Orientation Of Angular Momentum Research Articles

Inhibited rotational quenching in oriented ultra-high rotational states of CO2

We demonstrate long-lived rotational orientation of CO2 molecules originally prepared in an optical centrifuge. The optical centrifuge traps molecules in a strong optical field and spins them to high rotational states by angular acceleration of the optical field. In the case of CO2, the optical centrifuge creates ultra-high rotational states with J ≥ 220. Polarisation-dependent, high-resolution transient infrared (IR) absorption was used to measure the spatial orientation of CO2 molecules in the (0000, J = 76) state following the optical centrifuge pulse and subsequent collisional energy transfer. Transient Doppler-broadened line profiles show that CO2 molecules in J = 76 probed with an IR transition dipole parallel to the initial plane of rotation are more plentiful and have higher translational temperatures than molecules with an IR transition dipole perpendicular to this plane. Time-dependent data show that the initial angular momentum orientation persists even after thousands of collisions, indicating that molecules in an optical centrifuge behave as quantum gyroscopes. These observations demonstrate that the optical centrifuge prepares an anisotropic rotational distribution and that molecules in oriented, ultra-high angular momentum states require many more collisions to randomise their orientation than do those in low rotational states.

Polarization of molecular angular momentum in the chemical reactions Li + HF and F + HD

The quantum mechanical approach to vector correlation of angular momentum orientation and alignment in chemical reactions [G. Balint-Kurti and O. S. Vasyutinskii, J. Phys. Chem. A 113, 14281 (2009)] is applied to the molecular reagents and products of the Li + HF [L. Gonzalez-Sanchez, O. S. Vasyutinskii, A. Zanchet, C. Sanz-Sanz, and O. Roncero, Phys. Chem. Chem. Phys. 13, 13656 (2011)] and F + HD [D. De Fazio, J. Lucas, V. Aquilanti, and S. Cavalli, Phys. Chem. Chem. Phys. 13, 8571 (2011)] reactions for which accurate scattering information has become recently available through time-dependent and time-independent approaches. Application of the theory to two important particular cases of the reactive collisions has been considered: (i) the influence of the angular momentum polarization of reactants in the entrance channel on the spatial distribution of the products in the exit channel and (ii) angular momentum polarization of the products of the reaction between unpolarized reactants. In the former case, the role of the angular momentum alignment of the reactants is shown to be large, particularly when the angular momentum is perpendicular to the reaction scattering plane. In the latter case, the orientation and alignment of the product angular momentum was found to be significant and strongly dependent on the scattering angle. The calculation also reveals significant differences between the vector correlation properties of the two reactions under study which are due to difference in the reaction mechanisms. In the case of F + HD reaction, the branching ratio between HF and DF production points out interest in the insight gained into the detailed dynamics, when information is available either from exact quantum mechanical calculations or from especially designed experiments. Also, the geometrical arrangement for the experimental determination of the product angular momentum orientation and alignment based on a compact and convenient spherical tensor expression for the intensity of the resonance enhanced multiphoton ionization (REMPI 2 + 1) signal is suggested.

Spatial distribution of optically induced atomic excitation in a dense and cold atomic ensemble

On the basis of our general theoretical results developed previously in JETP 112, 246 (2011), we calculate the spatial distribution of atoms excited in a dense and cold atomic cloud by weak monochromatic light. We also study the atomic distribution over different Zeeman sublevels of the excited state in different parts of the cloud. The dependence of this distribution of atomic excitation on the density of the atomic ensemble and the frequency of external emission is investigated. We show that in the boundary regions of the cloud the orientation and alignment of atomic angular momentum takes place. Analysis of the spatial distribution of atomic excitation shows no noticeable signs of light localization effects even in those parameter regimes where the Ioffe-Regel criterium of strong localization is satisfied. However, comparative calculations performed in the framework of the scalar approximation to the dipole-dipole interaction reveals explicit manifestation of strong localization under some conditions.

