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Applied Optics

The Spectroscopic Properties of the Ionic Beam Source

William S. Bickel

Published Aug 1, 1967• 29 citations
Publisher

Abstract

A relatively new spectroscopic source, consisting of a beam of high velocity, unidirectional and monoenergetic atoms and ions that have been electronically excited by a thin foil, is described. Special attention is directed toward the properties of spectroscopic interest; some of these properties include particle densities, source purity, operating parameters, and the nature of the radiation emitted. The application of the source to the study of atomic excited state lifetimes is described with specific emphasis on the techniques of obtaining quantitative data using spectrometers, photomultipliers, and interference filters Also included is a complete bibliography of work done up to the present date.

DOI
10.1364/ao.6.001309

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A relatively new spectroscopic source, consisting of a beam of high velocity, unidirectional and monoenergetic atoms and ions that have been electronically excited by a thin foil, is described. Special attention is directed toward the properties of spectroscopic interest; some of these properties include particle densities, source purity, operating parameters, and the nature of the radiation emitted. The application of the source to the study of atomic excited state lifetimes is described with specific emphasis on the techniques of obtaining quantitative data using spectrometers, photomultipliers, and interference filters Also included is a complete bibliography of work done up to the present date.
Electrons emitted from single-atom tips are analysed in a model with the tip as a round tubelike constriction. The source creates two-dimensional minimum wavepackets whose temporal evolution agrees well with observed beam divergences. With few-atom tips interference effects may arise due to the electron's non localized emission from several atoms. The phase space density of emitted electrons is around 10 −2 h −3 , and electron-electron correlation experiments become feasible. In ion field emission mode observed beam divergences are dominated by thermal broadening and other effects rather than quantum mechanical principles. Applications include electron interferometers of lower energy and wider beam separation than hitherto in use, as well as proton and possibly other ion interferometers. Highly focused low energy beams could also be produced.
A density-matrix approach has been developed for the angular distribution and polarization of radiative emission during single-photon atomic transitions for a general set of steady-state excitation processes in an arbitrary arrangement of static (or quasistatic) electric and magnetic fields. Particular attention has been directed at spectroscopic observations in the intense fields of the high-power ion diodes on the Particle Beam Fusion Accelerator II (PBFA II) and SABRE devices at Sandia National Laboratories and at magnetic-field measurements in tokamak plasmas. The field-dependent atomic eigenstates are represented as expansions in a complete basis set of field-free bound and continuum eigenstates. Particular emphasis has been given to directed-electron collisional excitations, which may be produced by an anisotropic incident-electron velocity distribution. We have allowed for the possibility of the coherent excitation of the nearly degenerate field-dependent atomic substates, which can give rise to a complex spectral pattern of overlapping Stark-Zeeman components. Coherent excitations may be produced by a beam of electrons that are spin-polarized at an angle with respect to the propagation direction or by nonparallel electric and magnetic fields. Our main result is a general expression for the matrix elements of the photon-polarization density operator representing the total intensity, angular distribution, and polarization of the atomic radiative emission. For the observation of radiative emission in the direction of the magnetic field, the detection of linearly polarized emission, in addition to the usual circularly polarized radiation, can reveal the presence of a perpendicular electric field or a coherent excitation mechanism.
Particle properties of radiation One of the most important properties of macroscopic material systems is their ability to emit radiation spontaneously. According to quantum mechanics, the emission process is realized in the following way: an atom (or a molecule) makes a transition from a higher lying energy level (to which it was brought, for example, by an electron collision) to a lower lying energy level without any noticeable external influence (in the form of an existing electromagnetic field), and the released energy is emitted in the form of electromagnetic radiation. The discrete energy structure of the atom dictated by the laws of quantum mechanics is imprinted also on the emission process (quantization of the emission energy), since the energy conservation law is also valid for single (individual) transitions. Hence, a single photon, in the sense of a well defined energy quantum, is always emitted. The emitted quanta can be directly detected by a photodetector. (Strictly speaking, identifying a registered photon with an emitted one is possible only when it is guaranteed that the observed volume contains only a single atom. (For details see Sections 6.8 and 8.1.) Under realistic conditions, the experiment can be performed in the following way. First, a beam of ionized atoms is sent through a thin foil; the emerging beam then consists of excited atoms. (This procedure is known as the beam–foil technique.) A detector is placed at a distance d from the foil to detect light emitted sideways by the atomic beam (Fig. 6.1).