In the year of 2019, we celebrate the centenary of the publication of the remarkable paper by Otto Stern and Max Volmer concerning the kinetics analysis of fluorescence quenching. Their achievement changed enormously many fields of science with great impact in the studies of electronic spectroscopy and kinetics of organic, inorganic and biological compounds in condensed phase. The development of molecular photochemistry was in great part based on the use of the Stern-Volmer (SV) approach to investigate bimolecular interaction in the electronic excited-state. This paper reviews and summarizes the assumptions behind the Stern-Volmer equation and the extensions of the theory to other important situations. Discussions about the use of the SV approach to investigate probe and quencher distribution, association, diffusion and reaction at the molecular level obtained from advanced fluorescence microscopy methods are also included.

The Centenary of the Stern-Volmer Equation of Fluorescence Quenching: From the single line plot to the SV quenching map

Ein Weg zur experimentellen Prüfung der Richtungsquantelung im Magnetfeld

The Stern-Gerlach-Experiment (SGE) performed in 1922 is a seminal benchmark experiment of quantum physics providing evidence for several fundamental properties of quantum systems. Based on the knowledge of today we illustrate the different benchmark results of the SGE for the development of modern quantum physics and chemistry. The SGE provided the first direct experimental evidence for angular momentum quantization in the quantum world and therefore also for the existence of directional quantization of all angular momenta in the process of measurement. Furthermore, it measured for the first time a ground state property of an atom, it produced for the first time a fully “spin-polarized” atomic beam, and it also revealed the electron spin, even though this was not realized at the time. The SGE was the first fully successful molecular beam experiment where the kinematics of particles can be determined with high momentum-resolution by beam measurements in vacuum. This technique provided a kind of new kinematic microscope with which inner atomic or nuclear properties could be investigated. Historical facts of the original SGE are described together with early attempts by Einstein, Ehrenfest, Heisenberg, and others to reveal the physical processes creating directional quantization in the SGE. Heisenberg’s and Einstein’s proposals of an improved multi-stage SGE are presented. The first realization of these proposed experiments by Stern, Phipps, Frisch and Segre is described. The experimental set-up suggested by Einstein can be considered as an anticipation of a Rabi-apparatus with varying fields. Recent theoretical work by Wennerstrom and Westlund, by Devereux and others, is mentioned in which the directional quantization process and possible interference effects of the two different spin states are investigated. In full agreement with the results of the new quantum theory directional quantization appears as a general and universal feature of quantum measurements. One experimental example for such directional quantization in scattering processes is shown. Last not least, the early history of the “almost” discovery of the electron spin in the SGE is revisited.

The Stern-Gerlach experiment revisited

I will argue, pace a great many of my contemporaries, that there's something right about Boltzmann's attempt to ground the second law of thermodynamics in a suitably amended deterministic time-reversal invariant classical dynamics, and that in order to appreciate what's right about (what was at least at one time) Boltzmann's explanatory project, one has to fully apprehend the nature of microphysical causal structure, time-reversal invariance, and the relationship between Boltzmann entropy and the work of Rudolf Clausius.

In Praise of Clausius Entropy: Reassessing the Foundations of Boltzmannian Statistical Mechanics

A joint paper by Albert Einstein and
Paul Ehrenfest, published just weeks
after the results of the Stern-Gerlach
experiment became known, shows
with remarkable clarity and prescience
the unsurmountable difficulties
that the experiment posed for
any classical interpretation. With a
focus on the measurement process,
rather than on the underlying theoretical
alternatives for this experimentum
crucis, the authors almost
anticipate what would later be recognized
as a central conceptual difficulty
of quantum mechanics, i.e., the
quantum measurement problem.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/andp.201300708

Einstein, Ehrenfest, and the quantum measurement problem

We review the work and life of Otto Stern who developed the molecular beam technique and with its aid laid the foundations of experimental atomic physics. Among the key results of his research are: the experimental test of the Maxwell-Boltzmann distribution of molecular velocities (1920), experimental demonstration of space quantization of angular momentum (1922), diffraction of matter waves comprised of atoms and molecules by crystals (1931) and the determination of the magnetic dipole moments of the proton and deuteron (1933).

/pdf/otto-stern-1888-1969-the-founding-father-of-experimental-3v75uf2jjm.pdf

J.C.A. Lower

Papers

Otto Stern (1888–1969): The founding father of experimental atomic physics