Optical Method of Determining the Concentration of Atoms and Molecules in the Ground State

The phenomenon of anomalous dispersion has been used quite fruitfully by Rozhdestvensky and his pupils, who evolved the famous "rtacken-Methode." This method makes it possible to measure with great accuracy considerable optical densities, but involves quite formidable work on spectrograms. The theory of the 'Hacken-Methode" for determining the optical density of molecular substances has not yet been evolved, and so far its use is limited to the study of atomic vapors of certain metals. The method we are suggesting can in many cases simplify measuring and enable us to use the phenomenon of anomalous dispersion to study the processes occurring in substances with a molecular structure. The method consists of the following. We know that when a layer of a substance is introduced into one branch of the Rozhdestvensky interferometer crossed with a spectrograph and when light is transmitted through an optical system from the source of a continuous spectrum, then dispersion curves are observed in the spectrograph near the absorption lines or bands. Mien a layer of the same substance and of equal thickness is introduced into the other branch, then the interference fringes near the absorption lines will straighten out, since the optical lengths of the paths traversed by the two interfering pencils will be equal for all wavelengths. Let us now assume that a certain process, accompanied by a change in the concentration of absorbing particles, takes place in the first layer. Then the interference fringes will be distorted near the absorption lines or bands. The interference fringes can be straightened again by a corresponding change in the thickness of the second layer. Here, the layer acts as a compensator. It is evident that a relative change in the concentration of absorbing particles caused by the process in the first layer will be equal to the relative change in thickness of the compensator. Thus the determination of the change in the concentration of absorbing particles in the given layer of the substance can be reduced to observing the interference pattern and to measuring the linear dimensions of the compensator. In many cases it may be more convenient to change the density of the substance in the compensator than to change the dimensions of the compensator. Thus in studying solutions one can change the concentration of the molecules of a given kind in a solution. Mien a substance is studied in a vapor state, the compensation can be made by changing the density of the vapor. If a substance consists of particles of different kinds, their concentrations can be determined selectively. The proposed method can obviously be used for studying