement dynamics previously observed for F + C6H5Cl.

A model for texture evolution in a growing film

A model was developed in order to calculate texture evolution, where two major orientations may be present. The tendency for the lowest possible surface energy was taken as the driving force for texture changes during deposition. It was found that an initially isotropic angular distribution evolves towards a bimodal one and finally to a texture with a sharp distribution around the direction of lowest surface energy. A continuous sharpening of the angular distribution around the main directions was found in all cases. In some cases the texture can turn over from one orientation to the other one. All these effects could be observed in the experimental study of TiN layers deposited on various substrates using the PVD technique.

A model for texture evolution in a growing film

A model was developed in order to calculate texture evolution, where two major orientations may be present. The tendency for the lowest possible surface energy was taken as the driving force for texture changes during deposition. It was found that an initially isotropic angular distribution evolves towards a bimodal one and finally to a texture with a sharp distribution around the direction of lowest surface energy. A continuous sharpening of the angular distribution around the main directions was found in all cases. In some cases the texture can turn over from one orientation to the other one. All these effects could be observed in the experimental study of TiN layers deposited on various substrates using the PVD technique.

Unsteady plasma ejections from hollow accretion columns of galactic neutron stars as a trigger for gamma-ray bursts

We propose a model of gamma-ray bursts (GRBs) based on close Galactic neutron stars with accretion disks. We outline a simple mechanism of unsteady plasma ejections during episodic accretion events. The relative kinetic energy of ejected blobs can be converted into gamma-rays by internal shocks. The beaming of gamma-ray emission can be responsible for the observed isotropic angular distribution of GRBs.

Testing Isotropy versus Clustering of Gamma-Ray Bursts

view Abstract Citations (9) References (11) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Testing Isotropy versus Clustering of Gamma-Ray Bursts Efron, Bradley ; Petrosian, Vahe Abstract The cosmological origin for gamma-ray bursts has been gaining popularity primarily because they manifest nearly isotropic angular distribution. Analyses of the data using different statistical tests have claimed contradictory results on the degree and significance of deviation of the angular distribution from isotropy. In this paper we apply several tests, which we believe are more powerful than those applied previously, to the BATSE 1B and 2B catalogs. In the 1B catalog we find that the excess of sources at small angles claimed by Quashnock & Lamb (1993b) is significant and can be interpreted as evidence that 15 sources have repeated twice. We also find some weak evidence for clustering of sources on a scale of about 30°. These features, however, are absent in the 2B catalog. We also investigate dependence of the angular distribution on burst intensity and find no strong correlation or deviation from isotropy. In a separate paper (Petrosian & Efron 1995) we search for a correlation between temporal and angular distribution of the bursts and find some marginal evidence for repeaters in both the 1B and 2B catalogs. Publication: The Astrophysical Journal Pub Date: August 1995 DOI: 10.1086/176048 Bibcode: 1995ApJ...449..216E Keywords: GAMMA RAYS: BURSTS; METHODS: STATISTICAL full text sources ADS |

The concerted photodissociation of azomethane at 193 nm

The 193 nm photodissociation of azomethane CH3N2CH 3 ~ CH 3 + N 2 + CH 3 has been investigated by photofragment translational spectroscopy. The time-of-flight (TOF) distribution of the CH 3 photofragments consists of a single and relatively narrow peak which indicates that the two methyl radicals are formed with equal or very similar velocity distributions. Based on this result and on the observed TOF distribution of the N 2 fragment, the rupture of the two C-N bonds is shown to be consistent with a concerted mechanism with the possibility of a symmetric three-body decay. Furthermore, the concerted mechanism implies that dissociation is preceded by rapid isomerisation from the initial trans form to a gauche or cis type structure. The isotropic (/3 = 0) angular distributions of CH 3 and N 2 are consistent with an electronic transition moment being oriented parallel to the N--N bond.

Muscle cross-bridges bound to actin are disordered in the presence of 2,3-butanedione monoxime

Electron paramagnetic resonance spectroscopy was used to monitor the orientation of muscle cross-bridges attached to actin in a low force and high stiffness state that may occur before force generation in the actomyosin cycle of interactions. 2,3-butanedione monoxime (BDM) has been shown to act as an uncompetitive inhibitor of the myosin ATPase that stabilizes a myosin.ADP.P(i) complex. Such a complex is thought to attach to actin at the beginning of the powerstroke. Addition of 25 mM BDM decreases tension by 90%, although stiffness remains high, 40-50% of control, showing that cross-bridges are attached to actin but generate little or no force. Active cross-bridge orientation was monitored via electron paramagnetic resonance spectroscopy of a maleimide spin probe rigidly attached to cys-707 (SH-1) on the myosin head. A new labeling procedure was used that showed improved specificity of labeling. In 25 mM BDM, the probes have an almost isotropic angular distribution, indicating that cross-bridges are highly disordered. We conclude that in the pre-powerstroke state stabilized by BDM, cross-bridges are attached to actin, generating little force, with a large portion of the catalytic domain of the myosin heads disordered.

