Depolarization Cross Sections Research Articles

Depolarization of spin-polarized hydrogen via collisions with chlorine atoms at ultrahigh density

Recently, the production of ultrahigh-density (∼1019cm−3) spin-polarized deuterium (SPD) atoms was demonstrated, from the photodissociation of deuterium iodide, but the upper density limit was not determined. Here, we present studies of spin-polarized hydrogen (SPH) densities up to 1020cm−3, by photodissociating 5 bar of hydrogen chloride with a focused 213 nm, 150 ps laser pulse. We extract the depolarization cross-section of hydrogen and chlorine atom collisions, which is the main depolarization mechanism at this high-density regime, to be σH-Cl=7(2)×10−17cm2. We discuss the conditions under which the ultrahigh SPH and SPD densities can be reached, and the potential applications to ultrafast magnetometry, laser-ion acceleration, and tests of polarized nuclear fusion.

Polar Liquid Cloud Base Detection Algorithms for High Spectral Resolution or Micropulse Lidar Data

AbstractLiquid layers in clouds affect their microphysical processes, as well as the surface energy budget. Studies focusing on these and other areas of research are often in need of skillful estimation of liquid‐bearing cloud layer boundaries. The bases of these layers are predominantly determined by ground‐based lidar instruments. Most studies requiring liquid cloud base height (LCBH) information use either fixed lidar parameter (depolarization and/or backscatter cross section) thresholds or cloud base height data products that do not distinguish between ice and liquid, all of which might introduce inconsistencies and errors in the resolved LCBH. In this paper, two explicit LCBH detection algorithms are presented. The first algorithm uses the high spectral resolution lidar (HSRL) data. Examination of this algorithm in multiple cases and scenarios during numerous days and first‐order comparison with microwave‐radiometer data show satisfactory results. The second algorithm incorporates widely available micropulse lidar (MPL) data for the LCBH detection. A 1‐year long comparison of data gathered at Barrow, Alaska, and McMurdo Station, Antarctica, which includes other cloud base detection methodologies (ceilometer, MPL value‐added product cloud base height, and LCBHs detected using a fixed MPL depolarization threshold), emphasizes the merits of the presented MPL algorithm. Examination of several unusual LCBH configurations suggests that the current practice of operating lidar at a tilting angle of 4° off zenith may not be sufficient to avoid specular reflection from oriented ice crystals. Data collected at Madison Wisconsin are used to show that specular reflection may impact measurements even at 4°.

The collisional depolarization of OH(A 2Σ+) and NO(A 2Σ+) with Kr

Quantum beat spectroscopy has been used to measure rate coefficients at 300K for collisional depolarization for NO(A(2)Σ(+)) and OH(A(2)Σ(+)) with krypton. Elastic depolarization rate coefficients have also been determined for OH(A) + Kr, and shown to make a much more significant contribution to the total depolarization rate than for NO(A) + Kr. While the experimental data for NO(A) + Kr are in excellent agreement with single surface quasiclassical trajectory (QCT) calculations carried out on the upper 2A(') potential energy surface, the equivalent QCT and quantum mechanical calculations cannot account for the experimental results for OH(A) + Kr collisions, particularly at low N. This disagreement is due to the presence of competing electronic quenching at low N, which requires a multi-surface, non-adiabatic treatment. Somewhat improved agreement with experiment is obtained by means of trajectory surface hopping calculations that include non-adiabatic coupling between the ground 1A(') and excited 2A(') states of OH(X/A) + Kr, although the theoretical depolarization cross sections still significantly overestimate those obtained experimentally.

