James Franck

Migration and Photochemical Action of Excitation Energy in Crystals

A crystal which has absorbed a light quantum can be treated either as an assembly of molecules or else as a giant molecule. If the exchange of excitation energy between crystal cells is slow as compared to the periods of vibration, the first description is preferable; if it is fast, the second picture is better. Both cases are discussed in connection with the following question: To what extent can excitation energy absorbed by an arbitrary cell of the crystal be used photochemically at a specific point which may be far removed from the absorbing cell? The results are applied to the behavior of polymerized pseudoisocyanines, to the hypothetical photosynthetic unit and to the theory of sensitized photographic plates.

Photosynthetic evolution of oxygen by flashes of light.

Abstract An attempt is made to reconcile the apparently contradictory observations in flash photosynthesis using short flashes ( ca . 10 −5 sec.) and long flashes ( ca . 10 −3 sec.). It is shown that the introduction of a new period of ca . 10 −4 sec. not only obviates these previous difficulties, but also is not in contradiction with any existing observations. In addition, a mechanism for the primary photochemical process, suggested by one of us (J. F.) and discussed in detail elsewhere, is presented. Using reversible phosphorescence quenching as a means of measuring very small concentrations of oxygen, we have also studied the evolution of oxygen under anaerobic conditions by single flashes of light. The biological material was the algae Scenedesmus obliquus in the presence of either carbon dioxide or the Hill reagent quinone. It has been observed that while a single, intense, half-millisecond flash evolves oxygen in the Hill reaction, it does not evolve oxygen in photosynthesis. A flash 50 times as long and of much lower intensity results in the evolution of oxygen in both systems. Further, in the Hill reaction, the yield from an intense half-millisecond flash is increased by a previous similar flash preceding it by one full second. This observation suggests that certain photoproducts are capable of surviving longer than the generally assumed 10 −2 sec. Various other observations on the effect of overlapping short and long flashes, and some plausible explanations, are described.

Remarks on the Fluorescence, Phosphorescence and Photochemistry of Dyestuffs

Many of the apparently conflicting facts of the photochemistry as well as of the phosphorescence and of the fluorescence quenching of dyes can be given a rational and unified interpretation if it be assumed that an electronically excited dye molecule can go over, by a process of internal conversion, to the electronic ground state of a reactive energy‐rich tautomer. The endothermic reversion of such a tautomer to the electronically excited state of the original molecule would explain the weak, temperature dependent phosphorescence observed in some systems. In the presence of a suitable reducing agent the reactive tautomer may be reduced to a semiquinone. This process and the likely subsequent reactions are sufficient to explain the majority of dye‐sensitized photo‐oxidations as well as the photo‐bleaching of dyes by reducing agents. In the case of chlorophyll, and possibly a few other dyes, it appears to be necessary to assume that the tautomer and a normal dye molecule can undergo a process of disproporti...

A THEORY OF LIGHT UTILIZATION IN PLANT PHOTOSYNTHESIS.

Abstract A model for the photochemical steps in photosynthesis is described in which only one type of reaction center participates in the chemical conversion of excitation energy. Two types of energy-collecting pigment systems can occur, but in distinction to most current models the energy is delivered to a unique type of chlorophyll-enzyme complex. Of the two photochemical steps in photosynthesis, one is mediated by the metastable triplet state of chlorophyll in this complex and the other requires singlet excitation for efficient operation. The nature of chlorophyll in the two pigment systems is identified on the basis of spectroscopic evidence. The far-red absorbing pigment, P700, is considered to be a crystallized chlorophyll that may participate in photosynthetic energy collection but is not essential for the overall process. The model is tested by comparisons with experimental evidence from fluorescence, afterglow, the Emerson effect, reversible bleaching, and alternate photochemical pathways.

Experimental and theoretical contribution to studies of the afterglow of chlorophyll in plant materials.

The afterglow of chlorophyll in plant cells and in chloroplasts, discovered and studied by Strehler and Arnold et al. (l-4), is of similar importance in attempts to understand the photochemical steps of photosynthesis as is the fluorescence of chlorophyll. The observations described in Part I of this paper are measurements of the afterglow intensity of algae observed in a phosphoroscope under a great variety of experimental conditions. The experiments were completed more than 3 years ago. Their publication has been postponed until a plausible theoretical interpretation of all afterglow phenomena could be presented. This interpretation is given in Part II. It is based on Arnold’s recent discovery (5) that the afterglow is associated with electron conductivity and on the general theory of the photochemical part of photosynthesis which one of us (6) has developed in recent years.