Atomic orientation following predissociation of the C 3Πg Rydberg state of molecular oxygen

(2 + 1) resonance enhanced multiphoton ionization in combination with time-of-flight mass spectroscopy (TOF-MS) has been used to detect both the O((3)P) and O((1)D) fragments produced as a result of predissociation of the C (3)Πg (v = 0) and (v = 1) Rydberg states of O2, accessed via two-photon absorption from the ground X (3)Σg(-) state. In particular, TOF profiles have been recorded at various fixed two-photon absorption wavelengths within the two bands, with circular polarized probe laser light used to probe the angular momentum orientation of these photofragments. All photofragments are found to display coherent orientation resulting from interference between two possible two-photon absorption pathways. The measured orientation is affected by rotational depolarization due to the long lifetime of the excited C state; once this effect is accounted for the orientation is found to be nearly constant over all dissociation wavelengths. The origin of the coherent orientation is attributed to two-photon absorption to different spin-orbit components of the C state.

Vector correlations in the O2(a1Δg, v = 1) fragment formed in the 265 nm photodissociation of ozone

Angular momentum polarisation and translational anisotropy have been measured for the J = 17 and 18 rotational levels in the O2(a1Δg, v = 1) fragment formed from the photodissociation of ozone at 265 nm. The results show similarities to those previously measured for the most populated vibrational level, v = 0, produced from photolysis in the Hartley band. Translational anisotropy parameters are very similar for both fragments, consistent with prompt dissociation from a bent intermediate. Angular momentum alignment and orientation, however, are found to be consistently lower for J = 17, compared with J = 18, and the magnitude of the depolarisation is consistent with previous results obtained for the odd and even J fragments in v = 0, produced with the same speed at longer photolysis wavelengths. The results indicate a vibrational-state-independent mechanism for the difference in polarisation between even and odd rotational states observed at relatively low fragment speeds.

Angular momentum polarization of the molecules in the Li + HF reaction

A theoretical analysis of the orientation and alignment of the rotational angular momenta of the reactants and products of the Li + HF(v r = 0, j r = 3) → LiF(v, j) + H reaction at a collision energy of E coll = 0.317 eV is performed. The polarization of the angular momentum of the molecules involved in the reaction is based on the technique of spherical tensor operators (state multipoles). Quantum-mechanical calculations of the S-matrix of the reaction are carried out using the wave packet method. In particular, the influence of the orientation of the angular momentum of the HF reactant on the differential cross section is examined. It is shown that the contribution to the differential cross section comes only from be reactants with an angular momentum perpendicular to the reaction plane. In addition, the angular dependence of the orientation and alignment of the angular momenta of the reaction products are examined. It is shown that, for an isotropic distribution of reactant molecules, the orientation of the angular momenta of the products differs from zero only in the direction perpendicular to the reaction plane. Different experimental geometries, based on radiation enhanced multiphoton ionization, are proposed to detect the predicted effects.

Field-free molecular alignment for probing collisional relaxation dynamics

We report the experimental study of field-free molecular alignment in CO${}_{2}$ gas mixtures induced by intense femtosecond laser pulses in the presence of collisional processes. We demonstrate that the alignment signals exhibit specific features due to nontrivial collisional propensity rules that tend to preserve the orientation of the rotational angular momentum of the molecules. The analysis is performed with a quantum approach based on the modeling of rotational $J$- and $M$-dependent state-to-state transfer rates. The present work paves the way for strong-field spectroscopy of collisional dynamics.