Determination of anisotropy-dependent inelastic collision cross sections in atomic sodium using linear polarization spectroscopy

We report on the measurement of the linear polarization degree of the $3s{\phantom{\rule{0.16em}{0ex}}}^{2}\phantom{\rule{-0.16em}{0ex}}{S}_{1/2}$ $\ensuremath{\rightarrow}$ $3p{\phantom{\rule{0.16em}{0ex}}}^{2}\phantom{\rule{-0.16em}{0ex}}{P}_{3/2}$ $\ensuremath{\rightarrow}$ $5s{\phantom{\rule{0.16em}{0ex}}}^{2}\phantom{\rule{-0.16em}{0ex}}{S}_{1/2}$ transition in atomic sodium. A linearly polarized pump laser creates an alignment in the excited ${}^{23}$Na $3p{\phantom{\rule{0.16em}{0ex}}}^{2}\phantom{\rule{-0.16em}{0ex}}{P}_{3/2}$ level. From the spectra, we extracted the inelastic collision cross section for the disalignment of the excited atoms by collisions with ground-level argon or xenon atoms. Measurements were done by monitoring the intensity of the cascade fluorescence from the $4p{\phantom{\rule{0.16em}{0ex}}}^{2}\phantom{\rule{-0.16em}{0ex}}{P}_{1/2}$ $\ensuremath{\rightarrow}$ $3s{\phantom{\rule{0.16em}{0ex}}}^{2}\phantom{\rule{-0.16em}{0ex}}{S}_{1/2}$ transition at 330.29 nm, in the presence of Ar and Xe for pressures ranging from 5.3 to 146.6 mbar. We demonstrated that the polarization measurement can provide important information about the alignment-dependent inelastic process in alkali-metal\char21{}rare-gas collisions. A depolarization cross-section value of 196 (22) \AA{}${}^{2}$ for Na-Ar and 357 (21) \AA{}${}^{2}$ for Na-Xe are obtained.

Perturber Dependence of Disalignment Cross Sections of the Argon 2p2Atoms Measured at Temperatures between 77 and 295 K

In a temperature controlled glow gaseous discharge of pure argon, or a mixture of argon and helium or neon at 77, 135, 190, 240, and 295 K, a linearly polarized laser pulse produces polarized argon atoms on the 2p 2 (in Paschen notation) level from the 1s 3 level. We measure the direct fluorescence from the 2p 2 level to the 1s 2 level with its polarization components resolved. From the temporal evolution of the intensities for these components, we determine the disalignment rate. We also determine the disalignment rate coefficients due to helium, neon and argon atom collisions by separating the effect of radiation reabsorption, and then determine the effective cross section of disalignment from the rate coefficient divided by the mean relative velocity of colliding atoms. It is found that perturber rare gas atoms with larger atomic number have larger depolarization cross section. This effect can be attributed to larger values of the dipole polarizability for perturbers with larger atomic number.

Depolarization of rotational angular momentum in CN(A2Π, v = 4) + Ar collisions

Angular momentum depolarization and population transfer in CN(A(2)Π, v = 4, j, F(1)e) + Ar collisions have been investigated both experimentally and theoretically. Ground-state CN(X(2)Σ(+)) molecules were generated by pulsed 266-nm laser photolysis of ICN in a thermal (nominally 298 K) bath of the Ar collision partner at a range of pressures. The translationally thermalized CN(X) radicals were optically pumped to selected unique CN(A(2)Π, v = 4, j = 2.5, 3.5, 6.5, 11.5, 13.5, and 18.5, F(1)e) levels on the A-X (4,0) band by a pulsed tunable dye laser. The prepared level was monitored in a collinear geometry by cw frequency-modulated (FM) spectroscopy in stimulated emission on the CN(A-X) (4,2) band. The FM lineshapes for co- and counter-rotating circular pump and probe polarizations were analyzed to extract the time dependence of the population and (to a good approximation) orientation (tensor rank K = 1 polarization). The corresponding parallel and perpendicular linear polarizations yielded population and alignment (K = 2). The combined population and polarization measurements at each Ar pressure were fitted to a 3-level kinetic model, the minimum complexity necessary to reproduce the qualitative features of the data. Rate constants were extracted for the total loss of population and of elastic depolarization of ranks K = 1 and 2. Elastic depolarization is concluded to be a relatively minor process in this system. Complementary full quantum scattering (QS) calculations were carried out on the best previous and a new set of ab initio potential energy surfaces for CN(A)-Ar. Collision-energy-dependent elastic tensor and depolarization cross sections for ranks K = 1 and 2 were computed for CN(A(2)Π, v = 4, j = 1.5-10.5, F(1)e) rotational/fine-structure levels. In addition, integral cross sections for rotationally inelastic transitions out of these levels were computed and summed to yield total population transfer cross sections. These quantities were integrated over a thermal collision-energy distribution to yield the corresponding rate constants. A complete master-equation simulation using the QS results for the selected initial level j = 6.5 gave close, but not perfect, agreement with the near-exponential experimental population decays, and successfully reproduced the observed multimodal character of the polarization decays. On average, the QS population removal rate constants were consistently 10%-15% higher than those derived from the 3-level fit to the experimental data. The QS and experimental depolarization rate constants agree within the experimental uncertainties at low j, but the QS predictions decline more rapidly with j than the observations. In addition to providing a sensitive test of the achievable level of agreement between state-of-the art experiment and theory, these results highlight the importance of multiple collisions in contributing to phenomenological depolarization using any method sensitive to both polarized and unpolarized molecules in the observed level.

