Recoil Direction Research Articles

Photofragment alignment in the photodissociation of I2 from 450to510nm

A combination of velocity map imaging and slicing techniques have been used to measure the product recoil anisotropy and angular momentum polarization for the photodissociation process I2-->I(2P(3/2))+I(2P(3/2)) and I2-->I(2P(3/2)))+I(2P(1/2)) in the 450-510 nm laser wavelength region using linearly polarized photolysis and probe laser light. The former channel is produced predominantly via perpendicular excitation to the 1Piu state, and the latter is predominantly parallel, via the B 3Pi(0u)+ state. In both cases we observe mostly adiabatic dissociation, which produces electronically aligned iodine atoms in the mid /m/=1/2 states with respect to the recoil direction.

Point-defect properties of, and sputtering events in, the {001} surfaces of Ni3Al. II. Sputtering events at and near surfaces

Atomic recoil events at and near {001} surfaces of Ni3Al due to elastic collisions between electrons and atoms have been simulated by molecular dynamics to obtain the sputtering threshold energy as a function of atomic species, recoil direction and atomic layer of the primary recoil atom. The minimum sputtering energy occurs for adatoms and is 3.5 and 4.5 eV for Al and Ni adatoms on the Ni–Al surface (denoted ‘M’), respectively, and 4.5 eV for both species on the pure Ni surface (denoted ‘N’). For atoms within the surface plane, the minimum sputtering energy is 6.0 eV for Al and Ni atoms in the M plane and for Ni atoms in the N surface. The sputtering threshold energy increases with increasing angle, θ, between the recoil direction and surface normal, and is almost independent of azimuthal angle, ϕ, if θ<60°; it varies strongly with ϕ when θ>60°, with a maximum at ϕ = 45° due to ⟨{110}⟩ close-packed atomic chains in the surface. The sputtering threshold energy increases significantly for subsurface recoils, except for those that generate efficient energy transfer to a surface atom by a replacement collision sequence. The implications of the results for the prediction of the mass loss due to sputtering during microanalysis in a FEG STEM are discussed.

Directional statistics for realistic weakly interacting massive particle direct detection experiments

The direction dependence of the event rate in WIMP direct detection experiments provides a powerful tool for distinguishing WIMP events from potential backgrounds. We use a variety of (non-parametric) statistical tests to examine the number of events required to distinguish a WIMP signal from an isotropic background when the uncertainty in the reconstruction of the nuclear recoil direction is included in the calculation of the expected signal. We consider a range of models for the Milky Way halo, and also study rotational symmetry tests aimed at detecting non-sphericity/isotropy of the Milky Way halo. Finally we examine ways of detecting tidal streams of WIMPs. We find that if the senses of the recoils are known then of order ten events will be sufficient to distinguish a WIMP signal from an isotropic background for all of the halo models considered, with the uncertainties in reconstructing the recoil direction only mildly increasing the required number of events. If the senses of the recoils are not known the number of events required is an order of magnitude larger, with a large variation between halo models, and the recoil resolution is now an important factor. The rotational symmetry tests require of order a thousand events to distinguish between spherical and significantly triaxial halos, however a deviation of the peak recoil direction from the direction of the solar motion due to a tidal stream could be detected with of order a hundred events, regardless of whether the sense of the recoils is known.

Velocity-map imaging study of the photodissociation of acetaldehyde

Velocity-map imaging studies are reported for the photodissociation of acetaldehyde over a range of photolysis wavelengths (317.5-282.5 nm). Images are obtained for both the HCO and CH3 fragments. The mean rotational energy of both fragments increases with photodissociation energy, with a lesser degree of excitation in the CH3 fragment. The CH3 images demonstrate that the CH3 fragments are rotationally aligned with respect to the recoil direction and this is interpreted, and well modeled, on the basis of a propensity for forming CH3 fragments with M approximately K, where M is the projection of the rotational angular momentum along the recoil direction. The origin of the CH3 rotation is conserved motion from the torsional and methyl-rocking modes of the parent molecule. Nonstatistical vibrational distributions for the CH3 fragment are obtained at higher energies.