Near-infrared vibronic spectrum of the CH2 b̃ 1B1←ã 1A1 transition

A Doppler-limited high resolution vibronic spectrum of the methylene (CH2) b̃ 1B1←ã 1A1 transition in the near-infrared wavelength region has been obtained using transient absorption techniques. The radical was produced by 308 nm excimer laser photolysis of ketene (CH2CO) in a flow system. The analysis of this spectrum confirms the reassignments of some previously observed bands as well as the presence of new vibronic bands predicted by a recent ab initio calculation [Green et al., J. Chem. Phys. 94, 118 (1991)]. We also measured the Doppler broadened profiles of low-J rotational lines of CH2 under collisionless conditions. From the line profile analysis, we find that the 308 nm photolysis of ketene produces fragments with isotropic and uncorrelated velocity and angular momentum distributions. The Doppler profiles also provide a view of the coincident product state distributions. The measured Doppler profiles are consistent with a rotational distribution of CO produced in coincidence with low energy CH2 states given by statistical phase space theory. The vibrationally excited coincident CO appears, however, to be produced about 2–3 times more efficiently than is predicted by various statistical theories. A frequency modulation (FM) technique has been adopted to improve the sensitivity of the transient absorption experiment. There is a significant improvement in the observed signal to noise ratio of the CH2 spectrum over the dual beam method previously used.

Angular distributions of reactive scattering arising from persistent complexes with dissociation at arbitrary angles to the inertial axes of the transition state

Angular distributions of reactive scattering arising from a long-lived collision complex dissociating via a transition state that approximates to a linear rotator are predicted for H atom displacement occurring over a distribution of bending angles β with respect to the rotator axis. A conical angular distribution may be generated when only a narrow range of bending angles contributes to the reactive scattering. However, the angular distribution may take on quite different character as the distribution of bending angles becomes broader. Thus, an isotropic angular distribution is generated by a bending angle distribution which follows the spherical polar weighting P(β) ∝ sin β. Forward and backward scattering is generated by bending angle distributions favouring less strongly bent displacement, while sideways scattering is generated by a bending angle distribution favouring more strongly bent displacement. The experimental angular distribution for OH + CO reactive scattering which shows mild forward and ba...

Model for the High-Energy Emission from Blazars

view Abstract Citations (756) References (57) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Model for the High-Energy Emission from Blazars Dermer, Charles D. ; Schlickeiser, Reinhard Abstract Approximate formulae for the instantaneous scattered radiation spectra are derived in the Thomson and Klein-Nishina regimes, assuming that the relativistic electrons are described by an isotropic angular and power-law energy distribution in the outflowing fluid frame. The soft photon source is assumed to be axially symmetric. We find that the Thomson-limit luminosity enhancement in the direction that the scattered radiation is most intense varies as Γ6 for photons entering from the side and as Γ2 for photons entering directly from behind, where Γ is the bulk Lorentz factor of the outflowing plasma cloud. Energy loss and flow are treated in a model where energetic electrons are instantaneously injected with a power-law energy distribution at some height above a point or disk photon source, and instantaneous and time-average spectra are calculated. We also treat the electron energy-loss rate and scattered photon spectra from photon fields that are isotropic in the stationary frame. We calculate broad-band spectra for the case where soft photons are produced by a cool blackbody outer disk extending to the innermost stable orbit of a Schwarzschild black hole, and fit these results to observations of high-energy radiation from 3C 279, 3C 273, and Mrk 421. The observed spectral softening in the regime between hard X-ray and >100 MeV gamma-ray energies is attributed to the transition from an uncooled to a cooling electron distribution, yielding a break of 0.5 units in the scattered photon spectra index. Publication: The Astrophysical Journal Pub Date: October 1993 DOI: 10.1086/173251 Bibcode: 1993ApJ...416..458D Keywords: GALAXIES: ACTIVE; GALAXIES: JETS; GALAXIES: QUASARS: GENERAL; GAMMA RAYS: THEORY; RADIATION MECHANISMS: MISCELLANEOUS full text sources ADS | data products SIMBAD (6) NED (3)

Studies of intermediate-mass fragment emission in the 3He+natAg, 197Au reactions between 0.48 and 3.6 GeV.

