I. Estermann

Beugung von Molekularstrahlen

ZusammenfassungTrifft ein Molekularstrahl (H2; He) auf eine Kristallspaltfläche (Li F) auf, so zeigen die von ihr gestreuten Strahlen in allen Einzelheiten eine Intensitätsverteilung, wie sie den von einem Kreuzgitter entworfenen Spektren entspricht. Die aus der Gitterkonstante des Kristalls berechnete Wellenlänge hat für verschiedene m und υ den von de Broglie geforderten Wert λ=h/m · v.

Über die magnetische Ablenkung von Wasserstoffmolekülen und das magnetische Moment des Protons. II

ZusammenfassungStrahlen aus Wasserstoffmolekülen wurden nach der Methode von Gerlach und Stern magnetisch abgelenkt und so ihr magnetisches Moment bestimmt. Die Messungen an Parawasserstoff ergaben das von der Rotation des Moleküls herrührende magnetische Moment zu etwa 1 Kernmagneton (1/1840 Bohrmaneton) pro Rotationsquant. Die Messungen an Orthowasserstoff ergaben das magnetische Moment des Protons zu 2 bis 3 Kernmagnetonen (nicht 1 Kernmagneton, wie bisher vermutet wurde).

Monochromasierung der de Broglie-Wellen von Molekularstrahlen

ZusammenfassungDie de Broglie-Wellen wurden auf zwei Wegen monochromasiert: 1. Ein gewöhnlicher Molekularstrahl (mit Maxwellverteilung der Geschwindigkeiten) von Heliumatomen wurde an einer LiF-Spaltfläche gebeugt; aus dem Beugungs-spektrum wurden Strahlen bestimmter Richtung, also Wellenlänge, ausgeblendet und die erfolgte Monochromasierung durch Beugung an einem zweiten Kristall nachgewiesen. 2. Ein gewöhnlicher Molekularstrahl wurde durch ein Zahnrad-system geschickt, das nur Atome eines bestimmten Geschwindigkeitsbereichs passieren ließ, und an einer LiF-Spaltfläche gebeugt. Die so gemessene Wellenlänge stimmte mit der aus der—grobmechanisch bestimmten — Geschwindigkeit berechneten (λ=h/mv) auf 1% überein.

Recent Research in Molecular Beams

Recent Research in Molecular Beam is a collection of scientific papers that have been inspired by Otto Stern, the founder of Molecular Beam Research. This book is composed of 10 chapters and begins with discussions on the early history of molecular beam research. The next chapters describe the velocity distribution measurements made on potassium molecular beams with a fixed-frequency, variable phase velocity selector, along with a brief consideration of the principles and concepts of electron magnetic moment and atomic magnetism. A chapter presents the atomic beam spectroscopic experiments on the metastable state of the hydrogen-like atoms that depend on a wholly different principle for the detection of transitions. This text further explores the effects of variations in the oscillatory field amplitudes, perturbations by neighboring resonances, perturbations by oscillatory fields, variations in the fixed field amplitudes, and phase shifts of the oscillatory fields. These topics are followed by a comparison of advantages and limitations of various techniques for spin property measurement as they apply in particular to radioactive nuclei, such as optical and molecular gas microwave spectroscopy, nuclear and paramagnetic resonance, and atomic beams. The remaining chapters examine fluid friction in a rarefied gas flow; some applications of molecular beam techniques to chemistry; and the polarized neutrons based on a Stern-Gerlach experiment. This work will be of great value to workers and researchers in molecular beam field.

Heat Conduction in Alloys at Low Temperatures

With a view to studying the mechanism of heat conduction in low conductivity alloys, a method has been devised by which the thermal conductivity of relatively small samples (⅛‐ to ¼‐inch diameter, 1 to 2 inches long) of various materials can be measured in the temperature regions obtainable with liquid nitrogen, liquid hydrogen, and liquid helium.Preliminary measurements on several commercial alloys (monel, inconel, and stainless steel) gave Wiedemann‐Franz ratios several times greater than the theoretical value of 2.45×10−8 watt‐ohm/deg2, the deviation being greater for annealed than for cold‐worked specimens. This has been interpreted in terms of appreciable lattice conduction of heat in these alloys.Following these results, samples of an alloy of 90 percent copper 10 percent nickel were prepared with varying amounts of cold‐work and with different grain sizes. Results with these samples were similar to those obtained with monel and inconel and confirm the hypothesis of lattice conduction. They also giv...