Hydrogen Migration in Acetonitrile in Intense Laser Fields Studied by Coincidence Momentum Imaging

The two-body Coulomb explosion of acetonitrile, CH3CN2+ → CH3-n+ + HnCN+ (n = 0–2), in intense laser fields (0.15 PW/cm2, 70 fs) is studied by the coincidence momentum imaging technique. It is found that the fragment ions for n = 0 are ejected mostly in the direction of the laser polarization vector with ⟨ cos2 θ ⟩ = 0.68, where θ is the angle between the fragment recoil direction and the laser polarization vector, while the angle distribution becomes more isotropic as n increases, i.e., ⟨ cos2 θ ⟩ = 0.49 for n = 1 and ⟨ cos2 θ ⟩ = 0.37 for n = 2. From this characteristic correlation between the anisotropy in the fragment ejection and the hydrogen migration, the Coulomb explosion dynamics competing with the hydrogen atom transfer from the methyl group to the nitrile group is investigated.

Examination of liquid metal surfaces through angular and energy measurements of inert gas collisions with liquid Ga, In, and Bi

Gas–liquid scattering experiments are used to measure the recoil directions and energies of neon, argon, and xenon atoms scattering from liquid gallium, indium, and bismuth. The angular and energy distributions vary systematically with the identity of the gas and liquid and with the incident gas energy and liquid temperature. We find that the gas atoms scatter into a narrower angular range from liquids with higher surface tension γ (γGa&amp;gt;γIn&amp;gt;γBi), while they transfer less energy to liquids of higher atomic mass (mBi&amp;gt;mIn&amp;gt;mGa). Comparisons of the angular distributions with scattering models suggest that lower surface tension liquids possess atomically rougher surfaces that redirect impinging atoms more broadly into space. The trend we observe linking broader angular distributions with lower energy transfer appears to be a microscopic manifestation of a general trend between decreasing surface tension and increasing atomic mass for main group metals such as Ga, In, and Bi.

DRIFT: a directionally sensitive dark matter detector

Directional Recoil Identification From Tracks-I (DRIFT) is the world's first WIMP dark matter detector with sensitivity to the directions of nuclear recoils. The distribution of WIMP induced nuclear recoil directions offers the most powerful way of positively identifying a WIMP signal. This paper discusses the DRIFT-I detector and considers future high spatial resolution readout schemes.

Molecular and laboratory frame photofragment angular distributions from oriented and aligned molecules

We present explicit expressions for the molecular frame and laboratory frame photofragment angular distributions from oriented parent molecules, in terms of the dynamically significant molecular frame angles between the recoil direction and the transition and permanent dipole moments of the molecule. We discuss how these angles can be measured from distinct experimental geometries. Explicit examples are given on the extracted information of the molecular frame photodissociation, especially in case of non-axial recoil dynamics.

Angular momentum alignment of Cl(2P3/2) in the 308 nm photolysis of Cl2 determined using Fourier moment velocity-map imaging

Velocity-map ion imaging, in conjunction with the recently-developed Fourier moment analysis procedure, has been used to determine angular momentum alignment parameters for the Cl(2P3/2) products of 308 nm Cl2 photolysis. The alignment parameters are in good agreement with previous measurements at similar energies and indicate strong alignment of the photofragment orbital angular momentum perpendicular to the recoil direction. The contributions of both adiabatic and non-adiabatic pathways to the dissociation dynamics have been quantified.

