Effect of the dimension of the phonon spectrum on the stability of condensed media states

The effect of dimension index d _f of the phonon spectrum, which is a structural characteristic in continual models, on the stability of states of condensed media is considered in the Einstein and Debye approximations. The estimate of the phase state stability is based on the Lindemann criterion generalized to arbitrary values of 0 ≤ d _f ≤ ∞. The problem of variation of physical characteristic of a substance by controlling the structure of its phonon spectrum is considered by analyzing the possibility of obtaining molecular hydrogen in the superfluid state. The Einstein and Debye models as applied to the problem on the dynamics of atomic oscillations are compared, and the divergence of the latter model for fractal dimensions d _f < 2 of the phonon spectrum is demonstrated, as well as the incompatibility of the Debye model at high temperatures and the model of a classical oscillator for all dimensions except d _f → ∞.     

Change in the states of optically excited Rb atoms near sapphire surface

The changes in the states of excited Rb atoms approaching a single-crystal sapphire surface have been investigated by methods of laser-excitation spectroscopy and luminescence of Rb vapor in a cell with sapphire windows, the gap between which was varied from 250 to 500 nm. Upon resonant excitation of Rb atoms by two semiconductor lasers with powers of 20 and 40 mW, luminescence from optically excited 5D _3/2 and 5D _5/2 states and optically unexcited 6P _1/2 and 6P _3/2 states is observed. It is established that the luminescence intensity from unexcited states is only a few times lower than that from excited states, with allowance for the fact that excited atoms are rapidly and almost completely quenched on the sapphire surface. The found anomalously strong luminescence from optically unexcited 6P _J states is explained by their nonradiative occupation near the sapphire surface from optically excited 5P _J states, in which atoms fail to reach the sapphire surface because of the repulsion from it. This repulsion is due to the polarization interaction between sapphire and the atoms in the 5P _J states near the surface. Nonradiative transition from the 5P _J state to the 6P _J ^?1 state is accompanied by excitation of two optical phonons in sapphire.     

A mixed problem for an inhomogeneous wave equation with a summable potential

A mixed problem for an inhomogeneous wave equation with fixed ends in the case of a summable potential is studied. Using the Krylov method for acceleration of the convergence of Fourier series, a classical solution under minimal conditions on the initial data and a generalized solution in the case of quadratic summable initial data and perturbing function are obtained.     

A study of laser action on a free-molecular flow of atoms in a capillary

We found experimentally that resonance laser irradiation of a rarefied flow of atoms in a capillary causes their density at the exit to decrease. We established a dependence of the change in atomic density on the excitation intensity. The observed effect is shown to be related to the flow heating caused by hyperelastic collisions of excited atoms with the capillary wall. We describe the gas heating in a capillary taking into account the energy relaxation of hot atoms as they collide with the wall. We show that our experimental results can be explained by assuming that the energy accommodation coefficient for hot atoms during their collisions with the wall is close to unity.     

Photostimulated diffusion of atoms adsorbed on a solid surface

A new light-induced effect—surface photodiffusion—is found in the processes of transfer on a solid surface. The essence of the effect is that the energy of electronic excitation of an adsorbed atom is transformed into that of its motion over the surface. It is shown that, under nonuniform illumination of the surface, the adsorbed atoms are moved away from the illuminated region, with the atomic concentration at the boundary between the illuminated and dark regions exceeding the equilibrium value. The dark and photoinduced diffusion coefficients are measured, and the value of the diffusion barrier height is estimated.     

Mixed problem for the wave equation with integrable potential in the case of two-point boundary conditions of distinct orders

We study a mixed problem for the wave equation with integrable potential and with two-point boundary conditions of distinct orders for the case in which the corresponding spectral problem may have multiple spectrum. Based on the resolvent approach in the Fourier method and the Krylov convergence acceleration trick for Fourier series, we obtain a classical solution u(x, t) of this problem under minimal constraints on the initial condition u(x, 0) = ?(x). We use the Carleson–Hunt theorem to prove the convergence almost everywhere of the formal solution series in the limit case of ?(x) ∈ L _p[0, 1], p > 1, and show that the formal solution is a generalized solution of the problem.     

Optical manifestations of self-diffusion of atoms over the surfaces of silver nanoparticles

The kinetics of the extinction spectrum of a thin island silver film heated in the temperature range 473–553 K is studied experimentally. The observed variations in the inhomogeneously broadened extinction spectrum are related to two types of changes in the morphology of islands, namely, (i) coalescence and fragmentation of the islands at the initial stage and (ii) subsequent rounding of individual islands. This conclusion is supported by the data of electron microscopy of the film at different stages of its heating. The long duration of the relaxation process is explained by specific features of mass transfer over the surfaces of granulated nanoparticles.     

Optical method for measuring structural parameters of island films

An optical method for determining characteristics of the submicron structure in island films is proposed. The method is based on the measurements of fluctuations of optical properties of a film detected when a focused laser beam is scanned over it. It is shown that the island size distribution can be determined in principle by the method proposed. Structural inhomogeneity of a Cs film on sapphire is revealed experimentally. Parameters of these inhomogeneities are measured as functions of the film thickness. The correspondence of the results of the optical method and the data of electron microscopy is tested for a In film on glass.     

Differential operator with irregular splitting boundary conditions

A finalized solution is obtained for the problem of expanding a differential operator, with arbitrary continuous coefficients and irregular splitting boundary conditions, into uniformly convergent series in eigenfunctions and associated functions.Translated from Matematicheskie Zametki, Vol. 19, No. 5, pp. 763–772, May, 1976.     

Nonradiative transition to the ground state and superelastic scattering of exited atoms upon impact on the surface of wide-bandgap dielectrics

The impact of excited cesium atoms on sapphire and glass surfaces have been experimentally studied. It is established that the probability of electron excitation quenching upon impact of an atom on the dielectric surface is close to unity. The velocity distribution of unexcited atoms upon scattering from the surface has been determined using the time-of-flight technique. The kinetic energies of most of these atoms are several tens of times greater than the energy of thermal motion of the excited atoms impinging on the surface. Conversion of the internal energy of atoms into their kinetic energy is explained in terms of nonradiative electron transitions with simultaneous excitation of lattice vibrations in the dielectric crystal. This mechanism of atomic excitation quenching near the dielectric surface explains the significant difference between the energies of atoms upon superelastic scattering and upon photodesorption from an adsorbed state.