Two-photon coherent control of atomic collisions by light with entangled polarization

We describe a new method of coherent optical control of internal dynamics of atomic collisions by means of two correlated light beams having entangled polarizations. We show that, if excitation of a colliding pair of atoms is by two photons having entangled polarizations, it is possible to redirect the output fragments of the collision into certain channels with a selected type of internal transition symmetry. The transition symmetry is defined in the body-fixed coordinate frame which has random and originally unknown orientation in space.     

The influence of cascade on the polarization of radiation produced in electronic or atomic collisions

A study is presented, in which we consider the influence of cascade effects on the polarization of light emission produced in electronic or atomic collisions. Cascade transitions are shown to cause a polarization transfer which may result in a total falsification and even in a sign reversal of the polarization. Closed expressions are derived for some special cases. A practical application is given in which the derived formulas are used to understand a serious discrepancy between theoretical predictions and experimental data in the case of the line polarization due to some 3p-3s transitions in neon induced by proton impact.     

Diffusion processes and atomic collisions of Na-rare gas systems studied by laser induced population grating method

Diffusion processes and atomic collisions of Na-He and Na-Ar systems have been studied by measurement of the time development of population gratings (Laser-Induced Population Grating; LIPG) induced in atomic levels by two intersecting resonant laser pulses and monitored through the Bragg diffraction of a probe laser beam. The diffusion process in the systems was observed through the decay of the LIPGs in the sodium ground-state $\text{(3}{}^2\text{S}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{)}$ sublevels. The pressure dependence of the diffusion constants were measured. The values showed rough agreement with the previously reported values which had been measured indirectly by the optical-pumping method. The gaussian process model and a simple collision kernel model were considered for the diffusion process. It was also found that with increasing buffer-gas pressure, the population gratings in the excited state $\text{3}{}^2\text{P}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ were transferred to $\text{3}{}^2\text{P}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, which was interpreted as an effect of fine-structure state-changing collisions.     

Low energy atomic collisions

The Schrödinger equation for the system H⁺-H developed in a previous paper is considered using new expansion functions for electronic states obtained from H₂ ⁺ molecular ion electronic eigenfunctions by a unitary transformation. These new functions have the advantage of remaining orthonormal at all internuclear separations and asymptotically becoming symmetrized atomic hydrogen states. Although they are eigenfunctions of the H₂ ⁺ hamiltonian only in the limit of large internuclear distance, the effect of the H₂ ⁺ hamiltonian on these functions is readily found.

Due to coupling which remains non-zero in the limit of large interproton distance, each independent formal solution of the H⁺-H equations involves more than one expansion state in this limit. These solutions may be expressed asymptotically as column vectors multiplied by incoming or outgoing spherical waves.

The formal theory of scattering as developed by Gell-Mann and Goldberger has been utilized along with the projection formalism of Feshbach to obtain the correct asymptotic form of the scattering wave function. The procedure employed involves formulating the problem in terms of two-potential scattering and requires application of renormalization techniques for treating level shifts produced by the infinite-ranged coupling. This asymptotic form may be used in imposing scattering boundary conditions on numerical solutions of coupled equations for H⁺-H scattering.

Finally, it is shown that one cannot interpret coefficients of all outgoing spherical waves as scattering amplitudes. In addition, new interference phenomena are found to result from the presence of the infinite-ranged coupling. The present formalism is shown to reduce to the usual perturbed stationary-states method in the approximation that the infinite-ranged coupling is neglected.     

Manipulation of cold atomic collisions by cavity QED effects

We show how the dynamics of collisions between cold atoms can be manipulated by a modification of spontaneous emission times. This is achieved by placing the atomic sample in a resonant optical cavity. Spontaneous emission is enhanced by a combination of multiparticle entanglement together with a higher density of modes of the modified vacuum field, in a situation akin to superradiance. A specific situation is considered and we show that this effect can be experimentally observed as a large suppression in trap-loss rates.     