Dynamical effects in nuclear collisions in the Fermi energy range: aligned breakup of heavy projectiles

Recent experimental results concerning heavy systems (Pb+Au, Pb+Ag, Pb+Al, Gd+C, Gd+U, Xe+Sn, …) obtained at GANIL with the INDRA and NAUTILUS 4π arrays will be presented. The study of reaction mechanisms has shown the dominant binary and highly dissipative character of the process. The two heavy and excited fragments produced after the first stage of the interaction can decay into various decay modes from evaporation to multifragmentation including fission. However, deviations from this simple picture have been found by analyzing angular and velocity distributions of light charged particles, and fragments. Indeed, there is a certain amount of matter in excess emitted between the two primary sources suggesting either the existence of a mid-rapidity source similar to the one observed in the relativistic regime (participants) or a strong deformation induced by the dynamics of the collision (neck instability). This last possibility has been suggested by analyzing in detail the angular distributions of the fragments. More precisely, we observe an isotropic component which is compatible with the prediction of statistical models and a second one corresponding to breakup aligned with the recoil direction of the projectile like source which should be compared with the predictions of dynamical calculations based on microscopic transport models.

The 5a1−1 photoionization of oriented CF3I molecules: Angular distributions of the ka1 and ke photoelectron continua

Molecule-frame photoelectron angular distributions are obtained by the measurement of electron–ion recoil vector correlations in the dissociative 5a1−1 photoionization of CF3I. The laboratory frame orientation of an ionized molecule is inferred from the CF3+ fragment ion recoil direction and the contributions of radiation polarized perpendicular and parallel to the molecular axis can be estimated. This allows the degenerate ka1 and ke continua to be distinguished, each having a distinctive angular distribution. Experimental data are compared with calculated fixed-molecule photoelectron angular distributions.

The breaking of the backward–forward symmetry in the angular distribution of mj-selected photofragments

We present calculations on the angular distributions of mj-selected photofragments (with mj being the projection of the fragments’ internal angular momentum on a space-fixed z-axis). We show that the proper analysis of such experiments [based on G. G. Balint-Kurti and M. Shapiro, Chem. Phys. 61, 137 (1981)], involves coherent λ≠λ′ terms (with λ—the “helicity”—being the projection of the fragment angular momentum on the recoil direction). The involvement of these coherent terms leads, amongst other things, to the creation of “orientation,” i.e., the backward–forward asymmetry, in the fragments’ angular distributions. Thus we demonstrate that the detection of polarization in one (internal) variable—the internal rotation, creates a correlated orientation in another (external) variable—the recoil angular distribution.

Simulation of threshold displacements in NiAl

Modified many-body interatomic potentials for modeling high-energy atomic collision in NiAl are constructed and described. The displacement threshold energy as a function of the primary recoil direction for Ni atoms and Al atoms in NiAl has been investigated by molecular dynamics simulations using the modified potentials. The defect structure induced by recoil events has been presented. The results show that the lowest threshold displacement energy, E d, is near the 〈1 0 0〉 orientation and the mean value of E d for an Al atom is higher than a Ni. A crowdion in the compact crystalline directions, 〈1 1 1〉, incorporating an extra Ni atom is the stable interstitial configuration.

Photofragment angular momentum distributions in the molecular frame: Determination and interpretation

Photolysis of a molecule typically yields open-shell photofragments having angular momenta. A procedure is described for the measurement of the photofragment angular momentum distribution in terms of polarization parameters aq(k)(p) which are expressed in the molecular frame and which may be related to the transition dipole matrix elements. The index (p) indicates either a parallel transition (∥), a perpendicular transition (⊥), or a mixed transition (∥,⊥) having both parallel and perpendicular character. This procedure has the advantage that it decouples the angular momentum distributions in the molecular frame from the photofragment angular distributions in the laboratory frame, which gives new insight into the photodissociation dynamics. For cases in which k⩽2 and with linearly polarized photolysis light, the photofragment angular momentum distribution arising from pure parallel transitions can be described with only one parameter, a0(2)(∥); photofragment angular momentum distributions arising from pure perpendicular transitions require only two parameters, a0(2)(⊥) and a2(2)(⊥); photofragment angular momentum distributions arising from mixed transitions, having both parallel and perpendicular character, can be described with five parameters: the two (coherent) interference terms Im[a1(1)(∥,⊥)] and Re[a1(2)(∥,⊥)] in addition to the three incoherent terms mentioned above. We describe procedures for the measurement of the complete angular momentum distribution of state-selected photofragments using laser detection (such as REMPI) and some form of laboratory velocity selection (such as time-of-flight mass spectrometry, Doppler spectroscopy, or ion imaging). The laser-detection probability of a single photofragment is presented in the form I=1+f[θε,Θ,Φ,β,aq(k)(p)], where θε is the angle between the recoil direction and the photolysis polarization, Θ and Φ are the spherical polar angles describing the orientation of the probe polarization about the recoil direction, and β is the spatial anisotropy parameter. The physical significance of the aq(k)(p) is discussed; in particular, the a0(k)(∥) and a0(k)(⊥) describe the photofragment m-state distribution along the recoil direction; the a2(k)(⊥) describe how broken cylindrical symmetry in the parent molecule is reflected in the photofragment angular momentum distribution in a plane perpendicular to the recoil direction; and the a1(k)(∥,⊥) are related to the asymptotic phase difference associated with the interfering channels, and are thus sensitive to the shapes of the dissociative surfaces.