A study of dynamic properties of exchange reaction H(D)+SH/SD by quasi-classical trajectory method

Sulfur in hydrogen combustion reaction chemistry, which plays an important role in meteorology, combustion reactions, and atmospheric pollution, has been extensively investigated recently. And its reverse reaction has also been a research object gradually. The research in this paper is based on the exact potential energy surface (L S J, Zhang P Y, Han K L, He G Z 2012 J. Chem. Phys. 136 094308), with using the method of quasi-classical trajectory on the exchange reaction of H (D)+SH/SD dynamic properties. In this paper, the scalar properties are calculated, including the cross section, rate constant, opacity function, product vibrational, rotational distributions, product scattering direction, rotational angular momentum orientation, and alignment properties. In this paper, how the collision energy and the isotope affect the reaction H (D)+SH/SD kinetic properties is analyzed in detail. The results show that as collision energy increases, the reaction cross section increases, product backscatter weakens gradually while the product rotational angular momentum alignment and orientation nature strengthen gradually. In addition, the isotope effect has a significant influence on the reaction kinetics. The reaction mechanism which is shown in the title and based on the reaction kinetics and the potential energy surface, is also discussed in this paper.

THE COSMIC HISTORY OF THE SPIN OF DARK MATTER HALOS WITHIN THE LARGE-SCALE STRUCTURE

We use N-body simulations to investigate the evolution of the orientation and magnitude of dark matter halo angular momentum within the large scale structure since z=3. We look at the evolution of the alignment of halo spins with filaments and with each other, as well as the spin parameter, which is a measure of the magnitude of angular momentum. It was found that the angular momentum vectors of dark matter haloes at high redshift have a weak tendency to be orthogonal to filaments and high mass haloes have a stronger orthogonal alignment than low mass haloes. Since z=1, the spins of low mass haloes have become weakly aligned parallel to filaments, whereas high mass haloes kept their orthogonal alignment. This recent parallel alignment of low mass haloes casts doubt on tidal torque theory as the sole mechanism for the build up of angular momentum. We see evidence for bulk flows and the broadening of filaments over time in the alignments of halo spin and velocities. We find a significant alignment of the spin of neighboring dark matter haloes only at very small separations, $r<0.3$Mpc/h, which is driven by substructure. A correlation of the spin parameter with halo mass is confirmed at high redshift.

Molecular alignment and orientation with a hybrid Raman scattering technique

We demonstrate a scheme for the preparation of molecular alignment and angular momentum orientation using a hybrid combination of two limits of Raman scattering. First a weak, impulsive pump pulse initializes the system via the nonresonant dynamic Stark effect. Then, having overcome the influence of the vacuum fluctuations, an amplification pulse selectively enhances the initial coherences by transient stimulated Raman scattering, generating alignment and angular momentum orientation of molecular hydrogen. The amplitude and phase of the resulting coherent dynamics are experimentally probed, indicating an amplification factor of $4.5$. An analytic theory is developed to model the dynamics.

Idealized models for galactic disc formation and evolution in ‘realistic’ ΛCDM haloes

We study the dynamics of galactic disc formation and evolution in ‘realistic’ Λ cold dark matter haloes with idealized baryonic initial conditions. We add rotating spheres of hot gas at z = 1.3 to two fully cosmological dark-matter-only halo (re)simulations. The gas cools according to an artificial and adjustable cooling function to form a rotationally supported galaxy. The simulations evolve in the full cosmological context until z = 0. We vary the angular momentum and density profiles of the initial gas sphere, the cooling time and the orientation of the angular momentum vector to study the effects on the formation and evolution of the disc. The final discs show exponential radial and (double)-exponential vertical stellar density profiles, and stellar velocity dispersions that increase with age of the stars, as in real disc galaxies. The slower the cooling/accretion processes, the higher the kinematic disc-to-bulge (D/B) ratio of the resulting system. We find that the initial orientation of the baryonic angular momentum with respect to the halo has a major effect on the resulting D/B. The most stable systems result from orientations parallel to the halo minor axis. Despite the spherical and coherently rotating initial gas distribution, the orientation of the central disc and of the outer gas components, and the relative angle between the components can all change by more than 90° over several billion years. Initial orientations perpendicular to the major axis tend to align with the minor axis during their evolution, but the sign of the spin can have a strong effect. Discs can form from initial conditions oriented parallel to the major axis, but there is often strong misalignment between inner and outer material. The more the orientation of the baryonic angular momentum changes during the evolution, the lower the final D/B. The behaviour varies strongly from halo to halo. Even our very simple initial conditions can lead to strong bars, dominant bulges, massive, misaligned rings and counter-rotating components. We discuss how our results may relate to the failure or success of fully cosmological disc formation simulations.

