Selective reflection of resonance radiation from excited media

According to quantum electrodynamics, the cross section for resonant scattering of radiation on an aggregate of excited atoms can be written as a sum of positive definite terms. This type of structure is not consistent with the Fresnel formulas for the reflection coefficient of radiation from thermally excited media. The difference shows up on a macroscopic level and indicates that semiclassical radiation theory cannot be used. A study of the correlation between elastic scattering and stimulated emission processes clarifies the reason for the discrepancies. The resulting singularities require summing of Feynman diagrams which appear beginning in the sixth order of perturbation theory. A lower bound estimate for the reflection coefficient from a plane layer is given, including processes which violate the statistics of radiation. The contribution of stimulated emission processes caused by the initially scattered photon are examined specifically. An experiment is proposed which would settle the choice of theories. Zh. éksp. Teor. Fiz. 113, 521–538 (February 1998)     

Kinetics of reflected resonance radiation from excited gas media

According to the consequences of quantum theory the distortion of an electromagnetic wave front, selectively reflected from thermally excited media of two-level molecules, is determined by ground state molecules. The excited molecules react practically noninertially to the external field.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 89–93, February, 1987.We appreciate the attention of the participants of the V. L. Bonch-Bruevich seminar.     

Transition radiation of a charge in media with a nonuniform potential

The influence of a potential barrier on the transition radiation in the form of volume and surface electromagnetic waves emitted by a charged particle crossing an interface between media is investigated. It is shown that the volume-wave radiation field arises not only as a result of the jump in the dielectric constant at the boundary but also as a result of the velocity jump and the reflection of an electron induced by the presence of a nonuniform potential barrier. The angular distribution of the transition radiation intensity is obtained. Zh. Tekh. Fiz. 68, 11–14 (January 1998)     

Casimir-Polder interaction of excited media

The potential of long-range interaction between two dissimilar atoms, one of which is excited, drops as 1/R ₂ with the distance for the Casimir-Polder limit of large distances in comparison with the wave-length of atom transitions (E.A. Power and T. Thirunamachandran, Phys. Rev. A 51, 3660 (1995)). It is shown that such a dependence, obtained with the help of perturbation technique, results in a divergence for the interaction potential between an excited atom and a medium of dilute gas. We develop a nonperturbative method based upon quantum Green’s functions (Yu. Sherkunov, Phys. Rev. A 72, 052703 (2005)) to calculate the interaction potential for an excited atom and a ground-state atom embedded in a dielectric medium, taking into account the absorption of photons in the dielectric medium. The exponential suppression of the interaction between the atoms is demonstrated. The force acting on an excited atom near the interface of dilute gas medium is calculated. The result is no more divergent. The force between gas media in Casimir-Polder regime is calculated as well. The text was submitted by the author in English.     

Velocity statistics in excited granular media

We present an experimental study of velocity statistics for a partial layer of inelastic colliding beads driven by a vertically oscillating boundary. Over a wide range of parameters (accelerations 3-8 times the gravitational acceleration), the probability distribution P(v) deviates measurably from a Gaussian for the two horizontal velocity components. It can be described by P(v) approximately exp(-mid R:v/v(c)mid R:(1.5)), in agreement with a recent theory. The characteristic velocity v(c) is proportional to the peak velocity of the boundary. The granular temperature, defined as the mean square particle velocity, varies with particle density and exhibits a maximum at intermediate densities. On the other hand, for free cooling in the absence of excitation, we find an exponential velocity distribution. Finally, we examine the sharing of energy between particles of different mass. The more massive particles are found to have greater kinetic energy. (c) 1999 American Institute of Physics.     

Transition radiation of sound under uniform motion of momentum and mass sources in inhomogeneous media

Theoretical investigations of transition radiation of sound under the uniform motion of small momentum and mass (heat) sources in a gas with chaotic inhomogeneities of density and temperature are briefly discussed. We analyze two basic methods for calculating the source losses by the radiation of acoustic waves. The first method is based on the calculation of the energy flux in the Born approximation over small perturbations of the parameters of the medium, and allows us, in particular, to clarify the problem of the angular distribution of the radiation. The second method is based on the use of the effective permittivity tensor, which links together the spatiotemporal Fourier transforms of the average momentum of a unit volume of the medium and the average (over the ensemble) velocity of the wave perturbations. The dependence of the radiation reaction force on the velocity of the motion of source is analyzed for the case of small-scale and large-scale inhomogeneities of the medium.Radiophysical Research Institute, Nizhny Novgorod. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 38, Nos. 1–2, pp. 44–55, January–February, 1995.     

Ionoluminescence in the presence of ionizing radiation

An increase in the ionoluminescence intensity from a ZnS-CdS:Ag sample by a factor of more than 20 upon additional electronic excitation by UV light has been found. The effect decreases in magnitude with an increase in the energy of bombarding H₂⁺ ions and peaks at an energy below 175 eV. This effect manifests itself in samples having a system of shallow electron traps and is due to the relaxation of highly excited anharmonic atomic vibrations, caused by the ion impact, through the electronic channel. A mechanism is proposed and computed for the phenomenon revealed.     

Modeling two-dimensional opto-acoustic studies of excited media

The use of the opto-acoustic effect in tomographic studies of excited media under strong absorption of the initiating laser radiation is discussed. We present the results of two-dimensional model experiments showing the possibility of good reconstruction of a secondary acoustic source even in the case of significant absorption of the initiating radiation. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 1, pp. 83–86, January, 1999.     

Transition rates and sequential decay of excited nuclei

We present two possible sequential decay mechanisms which correspond either to an inverse fusion process (detailed balance) or to a generalised fission process (transition state theory). We compare the corresponding transition rates and try to understand the implications of these two different choices on the behaviour of mass and charge multiplicities. Deviations from percolation calculation predictions and the experiment are also analysed.     

Transition radiation by neutrinos

We calculate the transition radiation process ν→νγ at an interface of two media. The neutrinos are taken to be with only standard-model couplings. The medium fulfills the dual purpose of inducing an effective neutrino-photon vertex and of modifying the photon dispersion relation. The transition radiation occurs when at least one of those quantities have different values in different media. The neutrino mass is ignored due to its negligible contribution. We present a result for the probability of the transition radiation which is both accurate and analytic. For Eν=1 MeV neutrino crossing polyethylene-vacuum interface the transition radiation probability is about ¹⁰−39 and the energy intensity is about ¹⁰−34 eV. At the surface of the neutron stars the transition radiation probability may be ∼¹⁰−20. Our result is by the three orders of magnitude larger than those of previous calculations.     

Charge-transfer reactions in clusters excited with synchrotron radiation

Charge-transfer reactions are observed in a photoluminescence study of NF₃\rm NF_3-doped free krypton clusters. They show up in emissions from Kr⁺F^-\rm Kr^{+}F^{-}free excimers ejected from the clusters, and from excited Kr₂⁺F^-\rm Kr_2^{+}F^{-}and Kr₂⁺(NF₃)_m^-{\rm Kr}_2^{+}({\rm NF}_3)_m^{-} (m 3(m\geq 1) solvated in the clusters. The results show that reaction dynamics in clusters differs considerably from that in the gas and solid phases.     

Transition radiation of multicharged ions

The problem of the transition radiation of multicharged ions at the interface between two media and in a thin plate under the charge-exchange conditions has been solved. It has been shown that the processes of pickup (or loss) of electrons by accelerated multicharged ions at the interface between two media significantly increases the yield of transition radiation.