Theoretical investigation of rotationally inelastic collisions of CH2(ã) with helium

Rotationally inelastic collisions of the CH(2) molecule in its ã(1)A(1) electronic state have been investigated. We have determined a potential energy surface (PES) for the interaction of rigid CH(2)(ã), frozen at its equilibrium geometry, with a helium atom, using a coupled-cluster method that includes all single and double excitations, as well as perturbative contributions of connected triple excitations [RSSCD(T)]. The PES is quite anisotropic, due to lack of electron density in the unoccupied CH(2) non-bonding orbital perpendicular to the molecular plane. Quantum scattering calculations have been carried out to compute state-to-state rotational energy transfer and elastic depolarization cross sections at collision energies up to 2400 cm(-1). These cross sections were thermally averaged to derive room-temperature rate constants. The total removal and elastic depolarization rate constants for the ortho k(a) = 1 levels agree well with recent experimental measurements by Hall, Sears, and their co-workers. We observe a strong even-odd alternation in the magnitude of the total rate constants which we attribute to the asymmetry splitting of the k(a) = 1 levels.

Depolarization in H2O–He collisions

Elastic tensor and elastic depolarization cross-sections for collisions of specified rotational levels of H2O with helium are determined using an accurate ab initio potential energy surface computed by Patkowski et al. The elastic depolarization cross-sections display large maxima at low collision energies due to shape resonances and generally decrease smoothly with increasing collision energy, with superimposed sharp peaks due to resonances with quasibound van der Waals levels associated with higher rotational asymptotes. The cross-sections show a striking dependence upon the k-projection quantum numbers of the rotational levels, as well as a general decrease in magnitude with increasing values of the total rotational angular momentum j.

Tensor cross sections and collisional depolarization of MgH by He atoms

Tensor cross sections for collisions of MgH ( X 2 Σ + ) molecule with helium atom are computed using a new accurate ab initio potential energy surface calculated at the RCCSD (T) level. In the calculations, the MgH bond length is fixed at its experimental value (3.269 bohr). Basis set superposition error is corrected and bond functions are placed at mid-distance between the MgH mass centre and the He atom, for a better description of the van der Waals interaction. The global minimum of the PES is 12.2 cm - 1 . State-to-state tensor and depolarization cross sections are calculated. The tensor cross sections and depolarizing cross sections are computed for kinetic energies up to E c ⩽ 1000 cm - 1 . The tensor cross sections are found to decrease with increasing values of K.

Spin depolarization of N2+ (2Σ+) in collisions with 3He in a magnetic field: general behaviour and zero energy Feshbach resonances

We give a brief account of the effect of a magnetic field on the spin-depolarization cross sections of N2+(2Σ) in collisions with 3He at very low collision energy. The fundamental states of the two nuclear spin isomers of N2+ are shown to respond quite differently to the application of the field. When the applied magnetic field increases, the spin depolarization cross sections are found to monotonously decrease for the fundamental para level N=0 of N2+, whereas they monotonously increase for the fundamental ortho level N=1 of N2+. This effect is found to be amplified around the resonances and an explanation based on the different mechanism of spin flipping acting for these two levels is proposed. We also explore the tuning of zero energy Feshbach resonances by examining the variation of the scattering length as a function of the magnetic field. We use the Smith Q matrix to calculate the lifetimes of the complexes associated with these resonances and discuss the law of variation as a function of the magnetic field of the Q matrix eigenvalues across such resonances.