Velocity-tunable slow beams of cold O2 in a single spin-rovibronic state with full angular-momentum orientation by multistage Zeeman deceleration

Cold samples of oxygen molecules in supersonic beams have been decelerated from initial velocities of 390 and 450 m s−1 to final velocities in the range between 150 and 280 m s−1 using a 90-stage Zeeman decelerator. (2 + 1) resonance-enhanced-multiphoton-ionization (REMPI) spectra of the 3sσ g 3Π g (C) two-photon transition of O2 have been recorded to characterize the state selectivity of the deceleration process. The decelerated molecular sample was found to consist exclusively of molecules in the J ′′ = 2 spin–rotational component of the X ground state of O2. Measurements of the REMPI spectra using linearly polarized laser radiation with polarization vector parallel to the decelerator axis, and thus to the magnetic-field vector of the deceleration solenoids, further showed that only the magnetic sublevel of the N′′ = 1, J ′′ = 2 spin–rotational level is populated in the decelerated sample, which therefore is characterized by a fully oriented total-angular-momentum vector. By maintaining a weak quantization magnetic field beyond the decelerator, the polarization of the sample could be maintained over the 5 cm distance separating the last deceleration solenoid and the detection region.

Quantum and classical approaches for rotational relaxation and nonresonant laser alignment of linear molecules: A comparison for CO2 gas in the nonadiabatic regime

A quantum approach and classical molecular dynamics simulations (CMDS) are proposed for the modeling of rotational relaxation and of the nonadiabatic alignment of gaseous linear molecules by a nonresonant laser field under dissipative conditions. They are applied to pure CO(2) and compared by looking at state-to-state collisional rates and at the value of <cos(2)[θ(z)(t)]> induced by a 100 fs laser pulse linearly polarized along z[overhead arrow]. The main results are: (i) When properly requantized, the classical model leads to very satisfactory predictions of the permanent and transient alignments under non-dissipative conditions. (ii) The CMDS calculations of collisional-broadening coefficients and rotational state-to-state rates are in very good agreement with those of a quantum model based on the energy corrected sudden (ECS) approximation. (iii) Both approaches show a strong propensity of collisions, while they change the rotational energy (i.e., J), to conserve the angular momentum orientation (i.e., M/J). (iv) Under dissipative conditions, CMDS and quantum-ECS calculations lead to very consistent decays with time of the "permanent" and transient components of the laser-induced alignment. This result, expected from (i) and (ii), is obtained only if a properly J- and M-dependent ECS model is used. Indeed, rotational state-to-state rates and the decay of the "permanent" alignment demonstrate, for pure CO(2), the limits of a M-independent collisional model proposed previously. Furthermore, computations show that collisions induce a decay of the "permanent" alignment about twice slower than that of the transient revivals amplitudes, a direct consequence of (iii). (v) The analysis of the effects of reorienting and dephasing elastic collisions shows that the latter have a very small influence but that the former play a non-negligible role in the alignment dynamics. (vi) Rotation-translation collisionally induced transfers have also been studied, demonstrating that they only slightly change the alignment dissipation for the considered laser energy conditions.

Vector correlations in the F + HD reaction

A theoretical study of the orientation of product rotational angular momenta for two chemical reaction channels: F + HD(ν r = 0, j r = 0) → HF(ν, j) + D and F + HD(ν r = 0, j r = 0) → DF(ν, j) + H at a E coll = 78.54 meV collision energy was performed. Angular momentum orientation was described on the basis of irreducible tensor operators (state multipoles) expressed through anisotropy transfer coefficients, which contained quantum-mechanical scattering T matrices determined on the basis of exact solutions to quantum scattering equations obtained using the hyperquantization algorithm. The possibility of the existence of substantial orientation of the angular momentum of reaction products j in the direction perpendicular to the scattering plane was demonstrated. The dependences of differential reaction cross sections and state multi-poles on the ν and j quantum numbers were calculated and analyzed. A experimental scheme based on the multiphoton ionization method was suggested. The scheme can be used to detect predicted reaction product angular momentum orientation.

