SINP scientists conducted theoretical studies of the atomic photoeffect with istotope separation

DSc. Alexey Grum-Grzhimaylo and PhD. Elena Gryzlova

Co-authors of an article about the studies of the atomic photoeffect with isotope separation, published in Physical Review Letters, DSc. Alexey Grum-Grzhimaylo and PhD. Elena Gryzlova told us about their research, motivation for these studies and its prospects.

Corr.: What is the main idea of your research?

Alexey Grum-Grzhimaylo: Let me start our story. For me these studies of Xenon atoms have been started over 15 years ago. The subject is the following. If the impulsive moment of the electron shell of an atom is nonzero, it produces magnetic field which interacts with magnetic moment of the nucleus. In scientism we speak about interaction of impulsive moment of the electronic shell with nucleus spin. This interaction is very-very weak (therefore it is called super-fine), but it influences on the direction of the polarization of atom's electon shell, causing its precession. If initially the electron shell was polarized, its polarization will be partially lost. It leads to absolutely different results in some seemingly pure atomic processes, depending on taking or not taking super-fine interaction into account. Atomic photoeffect is one of such processes. For instance, angular distributions of the electrons outcoming from the nucleus under the influecne of light.

Elena Gryzlova: Angular distribution is just probability of electrons' outcoming at different angles, i.e. the electons can outcome in all directions with the same probability, but can form some figure. This figure depends strongly on the electron shell's polarization. In our case this figure changes from four-petals camomile to the circle, if we speak about the measurements in one plane.

ALexey Grum-Grzhimaylo: SO we measure electrons which outcome from the atom under the influence of light, and the way in which they outcome depends strongly on the very-very weak interaction which is incomparably weaker than interaction between the electron and the nculeus, other electrons or light irradiating the atom. It seems to be some discrepancy. Therefore, when we obtained the first experimental results some of them looked although expected, but still amazing, because from the "square" point of view it seemed that there was no difference what Xenon isotope participated in the experiment on the atomic photoeffect.

But the difference is very strong, because spin of the nucleus influence on the electron cloud polarization although very weak, and in its turn polarization influence on the angular distribution of the outcoming electrons. Not only electrons' angular distribution but a number of other observed quantities (for instance, angular distribution and polarization of fluorescence, emmited by the atom, when atom's electron de-exitates) also depend on electron shell's polarization. By the way, the opposite influence - of the electron shell on the polarization of the nucleus - is used for long in the method of exited angular correlations of gammas reailized by SINP scientists V.S. Shpinel, A.A. Sorokin and their colleagues for the studies of the nearest electron surroundings of the nucleiin condensed media. Currently the scientists of the Laboratory of nuclear-spectroscopic methods G.K.Ryashy and A.V.Tsvyashchenko continue this study.

Corr.: What are your personal contributions to this researh?

Alexey Grum-Grzhimaylo: My story with Xenon is long, and there are a lot of publications on this topic. Directly to this study I made primary estimations of the incluence of super-fine interaction, and then I took part only as a consultant. Elena Gryzlova contributed to the study and began to make dynamical calculations, because when we succeeded in the measurements of photoeffect with isotope separation and separation of the results for the isotopes with zero nucleus spin, we found out that previously used models of the atomic photoeffect for the exited Xenon did not work properly. And it was necessary to improve the model for the description of the atomic part. Elena managed to improve the model, to mix necessary electron configurations, etc. She'll tell about this part of the study better.

Elena Gryzlova: Speaking about personal contribution, there are only two theoreticians participated in this study: ALexey and me. And our contribution is not needed to be separated. Theoretical discription included deifferent aspects. First, it was necessary to obtain general formula for the angular distributions. Second, to insert certain parameters for the needed Xenon states and amplitudes of the certain transitions into it. So at first it was necessary to obtain analytical equations and then to insert certain spectroscopic parameters in them. We made all the calculations separately and then compared the results.

Corr.: What was the motivation of the studies of this unexplored problem?

Alexey Grum_grzhimaylo: I can't say that this problem was "unexplored". The effect was discussed in literature, and its physics is quite clear. I'd say this problem became "burning". As we've already told, we had to take into account polarization of electron shell, but it was found to be poorly determined due to super-fine interaction. Such uncontrollability of polarization leads to different effects. In particular, for a number of cases we could not conduct a detailed study of such a fundamental process as Auger-effect, when information about Auger-effect is taken from fluorescence of the ion, stayed after Auger-decay.

In order to interpret obtained experimental results it was necessary to turn to sometemes ill-founded ideas, which took into account depolarization of electron shell during its interaction with magnetic moment of the neucleus for atomic processes. It disturbed to discover if one or another atomic model proper or not for the description of pure atomic phenomena, such as the elctrons angular distribution for the atomic photoeffect, or Auger-effect. In my practice there were many studies, in which depolarization was taken into consideration in some way for the data interpretation and some results were obtained, but the question still remained undertermined. Things with excited Xenon atom were in the worst way, although many experiments and appropriate theoretical calculations concerned it. And at last, friendly speaking, I've got tired of it.