Quantum stereodynamics of four-atom reactions: theory and application to H2+OH↔H2O+H

This article presents a theoretical quantum method for the description of the stereo- dynamics of four-atom reactions of the type AB(vAB, jAB)+CD(vCD, jCD)↔ABC(v1′, v2′, v3′, j′)+D. The method is based on density matrix techniques and angular momentum algebra, and requires knowledge of the scattering matrix. Reagents and products are treated on an equal footing, and the reaction stereodynamics can be analysed either in the rotational polarisation or in the molecular polarisation representation. The formalism is applied to the H2(0, jH2)+OH(0, jOH)↔H2O(0, 0, 0, 0)+H reaction at zero total angular momentum and reveals state-specific correlations between the polarisations of the diatomic molecules, their approach or recoil directions, and the reaction probability.

Helicity of orientation parameters of photofragments in fluorescence-imaging experiments

A detection scheme in fluorescence-imaging experiments is presented for determining helicity of orientation parameters of photofragments prepared by circularly polarized photolysis lasers. In a framework of density matrix theory, explicit fluorescence intensity formulas are derived for detection of right- and left-circularly polarized fluorescence photons in various transition sequences. A pattern recognition from fluorescence images of photofragments in a state of definite helicity with respect to their recoil directions has been established, where differences between left- and right-circularly polarized fluorescence image patterns should be taken. Information on the coherence among various magnetic sublevels of an angular momentum state can be obtained by monitoring fluorescence images in a ΔJ=−1 transition (P branch in absorption).

A combined polarized target/ionization chamber for measuring the spin dependence of nuclear muon capture in laser polarized muonic 3He

We describe a device used in experiment 1231 at LAMPF and in e683 at TRIUMF to study the spin dependence of the reaction μ − + 3He → 3H + v μ . In order to study the spin dependence, we needed to form and polarize muonic 3He and in the same volume detect the 1.9 MeV tritons created in the reaction. Furthermore, the recoil direction of the tritons had to be determined. The apparatus served both as a polarized target and as a detector. The detector, a gridded ion chamber, was incorporated inside a 51 polarized target that was filled with 8 atm of 3He and 100 Torr of N 2 and rubidium metal. At the operating temperature of the device (205–230°C) the rubidium number density was approximately 10 15 atoms/cm 3. Muons that stopped in the target formed muonic helium atoms, which were then polarized by collisions with optically pumped Rb vapor. Two high-powered GaAlAs diode laser arrays were used to polarize the Rb in the 100 cm 3 fiducial volume located inside the ion chamber. The ion chamber produced clean signals from the reaction tritons despite having to operate under the extreme conditions required for efficient optical pumping. The direction of the tritons was determined by analyzing the shapes of the ionization pulses. The muon polarization was measured using the decay electron asymmetry.