Development of two-color laser system for high-resolution polarization spectroscopy measurements of atomic hydrogen

We have developed a high-spectral-resolution laser system for two-photon pump, polarization spectroscopy probe (TPP-PSP) measurements of atomic hydrogen in flames. In the TPP-PSP technique, a 243-nm laser beam excites the two-photon 1S-2S transition, and excited n=2 atoms are then detected by polarization spectroscopy of the n=2 to n=3 transition using 656-nm laser radiation. The single-frequency-mode 243 and 656-nm beams are produced using injection-seeded optical parametric generators coupled with pulsed dye amplifiers. The use of single-mode lasers allows accurate measurement of signal line shapes and intensities even with significant pulse-to-pulse fluctuations in pulse energies. Use of single-mode lasers and introduction of a scheme to select nearly constant laser energies enable repeatable extraction of important spectral features in atomic hydrogen transitions.     

Control of near-threshold detachment cross sections via laser polarization

The behavior of near-threshold cross sections for dissociation of a target into a pair of particles, as described by Wigner's threshold law, can depend sensitively on the angular momentum of the particles. In this Letter, we investigate the near-threshold nonresonant two-photon detachment process in the negative ion of gold. The expected s-wave threshold behavior is observed with linearly polarized light. Closure of the s-wave channel is realized by using circular polarization, allowing the first observation of a d-wave threshold. Practical applications are discussed, including extensions which could prove valuable for investigations of negative ions with near-threshold structure.     

Collective effects in atomic collisions

As a first step towards a collective treatment of charge flow in atomic collisions, we constructab initio potential energy surfaces as a function of the internuclear distance and the charge asymmetry between the two atomic species. To this end, the charge asymmetry off its stability value for a given nuclear separation is imposed upon the system by a suitable constraint within a two-centre Hartree-Fock calculation. Energy surfaces are presented for the systems LiH, HF, LiF, and CO. This representation offers a conceptual framework for visualising ionic or covalent molecular states and trajectories describing charge-changing collisions.     

Nonadiabatic transitions upon atomic collisions

The standard interpretation of the adiabatic approach and the t matrix method for the calculation of the probabilities of nonadiabatic transitions and inelastic cross sections in atomic collisions is analyzed, and a model problem of collisions of hydrogen atoms with neutrons is considered. It is shown that the application of the standard approach results in a physically inconsistent result—considerable values of the excitation cross sections of hydrogen atoms by neutrons. The application of the t matrix method improves the result by two to three orders of magnitude even for a restricted basis set.     

Physics of atomic collisions in retrospect

The classic work by Mott and Massey, in which the scope of the physics of atomic collisions was defined, was published about 65 years ago. Since then, this field has undergone considerable development. In fact, all the theoretical methods named by Mott and Massey have been implemented to some extent. As for experiment, the measurements performed, which are differential with respect to several parameters, have provided for reliable testing of the mechanisms proposed. The physics of atomic collisions has been developed to the point that we can look back on the road taken and discover many achievements which have promoted its development. Progress in science is usually associated with highly concentrated efforts on the part of a critical number of investigators to solve a specific problem, which is widely regarded as being of great importance. Such a “breakthrough” is usually followed by rapid development of the field. In this respect, the physics of atomic collisions is no exception. It has known periods of highly concentrated efforts aimed at solving specific problems and breakthroughs followed by rapid development and subsequent periods of stagnation. The cycles have repeated: a new area for concentrated efforts is discovered, a breakthrough occurs, and a new concept is established. Some of these cycles are analyzed from the standpoint of their significance to atomic physics as a whole. Zh. Tekh. Fiz. 69, 22–26 (September 1999)     

Control of supercontinuum generation with polarization of incident laser pulses

Supercontinuum generation is dependent on the polarization state of the incident laser. The polarization of the generated supercontinuum is the same as that of the incident laser. The magnitude of the generated supercontinuum depends on the polarization of the incident laser and increases as the polarization changes from circular to linear, irrespective of the nature of the sample, be it isotropic, anisotropic, or chiral. In all samples, the polarization dependence indicates a preference for the linear component of the incident laser beam. The anisotropic sample shows an additional difference in the generated supercontinuum for the two perpendicular directions of the incident laser polarization. PACS 42.25.Ja; 42.65.Ky     