So we decided to carry out an experiment with isolated nuclear spin. As I've already told you, it is important for the atoms with non-zero nuclear spin and non-zero angular moment of the electron shell. A group of five experiemtnalists from France, Germany, Italy - our collaborators in the previous studies - started to prepare the experiments over three years ago. The experiments were very complicated, but nevertheless they turned out well. Then processing of the results took long time and our publication, about which we are speaking today, is the first one.

Corr.: How soon the articlewas accepted for publication?

Alexey Grum_grzhimaylo: I've published about ten papers in this journal, and this one was accepted most easily. All referees noted that it is worth sharing, and in Physical Review Letters in the atomic field of studies it happens very seldom. For the scientists it means that the study isreally very interesting and maybe become the first step in a new scientific direction. I expect that our paper will be followed with other publications on the atomic processes with isotope selection on nuclear spin.

Corr.: How do you think what other studies can be carried out?

Alexey Grum-Grzhimaylo: For instance, analogous study can be carried out in order to explore Auger-effect, where is the same problem with nuclear spin. Besides, I think it is possible to study the processes of slow atomic collisions, where electron shells polarization influence on the reaction, in the same way. As we have told the polarization depends on the nuclear spin...

Elena Gryzlova: There is also a possibility for determination of superfine interaction constants for the case, when the levels of superfine structure in the atom overlap. If the levels are isolated the constants can be measured, but if the levels are partially overlapped, such measurements are difficult to do because all the levels run into one another. In our paper we offered a method for determination of superfine interaction constant by measuring the electrons angular distribution for different isotopes for the photoeffect and by comparing these distributions. For isntance, there are six Xenon isotopes with three different values of spin. By the way, we found one constant, which was unknown.

Corr.: You told, that Xenon is the most favourable for the experiments. Why?

Alexey Grum-Grzhimaylo: There are several reasons for it. First of all, Xenon was already studied well, there were a lot of experiments with it, but the problem of depolarization of its excited states was very sharp. Why it was so well-characterized? Becuase it is an inertial gas, very practical for the experiments - it does not interact with other substances, does not block the experimental equipment, does not need any heater for its evaporation.

It is also very convinient for theory development. On the one hand it is an extensive mutli-electron system where we can apply different models and analyze how good they describe an experiment. On the other hand - this system is not so complicated, as a heavy with open shell. Xenon is a tradiational target for the atomic physics, which is actively studied for over 40 years. But the most important is that there are stable isotopes with different nuclear spins in Xenon, while, for instance, in Neon and Argon stabel isotopes have only zero nuclear spin.

Elena Gryzlova: There are a lot of stable isotopes in natural Xenon, and there were eight of them in the experiment, but concentration of two of them was very low.

Corr.: Please, tell us about the experiment.

Alexey Grum-Grzhimaylo: In order to conduct this experiment it was necessary to produce a sufficient number of Xenon excited atoms by a synchrotronous source of the third generation, i.e. a vacuum UV source - intensive, monochromatic, with varied polarization. Currently there are only dozens of such beams in the world, and none of them in Russia. Our experiment took place at the French synchrotrone SOLEIL. This source is an accumulator ring with spinning electron bunches, passing undulators, where radiation is formed.

Photo-ionization was produced by the beam of optic laser. Laser operated within optic range is not an exotic equipment. It was more exotic that both beams - produced by the accumulator ring and by the laser - were combined in a way, that they worked unitedly. Besides, up-to-date equipment was used for registration of the angular distributions of the outcoming electrons, which covered full sphere, and for their energy selection. At last, we needed to know what isotope emitted every detected electron. For the purpose we analyzed the produced Xenon ions registered in coincidence with electrons by mass-spectroscopy methods. It seems, that such a scheme was used for the first time. The measurements were carried out during two weeks. There were two weeks of almost uninterrupted work for a group of four-five scientists.

Elena Gryzlova: The experimentalists carried out giant volume of work and measured a lot of parameters, in particular, for different polarizations of synchrotronous radiation and laser, and for different configurations. Here theoreticins were also needed because for some configurations it was unknown what parameters and how many of them do we need to measure.

Currently we are working at a omprehensive paper. It will be devoted to the full review of all possible polarizations and their possible orientations. In this direction theory needs to go ahead and to offer formulas for obtaining data, and after it data obtained from the experiment will be compared with theoretical.

Corr.: I wish you success in your further studies!

Alexey Grum-Grzhimaylo and Elena Gryzlova: Thank you! We wish you and all SINP scientists Happy New Year!