Elastic Depolarization of OH(A) by He and Ar: A Comparative Study

Two color polarization spectroscopy has been employed to measure the collisional depolarization of OH(A(2)Sigma(+), v = 1) by He and Ar. Complementary experiments using Zeeman quantum beat spectroscopy have also been performed to determine separately the cross sections for rotational energy transfer (RET) out of selected rotational levels of OH(A, v = 0) + Ar, as well as those for elastic depolarization. This has been achieved by dispersing the emission, so as to observe a single fluorescence transition. Elastic depolarization of OH(A) by Ar is found to be significant with that for loss of rotational alignment exceeding that for loss of orientation. In the case of OH(A) + He, the polarization spectroscopy measurements suggest that elastic depolarization plays a relatively minor role in the loss of the polarization signal compared with RET. The experimental data for OH(A) + Ar are compared in detail with the results of quasi-classical trajectory calculations that accommodate the effects of electron spin. These classical calculations are assessed against the results obtained using full close-coupled open shell quantum mechanical scattering methods. Overall the level of agreement between the two experiments, and between experiment and theory, is very reasonable. Surprisingly, at low N the elastic depolarization cross sections for OH(A) + Ar are found to be quite similar in magnitude to those observed for OH(X) + Ar despite the fact that the well depth in the latter system is considerably smaller than that for OH(A)-Ar. However, for OH(A) + Ar the depolarization cross sections are insensitive to N in the range 1-14. It is proposed that this behavior partly reflects the relatively anisotropic nature of the potential energy surface, which exhibits deep wells of different depths at the two linear configurations OH(A)-Ar and Ar-OH(A), and partly the nature of elastic depolarizing collisions, which must occur with a velocity component perpendicular to the plane of rotation of the diatomic molecule.

Collisional depolarization of NO(A) by He and Ar studied by quantum beat spectroscopy

Zeeman and hyperfine quantum beat spectroscopies have been used to measure the total elastic plus inelastic angular momentum depolarization rate constants at 300 K for NO(A Σ2+) in the presence of He and Ar. In the case of Zeeman quantum beats it is shown how the applied magnetic field can be used to allow measurement of depolarization rates for both angular momentum orientation and alignment. For the systems studied here, collisional loss of alignment is more efficient than loss of orientation. In the case of NO(A) with He, and to a lesser extent NO(A) with Ar, collisional depolarization is found to be a relatively minor process compared to rotational energy transfer, reflecting the very weak long-range forces in these systems. Detailed comparisons are made with quantum mechanical and quasiclassical trajectory calculations performed on recently developed potential energy surfaces. For both systems, the agreement between the calculated depolarization cross sections and the present measurements is found to be very good, suggesting that it is reasonable to consider the NO(A) bond as frozen during these angular momentum transferring collisions. A combination of kinematic effects and differences in the potential energy surfaces are shown to be responsible for the differences observed in depolarization cross section with He and Ar as a collider.

Dependence of elastic depolarization cross sections on the potential: OH(X Π2)-Ar and NO(X Π2)-Ar

Elastic tensor and depolarization cross sections are computed for the collision of two exemplary diatomic molecules with (2)Pi electronic ground states-OH and NO-with argon. The interaction of a diatomic molecule in a Pi state with a spherical collision partner must be described by two potential energy surfaces (PESs), corresponding to the two asymptotically degenerate electronic states, of A(') and A(") symmetry. Quantum scattering calculations are most naturally based on the average (V(sum)) and half-difference (V(dif)) of these two PESs. When V(dif) is neglected, the OH(X (2)Pi)-Ar depolarization cross sections are found to be significantly reduced in magnitude, while the NO(X (2)Pi)-Ar cross sections are relatively unaffected. In addition, treating the molecules as closed-shell (1)Sigma(+) species with a corresponding rotational level structure and using (V(sum)) to model the PES, we predict depolarization cross sections which differ significantly from those based on full inclusion of the electronic degeneracy and fine structure of these (2)Pi molecules. This indicates that any single-PES-based simulation of the collisional depolarization of these two molecules would be subject to significant error.

