From Current Literature

A NEW METHOD FOR PRODUCING MOLECULAR BEAMS

A molecular beam is a very important experimental tool of modern physics. Thanks to it, diffraction of atoms and molecules was carried out, providing one of the experimental foundations of the wave theory of matter. With its aid the hyperfine structure of spectra is studied and the spins of nuclei are determined. It is also used in the resonance method for measuring the magnetic moments of nuclei and in numerous works in the field of the kinetic theory of matter.

To obtain a molecular beam, one usually uses the method of evaporation through a narrow aperture in an “oven” and the selection of a directed beam by means of a system of diaphragms. However, this simple method has a number of substantial shortcomings that limit the use of molecular beams:

1. The intensity of the beam is usually very small.
2. The production of monochromatic (in velocity) beams requires very complicated apparatus, which attenuates the beam still further.
3. Regulation of the velocity of the particles forming the beam is possible only by changing the temperature of the “oven.” Since gas continuously flows through the “oven,” it is very difficult to stabilize its temperature. Moreover, the use of high temperatures is limited by dissociation of the molecules of the substance under investigation.

Recently a new method has been proposed for producing molecular beams, eliminating these shortcomings[^1]. The essence of the new method consists in obtaining a molecular beam by neutralizing a beam of ions of the substance under investigation with slow electrons. A beam of ions, passing through a region of space filled with electrons moving in a perpendicular direction, captures them and is transformed into a neutral beam, whose particles continue to move practically with the same velocity that they had in the ion beam, and in the same direction. The intensity of the resulting molecular beam is determined by the intensity of the ion source and by the coefficient of neutralization of the ions. The latter depends on the residence time of the ions in the region where the negative space charge is produced (i.e., on the velocity of the ions), on the density of the negative space charge, and on the velocity of the electrons. Ions that were not neutralized while passing through the region of negative charge are easily removed from the beam by means of a deflecting electric or magnetic field.

The use of ions makes it possible to avoid the above-mentioned shortcomings of the molecular-beam method. Ion sources create far more powerful beams than effusion ovens, and with smaller apertures; in this way the beam intensity can be readily varied. The velocity of the ions is determined by the potential of the accelerating electric field and likewise can be smoothly

change. The monochromatization and focusing of the beam are considerably simplified: first, with the aid of any electric velocity filter (for example, a filter with mutually perpendicular electric and magnetic fields),² monochromatization and focusing of the ion beam are carried out, and then the monochromatic ion beam is neutralized and converted into a monochromatic molecular beam. Consequently, in the new method the entire process of obtaining a molecular beam is “electrified,” which makes it possible to use the rich technology and methodology developed in the field of mass spectrometry.

It should be noted that, with such a method of producing molecular beams, the velocities of the particles forming the beam will be considerably higher than the thermal velocities of ordinary molecular beams. Therefore, in order to obtain velocities lower than the velocity imparted by the ion source at the minimum accelerating potential difference, it is necessary to slow the ions by means of an appropriate electric field.

A calculation that has been made shows that an ion source producing an ion current of density \(2 \cdot 10^{-5}\ \mathrm{A/cm^2}\), under identical geometrical conditions, proves to be approximately 100 times more intense than an effusion “oven.” At present there exist ion sources that make it possible to obtain considerably higher current densities.

To investigate the formation of a molecular beam by neutralizing an ion beam, and to study the dependence of the neutralization coefficient on the density of the electron space charge, an apparatus was constructed consisting of an ion source, an electron “gun” emitting electrons in a direction perpendicular to the ion beam, an ion receiver, and a molecular-beam indicator. The experiments were carried out with nitrogen ions accelerated by a potential difference of 1000 eV. A beam of ions of density \(i^+ \sim 1.3 \cdot 10^{-7}\ \mathrm{A/cm^2}\) \((n^+ \sim 1 \cdot 10^4\ \text{ions}/V^3)\) was crossed by an electron beam, the electron energy being 50 eV, while the electron-current density could be varied within the limits from 0 to \(1.7 \cdot 10^{-3}\ \mathrm{A/cm^2}\) \((n^- \sim 2 \cdot 10^7\ \text{electrons}/\mathrm{cm^3})\). The dependence of the ion-current magnitude on the electron-beam density was measured (at constant electron velocity). The experiment showed that the intensity of the ion beam remained practically constant up to an electron-current density of \(0.1 \cdot 10^{-3}\ \mathrm{A/cm^2}\), after which it decreased sharply (by \(4 \tfrac{1}{2}\) times in the interval from \(0.1\) to \(0.36 \cdot 10^{-3}\ \mathrm{A/cm^2}\)); and at an electron-current density of \(1.5 \cdot 10^{-3}\ \mathrm{A/cm^2}\) the ion current at the receiver ceased, i.e. the ion beam was completely neutralized. If one passes from current densities to volume concentrations of electrons and ions, it turns out that in each \(\mathrm{cm^3}\) of space where neutralization occurs, for the complete conversion of the ion beam into a molecular beam it was necessary to have about 2000 slow electrons for each nitrogen ion. It is obvious that for other values of the velocities of the ions and electrons this ratio will change.

Thus, the preliminary experiments confirmed the possibility of obtaining a molecular beam by neutralizing an ion beam with slow electrons.

V. Leshkovtsev

References

1. D. L. Simonenko, ZhETF 20, 385 (1950).
2. E. V. Shpolsky, Atomic Physics, Vol. I, § 9, Gostekhizdat (1949).