ON THE ORIGIN OF THE ANGULAR MOMENTUM PROPERTIES OF GAS AND DARK MATTER IN GALACTIC HALOS AND ITS IMPLICATIONS

We perform a set of non-radiative hydrodynamical simulations of merging spherical halos in order to understand the angular momentum (AM) properties of the galactic halos seen in cosmological simulations. The universal shape of AM distributions seen in simulations is found to be generically produced as a result of mergers. The universal shape is such that it has an excess of low AM material and hence cannot explain the exponential structure of disk galaxies. A resolution to this is suggested by the spatial distribution of low AM material which is found to be in the centre and a conical region close to the axis of rotation. A mechanism that preferentially discards the material in the centre and prevents the material along the poles from falling onto the disc is proposed as a solution. We implement a simple geometric criteria for selective removal of low AM material and show that in order for 90% of halos to host exponential discs one has to reject at least 40% of material. Next, we explore the physical mechanisms responsible for distributing the AM within the halo during a merger. For dark matter there is an inside-out transfer of AM, whereas for gas there is an outside-in transfer, which is due to differences between collisionless and gas dynamics. We also explain the apparent high spin of dark matter halos undergoing mergers and show that a criteria stricter than what is currently used, would be required to detect such unrelaxed halos. Finally, we demonstrate that the misalignment of AM between gas and dark matter only occurs when the intrinsic spins of the merging halos are not aligned with the orbital AM of the system. The self-misalignment (orientation of AM when measured in radial shells not being constant), which could be the cause of warps and anomalous rotation in disks galaxies, also occurs under similar conditions.

Nonredundant Single-Gimbaled Control Moment Gyroscopes

Abstract : Two objectives dominate consideration of control moment gyroscopes for spacecraft maneuvers: high torque (equivalently momentum) and singularity-free operations. This paper adds to the significant body of research toward these two goals using a minimal three-control-moment-gyroscope array to provide significant singularity-free momentum performance increase spherically (in all directions) by modification of control-moment-gyroscope skew angles, compared with the ubiquitous pyramid geometry skewed at 54.73 deg. Spherical 1H (one control moment gyroscope s worth momentum) singularity-free momentum is established with bidirectional 1H and 2H in the third direction in a baseline configuration. Next, momentum space reshaping is shown via mixed skew angles permitting orientation of maximum singularity-free angular momentum into the desired direction of maneuver (yaw in this study). Finally, a decoupled gimbal angle calculation technique is shown to avoid loss of attitude control associated with singular matrix inversion. This technique permits 3H (maximal) yaw maneuvers without loss of attitude control despite passing through singularity. These claims are demonstrated analytically, then heuristically, and finally validated experimentally.

New findings regarding the NO angular momentum orientation in Ar–NO(2Π1/2) collisions

This article reports a theoretical study of the stereodynamics of Ar + NO(X(2)Π, v = 0, j = 1/2, Ω = 1/2, ε = ±1) rotationally inelastic collisions. First, quantum scattering data are used to calculate all differential polarisation moments of the reagent and product molecules; this leads to the observation that the orientations of the reagent and product angular momenta are very strongly correlated. Next, canonical collision mechanisms theory [Aldegunde et al., Phys. Chem. Chem. Phys., 2008, 10, 1139] is used to separate and characterise the stereodynamics of the two independent collision mechanisms that contribute to the collision dynamics; this leads to the observation that the average product orientation is determined by the relative contributions of the two canonical mechanisms, which have comparable importance but are associated with starkly contrasting angular momentum orientations. These observations lead to a new and rigorous explanation of the experimental results reported a decade ago by Lorenz et al. [Science, 2001, 293, 2063]. The central fact of the new explanation is the incoherent, interference-free superposition of two independent collision mechanisms. This makes the new explanation radically different from the only one previously suggested, namely that the experimental observations might be due to quantum interference in a single collision mechanism.