Tensor cross sections and the collisional evolution of state multipoles: OH(XΠ2)–Ar

By means of a kinetic analysis, we show that the overall rate constant for the collisional loss of orientation or alignment of a rotational level is the sum of the rate constant for elastic depolarization and the sum of the rate constants for all rotationally inelastic transitions out of the level under consideration. An expression for the depolarization cross section is derived in terms of tensor cross sections, and the relationship of depolarization to m-resolved transitions is discussed. We use this formalism in simulations, based on high-quality ab initio potential energy surfaces, of the depolarization of the open-shell molecule OH(X (2)Pi) through collisions with Ar. Good agreement is seen with the results of the two-color polarization spectroscopy experiments of Paterson et al. [J. Chem. Phys. 129, 074304 (2008)]. In addition, we show that the major contribution to elastic collisional depolarization occurs not from weak, glancing collisions but from encounters which probe the inner wall of the potential energy surface.

Collisional depolarization of OH(A) with Ar: Experiment and theory

Zeeman quantum beat spectroscopy has been used to measure the 300 K rate constants for the angular momentum depolarization of OH(A (2)Sigma(+)) in the presence of Ar. We show that the beat amplitude at short times, in the absence of collisions, is well described by previously developed line strength theory for (1+1) laser induced fluorescence. The subsequent pressure dependent decay of the beat amplitude is used to extract depolarization rate constants and estimates of collisional depolarization cross sections. Depolarization accompanies both inelastic collisions, giving rise to rotational energy transfer, and elastic collisions, which change m(j) but conserve j. Previous experimental studies, as well as classical theory, suggest that elastic scattering contributes around 20% to the observed total depolarization rate at low j. Simulation of the experimental beat amplitudes, using theoretical calculations presented in the preceding paper, reveals that depolarization of OH(A) by Ar has a rate constant comparable to, if not larger than, that for energy transfer. This is consistent with a significant tilting or realignment of j(') away from j on collision. The experimental data are used to provide a detailed test of quantum mechanical and quasiclassical trajectory scattering calculations performed on a recently developed ab initio potential energy surface of Kłos et al. [J. Chem. Phys. 129, 054301 (2008)]. The calculations and simulations account well for the observed cross sections at high N, but underestimate the experimental results by between 10% and 20% at low N, possibly due to remaining inaccuracies in the potential energy surface or perhaps to limitations in the dynamical approximations made, particularly the freezing of the OH(A) bond.

Polarization spectroscopy to determine alignment depolarization of the [formula omitted] atoms using a pump-probe laser technique

We have experimentally investigated alignment depolarization cross section of the excited J = 3 / 2 133Cs atoms, by collisions with ground level krypton atoms, from a two-photon two-color linear polarization spectra. To measure the linear polarization degree of the 6 s 2 S 1 / 2 → 6 p 2 P 3 / 2 → 10 s 2 S 1 / 2 double-resonance transition, a nanosecond pulse pump-probe laser technique was employed. Alignment was optically induced by linear polarization of the pump laser. By analyzing the intensity of the cascade fluorescence signal, monitored from the 9 p 2 P 1 / 2 → 6 s 2 S 1 / 2 transition at 361.73 nm, in the presence of Kr pressures ranging from 7 mbar to 133 mbar a depolarization cross section value of 294 ( 45 ) Å 2 is obtained.