Beziehung zwischen Absorptionsspektren und chemischer Bindung bei Alkalihalogeniddämpfen

ZusammenfassungEs werden die Absorptionsspektren mehrerer Alkalihalogenide untersucht. Nach h rer Lage und Struktur können sie verschiedenen photochemischen Dissoziationsprozessen dieser Moleküle zugeordnet werden. Verallgemeinernd ergibt sich, daß Moleküle, die aus zwei einwertigen Ionen aufgebaut sind, photochemisch sowohl in zwei normale Atome wie in ein normales und ein angeregtes Atom zerlegt werden können. Bei sogenannten Atomverbindungen (s. die folgende Arbeit) fehlt die erstere Möglichkeit.

Fluorescence of Chlorophyll in Its Relation to Photochemical Processes in Plants and Organic Solutions

The intensity of the fluorescence of chlorophyll in organic solutions in its relation to photochemical reactions can be explained by the assumption that the excited chlorophyll, which is free from adsorbed molecules, has a small probability of reemitting light as fluorescence, and a greater probability of predissociating into a hydrogen atom and monodehydrochlorophyll. In the presence of oxygen the product of dissociation will react with it. If acceptor molecules (RH) for oxygen are added to the solution, they will take over the excitation energy and protect the chlorophyll against oxidation, while they themselves will be oxidized; the first step being the dissociation of RH into R and H. The energy transfer may take place by impact of the second kind or as an intramolecular energy exchange within a complex molecule H Chph RH which will dissociate into H Chph R and H. The formation and consumption of the complex radical H Chph R is responsible for the change of intensity of the fluorescence with the time of irradiation which occurs in the presence of oxygen and some acceptor molecules in organic solutions. Analogous intensity time relations have been found and qualitatively described by Kautsky and his co‐workers for the fluorescence of living leaves in the presence of oxygen. Quantitative measurements of these curves under various conditions form the experimental part of this paper. All observations can be interpreted by the assumption that not only photosynthesis but also photoxidation of organic substances adsorbed to chlorophyll takes place in the plant. The hypothesis is suggested that photoxidation is responsible for the so‐called light saturation of photosynthesis in living plants.

Phosphorescence of Adsorbed Trypaflavine and Its Quenching by Oxygen

Dyes adsorbed at, or imbedded in, solids have a greater yield of fluorescence and phosphorescence than they do in liquid solutions. As an hypothesis, it is suggested that hindrance of the natural movement of the atoms in the dye molecule causes a delay in the internal conversion of excitation energy into molecular oscillation energy, thus favoring the re‐emission of light. The green phosphorescence of trypaflavine adsorbed on silica gel, is half quenched by oxygen at 5×10−5 mm pressure; the quenching at higher oxygen pressures is much smaller than would be expected. This fact indicates that two long‐lived excitation states (tautomers of trypaflavine) share in causing the green phosphorescence. The first (M1) is very sensitive to oxygen, whereas the second (M2) is insensitive. The phosphorescence still visible at the higher oxygen pressures owes its origin entirely to M2. The quenching in this pressure range is due to the oxygen sensitivity of F, the unstable state responsible for the green fluorescence. W...

An Attempted Theory of Photosynthesis

An attempt is made to explain quantitatively many observations described in the literature on the photosynthetic production of oxygen in its dependence on light intensity, time of irradiation, etc. Four photochemical steps and two dark reactions are assumed, in which among others, a peracid, formic acid and a peraldehyde occur. These are the same intermediate compounds as in auto‐oxydation processes, so that the similarity between these two inverse processes is striking. Light saturation is explained by back chain reactions initiated by photolytical decomposition of the per‐compounds. The agreement between observations and calculations is good. The picture gained for the photosynthesis of CO2 can be applied in the same way for that of plant acids but the plant acids can also be photooxidized in a reaction sensitized by chlorophyll.

Anregung von Spektren des Wasserstoffs durch Elektronenstoß

ZusammenfassungEs werden Experimente beschrieben, die zeigen, daß Elektronen, die eine genügende kinetische Energie besitzen, in einem Elementarakt normale Wasserstoffmoleküle in ein normales und ein angeregtes Atom dissoziieren können. Die Dopplerbreite der Balmerlinien, die auf diese Weise angeregt werden, ist größer als die Dopplerbreite bei Anregung von freien Atomen. Dieser Effekt ist hervorgerufen durch die Relativenergie, mit der die Atome der dissoziierenden Moleküle auseinanderfahren. Es werden theoretische Betrachtungen angestellt über den Dissoziationsprozeß, die gleichzeitig Anhaltspunkte zum Verständnis eines kontinuierlichen Wasserstoffspektrums bieten können.