Orientation effects in Cl + H2 inelastic collisions: characterization of the mechanisms

Based on quantum mechanical scattering (QM) calculations, we have analyzed the polarization of the product hydrogen molecule in Cl + H(2) (v = 0, j = 0) inelastic collisions. The spatial arrangements adopted by the rotational angular momentum and internuclear axis of the departing molecule have been characterized and used to prove that two distinct mechanisms, corresponding to different dynamical regimes, are responsible for the inelastic collisions. Such mechanisms, named as low-b and high-b, correlate with well defined ranges of impact parameter values, add in an essentially incoherent way, and can be clearly differentiated through the quantum mechanical polarization moment that measures the orientation of the products rotational angular momentum with respect to the scattering plane. Other directional effects turn out to fail when it comes to distinguishing the mechanisms. Quasiclassical trajectories (QCT) calculations have been used as a supplement to the purely quantum mechanical analysis. By combining QM and QCT results, which are in very good agreement, we have succeeded in obtaining a clear and meaningful picture of how the two types of collisions take place.

Only marginal alignment of disc galaxies

Testing theories of angular-momentum acquisition of rotationally supported disc galaxies is the key to understand the formation of this type of galaxies. The tidal-torque theory tries to explain this acquisition process in a cosmological framework and predicts positive autocorrelations of angular-momentum orientation and spiral-arm handedness on distances of 1Mpc/h. This disc alignment can also cause systematic effects in weak-lensing measurements. Previous observations claimed discovering such correlations but did not account for errors in redshift, ellipticity and morphological classifications. We explain how to rigorously propagate all important errors. Analysing disc galaxies in the SDSS database, we find that positive autocorrelations of spiral-arm handedness and angular-momentum orientations on distances of 1Mpc/h are plausible but not statistically significant. This result agrees with a simple hypothesis test in the Local Group, where we find no evidence for disc alignment. Moreover, we demonstrate that ellipticity estimates based on second moments are strongly biased by galactic bulges, thereby corrupting correlation estimates and overestimating the impact of disc alignment on weak-lensing studies. Finally, we discuss the potential of future sky surveys. We argue that photometric redshifts have too large errors, i.e., PanSTARRS and LSST cannot be used. We also discuss potentials and problems of front-edge classifications of galaxy discs in order to improve estimates of angular-momentum orientation.

Collisional angular momentum depolarization of OH(A) and NO(A) by Ar: A comparison of mechanisms

This paper discusses the contrasting mechanisms of collisional angular momentum depolarization of OH(A(2)Σ(+)) and NO(A(2)Σ(+)) by Ar. New experimental results are presented for the collisional depolarization of OH(A) + Ar under both thermal and superthermal collision conditions, including cross sections for loss of both angular momentum orientation and alignment. Previous work on the two systems is summarized. It is shown that NO(A) + Ar depolarization is dominated by impulsive events in which the projection of the angular momentum, j, along the kinematic apse, a, is nearly conserved, and in which the majority of the trajectories can be described as "nearside." By contrast, at the relatively low collision energies sampled at 300 K, OH(A) + Ar depolarization is dominated by attractive collisions, which show a preponderance of "farside" trajectories. There is also evidence for very long-lived, complex type trajectories in which OH(A) and Ar orbit each other for several rotational periods prior to separation. Nevertheless, there is still a clear preference for conservation of the projection of j along the kinematic apse for both elastic and inelastic collisions. Experimental and theoretical results reveal that, as the collision energy is raised, the depolarization of OH(A) by Ar becomes more impulsive-like in nature.