Anisotropy-dependent circular polarization spectroscopy

Experimental and theoretical study of atomic collisions with neutral atoms, molecules, and ions is a key to understand the energy and charge transfer processes, depolarization cross sections, dynamical evolution of the state multipoles, and the accurate description of the intermediate levels of the collision complex 1‐5. Although the study of collision dynamics is not new, significant advances in techniques for laser cooling and trapping of atoms, molecules and ions has enormously increased the interest in this area of research 6,7. For example, collisions involving cold Rydberg atoms 8‐10 has been a great interest in many fields of research due to the interesting properties of these atoms in astrophysical and laboratory plasmas. Also, high-resolution photoassociation spectroscopy of ultracold molecules 11‐16 has become an important subject to accurately determine the potential curves of these molecules in atomic and molecular physics. The measurement of collisional depolarization cross section between an alkali metal and rare gas atom provides valuable information about the interaction between the two species such as relaxation rates of the electronic moments. Experimental investigations of collisional depolarization in 6p 2 P 3/2 level cesium atoms, up to now, were carried out using incoherent light sources as optical excitation 17,18. Guiry et al. 17 has experimentally studied the disorienta

A Theoretical Study of Spin Filtering and its Application to Polarizing Antiprotons

THESIS 8639%%%%There has been much recent research into possible methods of polarizing an antiproton beam, the most promising being spin filtering, the theoretical understanding of which is currently incomplete. The method of polarization buildup by spin filtering requires many of the beam particles to remain within the beam after repeated interaction with an internal target in a storage ring. Hence small scattering angles, where we show that electromagnetic effects dominate hadronic effects, are important. All spin-averaged and spin-dependent electromagnetic cross-sections and spin observables for elastic spin 1/2 - spin 1/2 scattering, for both point-like particles and non-point-like particles with internal structure defined by electromagnetic form factors, are derived to first order in QED. Particular attention is paid to spin transfer and depolarization cross-sections in antiproton-proton, antiproton-electron and positron-electron scattering, in the low |t| region of momentum transfer. A thorough mathematical treatment of spin filtering is then presented, identifying the key physical processes involved and highlighting the dynamical properties of the physical system. We present and solve sets of differential equations which describe the buildup of polarization by spin filtering in many different scenarios of interest. The advantages of using a lepton target are outlined, and finally a proposal to polarize antiprotons by spin filtering off an opposing polarized electron beam is investigated.

Collisional depolarization of Zeeman coherences in theCs1336pP3∕22level: Double-resonance two-photon polarization spectroscopy

An experimental investigation of the Cs $6s\phantom{\rule{0.2em}{0ex}}^{2}S_{1∕2}\ensuremath{\rightarrow}6p\phantom{\rule{0.2em}{0ex}}^{2}P_{3∕2}\ensuremath{\rightarrow}10s\phantom{\rule{0.2em}{0ex}}^{2}S_{1∕2}$ double-resonance two-photon polarization spectrum has been made. In the investigation, a linear polarization degree was measured to extract the alignment depolarization cross section in the cesium $6p\phantom{\rule{0.2em}{0ex}}^{2}P_{3∕2}$ level due to collisions between ground-level argon and excited $6p\phantom{\rule{0.2em}{0ex}}^{2}P_{3∕2}$ cesium atoms over the Zeeman coherences. The spectra show strong depolarization of the Cs $6p\phantom{\rule{0.2em}{0ex}}^{2}P_{3∕2}$ level. The measured alignment depolarization cross section value of 186 $(58)\phantom{\rule{0.3em}{0ex}}{\mathrm{\AA{}}}^{2}$ is in good agreement with the experiment and theoretical predictions.

Collisional depolarization of OH(A) studied by Zeeman quantum beat spectroscopy

The depolarization of the rotational angular momentum of electronically excited OH(2Σ) radicals through collisions with water molecules has been measured using Zeeman quantum beat spectroscopy. The new data have permitted the evaluation of OH(A) state-specific quenching and angular momentum depolarization cross-sections for superthermal OH(A) radicals with mean relative velocities centred around 3500 m s-1. The quenching cross-sections are compared both with values available in the literature, and with predictions based on a simple harpoon model, and are found to be in good qualitative accord with previous findings. For the lowest rotational levels studied, the depolarization cross-sections (which include contributions from both elastic and inelastic processes in OH(A)) are found to approach ∼100 Å2, only slightly below the high-temperature cross-sections for rotational energy transfer determined elsewhere. The data suggest that under the present conditions rotational energy transfer is accompanied by significant depolarization. The cross-section for the translational moderation of superthermal OH(X) by water is also determined in the present study.