Intermediate-mass fragments formed in reactions of $^{3}\mathrm{He}$ ions with $^{\mathrm{nat}}\mathrm{Ag}$ and $^{197}\mathrm{Au}$ targets have been studied at five energies between 0.48 and 3.6 GeV. Inclusive measurements show that as the bombarding energy increases, there is a strong enhancement in fragment cross sections and a trend toward isotropic angular distributions. Between 0.90 and 1.8 GeV, a change in the emission mechanism is suggested by (1) kinetic energy spectra with high-energy tails that become distinctly flatter, (2) a broadening of the spectral Coulomb peaks toward lower energies, and (3) charge distributions that become constant, exhibiting a power-law exponent \ensuremath{\tau}\ensuremath{\approxeq}2.0. Exclusive studies of the $^{3}\mathrm{He}$${+}^{\mathrm{nat}}$Ag system at 0.90 and 3.6 GeV detected multifragment events with multiplicities up to four. The probability for high-multiplicity events increases about 40-fold between 0.90 and 3.6 GeV. At both energies, the kinetic energy spectra depend on multiplicity, especially when triggering on backward-emitted fragments. For multiplicity three events, a rapidity analysis of the data at 3.6 GeV is consistent with a single, relatively low source velocity, ${\mathit{v}}_{\mathit{S}}$\ensuremath{\approxeq}0.4 cm/ns. The data are compared with predictions of a coplanarity-sphericity calculation, the sequential statistical decay code GEMINI, and a hybrid intranuclear cascade/percolation model.

Orientation effects in QSO spectra

view Abstract Citations (22) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Orientation Effects in QSO Spectra Francis, Paul J. Abstract The consequences of anisotropic continuum emission for the equivalent- width distribution of QSOs are investigated and compared with observations from a large QSO survey. Isotropic line emission is assumed, and a variety of continuum anisotropy models are investigated. Far fewer high equivalent width QSOs are observed than most models predict. Both thick and thin accretion disks are ruled out, unless their radiation is reprocessed into a more isotropic angular distribution, or an obscuring torus conceals the broad-line region of nearly edge-on QSOs. Publication: The Astrophysical Journal Pub Date: March 1993 DOI: 10.1086/172344 Bibcode: 1993ApJ...405..119F Keywords: Accretion Disks; Line Spectra; Luminosity; Quasars; Anisotropic Media; Astronomical Models; Astrophysics; ACCRETION; ACCRETION DISKS; GALAXIES: QUASARS: GENERAL full text sources ADS |

Model for the flow of a yield fluid through a porous medium

The study of the flow of a Newtonian fluid in a porous medium and the dispersion of markers has been the subject of numerous works after Taylor and Saffman. Characteristics of the flow of a non-Newtonian fluid have not been investigated as much. We attempt to generalize the Saffman modelling to the case of a Bingham fluid. The porous medium is assumed to be statistically homogeneous and isotropic. Dispersion is completely described by a lateral and a longitudinal dispersion coefficient. Pores are represented by monodispersed capillary tubes with an isotropic angular distribution.

Mechanism of the ultraviolet photodissociation of chloroethylenes determined from the Doppler profiles, spatial anisotropy, and power dependence of the photofragments

Doppler profiles of chlorine and hydrogen atomic fragments produced in the photodissociation of mono- and dichloroethylenes at 193 nm have been measured in a pump-and-probe experiment using 2+1 resonance-enhanced multiphoton ionization. In a second experiment, the angular distributions of the Cl fragments produced from chloroethylenes at 235 and 238 nm were measured using a perfect-focusing mass spectrometer. In a third experiment, we measured the power dependence of the relative yields of H, Cl, HCl, and HCl+ produced from vinyl chloride at 193 nm. For Cl detachment, two primary processes have been confirmed. One produces an isotropic angular distribution of photofragments, while the other produces an anisotropic distribution. For H atom detachment, an isotropic angular distribution and a Boltzmann velocity distribution were found. The ratio of yields of the Cl and H fragments was found to be 4±1 for CH2CCl2 and higher than 10 for t-CHClCHCl and CCl2CClH. The H, Cl, and HCl yields were found to be first order in laser intensity, while the HCl+ yield was found to be third order. Saturation measurements of the ion yield indicate that the latter results from a 1+1+1 resonance-enhanced process involving a bound state of the parent molecule. This intermediate state may also be responsible for producing the statistical component of the Cl atom product.

Evidence for trapped anomalous cosmic ray oxygen ions in the inner magnetosphere

A series of measurements of 5–30 MeV/nucleon oxygen ions made with track detector stacks on Cosmos satellites show isotropic angular distributions during solar energetic particle events. Solar‐quiet times, on the other hand, have highly anisotropic distributions suggestive of a trapped‐particle component. Detailed Monte Carlo simulations confirm this interpretation and allow us to measure the trapped and cosmic‐ray contributions to the observed fluxes. Our data are fully consistent with anomalous cosmic‐ray ions, rather than radial diffusion from the outer zone, as the source of the trapped particles.

Angular distribution of near-zero kinetic energy photoelectrons from the lowest rotational states of the NO A 2Σ + state

We report the first rotationally resolved angular distributions of near-zero kinetic energy photoelectrons produced from ionization of the lowest rotational states ( N A=0, 1, 2 or 3) of the nitric oxide A 2Σ + (ν A=0) state. The angular distributions, for the ion in the X + 1Σ + (ν +=0, N +=0, 1, 2 or 3) state, agree well with a theoretical model proposed by McKoy and co-workers, with the exception of the ionizing N + = 0← N A=1 rotational transition. The latter transition exhibits an isotropic angular distribution, which seems to indicate some hitherto unobserved threshold dynamics.