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Physical Review A

Interaction of the Landau orbitals of atomic ions in a magnetic field with electronic motion

Peter Schmelcher, Lorenz S. Cederbaum

Published Jan 1, 1991• 41 citations
Publisher

Abstract

For neutral particle systems in a homogeneous magnetic field, there exists a constant of motion, the pseudomomentum, which allows a complete pseudoseparation of the center-of-mass motion. For ions such a separation is not possible, and the Landau orbit of the center-of-mass and internal (electronic) motion are intimately coupled. We investigate different physical situations for which this interaction becomes strong at laboratory magnetic-field strengths. In particular, we find that the Landau orbit itself can change substantially upon this interaction.

DOI
10.1103/physreva.43.287

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For neutral particle systems in a homogeneous magnetic field, there exists a constant of motion, the pseudomomentum, which allows a complete pseudoseparation of the center-of-mass motion. For ions such a separation is not possible, and the Landau orbit of the center-of-mass and internal (electronic) motion are intimately coupled. We investigate different physical situations for which this interaction becomes strong at laboratory magnetic-field strengths. In particular, we find that the Landau orbit itself can change substantially upon this interaction.
For neutral molecules in a homogeneous magnetic field there exists a constant of motion, the pseudomomentum, which allows a complete pseudoseparation of the center-of-mass motion. For molecular ions a complete set of constants of motion is not known, neither exactly nor approximately. In this paper we investigate the structure an approximate constant of motion for a molecular ion can have in general and arrive at an interesting formula. An explicit approximate constant of motion that obeys this structure formula is derived from a physically intuitive picture that says that an approximately conserved quantity can be obtained as the pseudomomentum relative to the instantaneously fixed-nuclei center of charge of the system. Some special examples are discussed in detail.
The approximate constant of motion introduced previously for the description of a charged system in a homogeneous magnetic field is interpreted physically as the kinetic momentum of the collective motion. The algebra satisfied by this operator and by the exact constants of motion describes the behavior in a magnetic field of a single particle possessing the total charge of the system. With this algebra, we generalize the approximate constant of motion and obtain a family of operators depending on arbitrary parameters. Canonical transformations based on these new operators separate the Hamiltonian into collective, internal, and coupling terms. These terms take the same form for charged and neutral systems although the collective energy represents different physical behaviors. The coupling between the internal and collective motions is small for some choices of the parameters if at least one particle is much heavier than the other ones.
In a magnetic field the center of mass (c.m.) motion of an atom or molecule couples to the electronic motion. It is demonstrated that this coupling dramatically influences the properties of negative ions. Neglecting c.m. effects the external field gives rise to a series of infinitely many bound states of the ion. Center of mass effects terminate this series and turn bound states into short-lived resonance states. Whether bound states exist at all, their number and properties as well as the lifetimes of the resonance states depend on the neutral system to which an electron is attached, and on the magnetic field.
The techniques of center of mass (c.m.) separation in a homogeneous magnetic field are tested on an exactly solvable problem: a bound system of arbitrary particles interacting with a general quadratic potential. An effective mass increasing with the field strength appears naturally in the collective energy and shows striking similarities for charged and neutral systems. In the charged case, the internal motion cannot be separated from the c.m. motion. These properties and several techniques discussed in the present paper are also valid for a Coulomb interaction. The present exact model provides a test of approximate separation methods employed in the Coulomb case.
We calculate the energy levels of He+ ion placed in a uniform magnetic field directed perpendicular to the direction of its center of mass (CM) velocity vector, correct to relative order . Our calculations are within the frame work of an approximately relativistic theory, correct to relative order , of a two-particle composite system bound by electromagnetic forces, and written in terms of the position, momentum and spin operators of the constituent particles as proposed by Krajcik and Foldy, and also by Close and Osborn. Since the He+ ion has a net electric charge, the total or the CM momentum is not conserved and a neat separation of the CM and the internal motion is not possible. What is new in our approach is that, for the basis states in a first order degenerate perturbation theory to calculate the effects of the external magnetic field, we use the direct product of the coherent state of the Landau Hamiltonian of the He+ ion in a uniform magnetic field and of the simultaneous eigenstate of the internal Hamiltonian h, j2, l2, s2 and jz, where j, l and s are the internal total, orbital and spin angular moments of the He+ ion. The coherent state is an excellent approximation to the expected classical circular motion of the center of mass (CM) of the He+ ion. In addition to the Z2 a2 corrections to the usual nonrelativistic results, including the small corrections due to the nuclear motion, we also obtain corrections which depend on the kinetic energy (ECM ) of the CM circular motion of the He+ ion, in a nontrivial way. Even though these corrections are proportional to , where M is the mass of the He+ ion, and are small for nonrelativistic CM motion, the results should be verifiable in careful experiments. Our results may also have application in astrophysical observations of the spectral lines of He+ ions in magnetized astrophysical objects.
In the presence of a magnetic field atomic ions are shown to generate monochromatic radiation with the frequency $\ensuremath{\omega}=QB/M$, depending on the field strength B and the ion mass M and charge Q. The origin of the effect is the interaction of the bound electrons with the oscillating field of the Stark term coupling the center of mass and electronic motions of an ion due to the nonseparability of the collective and internal degrees of freedom in the system. The intensity of the radiation is defined by the velocities of the ions and the magnetic field strength.