Specific Heat of Germanium between 20°K and 200°K

High precision measurements of the specific heat of germanium were carried out in the temperature region between 20°K and 200°K. Three samples of germanium were investigated: an extremely pure sample, one of intermediate purity (1018 impurity centers/cm3), and one with an addition of 0.006 atomic percent aluminum (2×1020 impurity centers/cm3). No anomaly such as reported by Cristescu and Simon was found in any of the samples. However, a small dependence of the specific heat on the concentration of impurity centers was observed.

The Deflection of Molecular Rays in an Electric Field: The Electric Moment of Hydrogen Chloride

A comparison of the method for determining electric moments of molecules by deflection of molecular beams by inhomogeneous electric fields with the dielectric constant method shows that the molecular beam method has certain advantages. This method determines the electric moment of gases directly and is able to detect the effect of higher rotational states of non‐gyroscopic molecules. Moreover, a deviation of the axis of the dipole moment from that of the axis of rotation by other than 90° should be detected. The additional energy due to an electric field is proportional to the square of the field intensity and is a function of the quantum numbers m and j, if the electric moment is perpendicular to the axis of rotation. Otherwise there will be a linear dependence on the intensity of the electric field. An experimental method of the Rabi type for studying the HCl molecule is described. The patterns produced by the undeflected and deflected molecules are given and from these patterns the electric moment of t...

History of molecular beam research: personal reminiscences of the important evolutionary period 1919--1933

Dr. Immanuel Estermann was engaged in writing a book on the History of the Molecular Beam Method when he died in Haifa, Israel on 31 March 1973. The original plan of the manuscript was to cover three distinct historical periods: the first, the years 1911–1933, when the center of research in molecular beams was at the University of Hamburg; the second, from 1933 to the outbreak of World War II, when the most active laboratory working in the field was at Columbia University; and the postwar period, when many laboratories on both sides of the Atlantic became actively engaged in this field. The material in this paper deals primarily with the early historical period and would have formed the essence of the first two chapters of the book. In addition to presenting interesting historical facts on a crucial period in the development of quantum physics, it contains some amusing historical sidelights on the research personalities that dominated that period.

A Torsion Balance for Measuring Forces in Low Density Gas Flows

A torsion balance, designed to measure the force on a flat plate (or other simple geometry) in a low density gas flow, is described. The balance has been used with two interchangeable torsion member sizes, with diameters of 0.00064″ and 0.00090″, which measured forces up to a maximum of 0.6 milligram and 2.4 milligrams, respectively. Calibrations (obtained in still air using weights) were reproducible within ±0.005 mg and ±0.04 mg for the two sizes. Data obtained for a flat plate in a gas stream under large molecular mean free path conditions are presented.

Applications of Molecular Beams to Problems in Rarefied Gas Dynamics

In classical aerodynamics the air is considered as a continuum and its flow characteristics are described by the equations of fluid mechanics. From the standpoint of kinetic theory, the validity of these equations is based on the physical assumption that the behavior of the fluid is determined almost exclusively by the interactions between the individual molecules and only to a very minor, generally negligible, ex tent by the interaction between molecules and solid boundaries. Another expression of this physical situation is contained in the statement that under the conditions encountered in classical fluid mechanics, the mean free paths between individual molecules is very small compared with the dimensions of the bodies in contact with the fluid. If, however, the density of the gas is gradually being decreased to reach such values as, for instance, exist in tne upper atmosphere, the mean free path which is inversely proportional to the density will gradually increase until the condition mentioned above is no longer valid. We are then reaching the regime known as rarefied gas dynamics, which is characterized by a mean free path λ between the molecules of the gas of the same order or even larger than the critical dimension d of the system. The ratio between this mean free path λ and the characteristic dimension K=λ/d, commonly called the Knudsen number, divides gas dynamics into the various flow regimes, values of K 10 free molecular flow, and intermediate values the re gions commonly called slip and transition flow. In this series of lectures we will be concerned only with the regions of very large Ks, that is with free molecular flow, also called Knudsen flow. In this flow regime individual molecules will make many collisions with the solid walls bordering on the system before they will have a chance to collide with one another. As a result, the flow characteristics are determined by the interactions between solid surfaces and gas molecules while the continuum quantities such as viscosity and heat conductivity lose their importance.