Feynman diagram approach to atomic collisions

A quantum field theoretic formulation of atomic collision phenomena involving non-relativistic free and bound systems is developed and a calculational procedure in terms of Feynman diagrams is prescribed. Matrix elements of several atomic collision processes have been calculated. In most cases standard quantum mechanical results are reproduced. But in some cases new terms appear in the scattering matrix whose contribution though negligibly small in the low energy region, become important at higher energies. A large portion of this work formed material for an invited talk delivered by one of us (T.P.) at the Second National Workshop on Atomic and Molecular Physics at Visva-Bharati, Santiniketan (India) held from 18–23 November 1979.     

Feynman diagram method for atomic collisions

Feynman diagram method of treating atomic collision problems in perturbation theory is presented and matrix elements are calculated for a number of processes. The result for the resonant charge transfer in hydrogen is identical to the well known OBK value. However, in processes like collisional ionisation, the results are different from those obtained by conventional methods.     

Distributed feedback laser action by polarization modulation

Laser action was generated in dye-doped sol–gel glass waveguides and in liquid solutions by crossing two beams from a frequency-doubled neodymium:YAG laser. The angle between the polarization directions of the two beams was varied continuously from 90. The case of 0° corresponded to pure intensity modulation. As the angle increased, the degree of intensity modulation decreased, resulting in the decline and finally the demise of the laser output. At 90°, corresponding to pure polarization modulation, distributed feedback laser action with narrow line-width output was again observed when the pump energy was increased three-fold. The laser output also showed different polarization characteristics when the feedback mechanism was changed from intensity modulation to polarization modulation. Received: 13 February 2003 / Revised version: 3 April 2003 / Published online: 22 May 2003 RID="*" ID="*"Corresponding author. Fax: 852/2603-5204, E-mail: dlo@sun1.phy.cuhk.edu.hk     

Spin-orbit quenching of positronium during atomic collisions

When positrons are injected into a gas, 75% of the positronium (Ps) is likely to be formed as long-lived ortho-Ps. The main decay mechanisms for the ortho-Ps have been assumed to be natural decay of ortho-Ps and pickoff annihilation of the positron during Ps-atom collisions. A third possibility for annihilation is ortho-Ps-->para-Ps conversion due to the spin-orbit interaction between the atom and colliding Ps. This extra quenching mechanism may explain a number of phenomena observed in the annihilation spectrum of Kr and Xe, including the very small Ps fraction of 3% seen for Xe.     

Quarkonium polarization in pp and p-nucleus collisions

The existing measurements of quarkonium polarization in proton-antiproton and proton-nucleus collisions are puzzling. We highlight issues which are often underestimated in the experimental analyses: the importance of the choice of the experimental acceptance on the comparison between experimental measurements and theoretical calculations. New measurements must provide more detailed information, such that physical conclusions can be derived without relying on model-dependent assumptions. We also describe a frame-invariant formalism which minimizes the dependence of the measurements on the experimental acceptance, facilitates the comparison with theoretical calculations, and probes systematic effects due to experimental biases.     

Λ0 polarization in ultrarelativistic nucleus-nucleus collisions

We study the polarization of Λ⁰’s produced in ultrarelativistic heavy-ion collisions. We argue that when the density of participants in the reaction is below the critical value for the production of a Quark-Gluon Plasma (QGP), recombination processes dominate the Λ⁰ production cross section and hence, Λ⁰’s are polarized. However, when the density of participants is above the critical value for QGP formation, coalescence processes dominate Λ⁰ production, giving rise to a reduction in the Λ⁰ polarization. For densities below the critical density for QGP production, we describe Λ⁰ polarization in terms of the DeGrand-Miettinen model and correct for the effects introduced by multiple scattering of the produced Λ⁰ within the nuclear environment.