Defect production by near-surface displacement cascades in α-zirconium

Recent computer simulations have shown that a nearby surface can engender a significant increase in the number of vacancy defects produced by displacement cascades in pure metals of cubic crystal structure (e.g. M. Ghaly and R.S. Averback, Phys. Rev. Lett. 72 (1994) 364). In the present work, the influence of a surface on the production of lattice defects by displacement cascades of 2 keV in energy in α-zirconium has been studied. This HCP metal is of interest because self-interstitials migrate preferentially along the basal planes, and so the crystallographic orientation of the surface may influence the final defect state of cascade damage. The primary recoil atom was chosen to be an atom on either a basal-or a prism-plane surface, and recoil directions making either a high or a low angle to the surface normal were considered. The resulting production of vacancies and interstitials below the surface has been compared with equivalent data for simulations of cascades in the bulk. The creation of vacancies is not strongly affected by the crystallography, but the number of interstitials, adatoms and sputtered atoms is. The production efficiency of the latter two forms is much larger for the prism-plane surface, whereas stable interstitial defects are more readily generated below a basal-plane surface. These results can be interpreted in terms of cascade mechanisms and defect properties in zirconium.

Photochemistry of adsorbed molecules. XVIII. Photodissociation and exchange reaction in CH3Br/MgO(001) at 193 nm

Methyl photofragments from the 193 nm photodissociation of CH3Br adsorbed on MgO(001) were studied by angularly resolved time-of-flight mass spectrometry. The translational energy distributions of the photofragments showed evidence of three pathways for the release of CH3. The pathways were termed “direct” [DIR], “indirect(1)” [IND(1)], and “indirect(2)” [IND(2)]. The DIR methyl translational energy distribution, P(ET′) peaked at 2.3 eV, with a full width at half-maximum (FWHM) of 0.65 eV, corresponding roughly to that reported for gas phase CH3 (peak=2.5 eV, FWHM=0.5 eV). The P(ET′) of the DIR pathway was consistent with CH3 escaping directly from the topmost molecular layer without collisions. For these DIR methyls, as previously demonstrated, the angular distribution reflected the prior Br–C bond direction. In the higher coverage range, 1.5–10 monolayers (ML) angular distributions, P(Θ), for the DIR methyls peaked at 22° to the normal with FWHM of 20°. At low coverage, 0.75 ML, the DIR peak shifted to ∼40° with doubled FWHM. The IND(1) methyls, despite the loss of 1.2 eV in a strong inelastic encounter, exhibited the same angular distributions and FWHM as the DIR component: 22° peak, 20° FWHM at high coverage; 40° peak, 40° FWHM at low coverage. The mechanism attributed to IND(1), which accounts for the retention of direction of methyl with concurrent substantial energy loss, is one proposed in a prior theoretical study [Barclay et al., J. Phys. Chem. 97, 12541 (1993)]: an exchange reaction favored by the adsorbate geometry; in the present case CH3+BrCH3′→CH3Br+CH3′. The second indirect channel, IND(2), exhibited broader translational energy distribution than DIR or IND(1) peaked at 0.6 eV lower energy than IND(1), and a broad angular distribution (cos2 Θ) peaked at the normal, characteristic of strongly inelastic encounters in which memory of the initial CH3 recoil direction is lost.

Photodissociation dynamics of HNO 3 at 266 nm

The photodissociation dynamics of HNO 3 at 266 nm has been investigated by measuring laser induced fluorescence spectra of the OH( 2п) fragments. The measured fractions of the available energy distributed among the fragements are ƒ t = 0.21, ƒ r ( OH) = 0.047 , ƒ int ( NO 2) = 0.74 , and negligible OH fragments are found in the vibrationally excited states. By analyzing Doppler profiles of the spectra, vector correlations in the photodissociation of HNO 3 have been measured. The measured correlations show that the transition is parallel ( β μv = 1.0) but the transition dipole moment and the recoil velocity of the fragment are not exactly parallel. The rotational angular momentum J of the product OH is aligned parallel to the recoil direction of the fragment. From the measured correlations and the large fraction of the internal energy in the NO 2 fragments, it has been concluded that the transition at 266 nm is described as vibronic to the pyramidal excited state of A″ symmetry. Together with the measured Λ-doublet distribution of the OH fragments, the dynamics of photodissociation is discussed in detail.