On the possibilities of sub-Doppler atomic spectroscopy in multilayer gas cells

Possibilities of high-resolution spectroscopy of atoms (or molecules) for optical pumping and probing with the use of a proposed cell with a series of plane-parallel thin gas layers between spatially separated regions of this cell are theoretically studied. It is shown that efficient velocity selection of optically pumped atoms is possible in view of their characteristic transit and collisional relaxation in such a cell, which leads to the formation of narrow sub-Doppler resonances in absorption of a probe monochromatic wave. The resolution of this spectroscopic method is analyzed in the cases of stationary and definite nonstationary optical pumping of atoms by broadband radiation for different geometrical parameters of these cells and pumping intensity. The proposed multilayer gas cell is a compact analog of a large number of parallel atomic (molecular) beams and can be the basis for new high-precision and compact optical frequency standards.     

Influence of transverse configuration of running laser beam on sub-Doppler structure of its absorption spectrum in thin gas cell

The theoretical investigation is conducted of the strong dependence of sub-Doppler resonances of absorption on the transverse intensity profile of a monochromatic laser beam propagating through a thin cell with rarefied gas medium. The resonances under study correspond to centers of quantum transitions and are caused by the specificity of the optical pumping of atoms during their flight between the walls of a thin cell whose internal thickness is many times smaller than its diameter. The detection of narrow high-contrast sub-Doppler absorption resonances, which are prospective for efficient laser-frequency stabilization, can be facilitated by the proper selection of the radial distribution of the light beam intensity in the cell’s transverse cross section.     

On the capabilities of sub-Doppler photoionization spectroscopy in thin gas cells

A high-sensitivity photoionization method of registration of narrow sub-Doppler resonances in the spectral distribution of a flow of metastable atoms (or molecules) excited from the ground quantum term by a monochromatic laser beam propagating at normal incidence through an ultrathin gas cell (with a micrometer-scale or even nanoscale gas layer thickness) is proposed. Based on density matrix equations for atomic particles, various mechanisms of broadening of the considered resonances, such as time-of-flight, field, and Doppler broadening, are analyzed. The requirements for laser beam parameters and gas cell dimensions that allow obtaining the narrowest resonances are established. The proposed method can be used in ultrahigh resolution spectroscopy of atoms and molecules, as well as high-precision optical frequency standards.     

Narrowing of sub-Doppler saturated-absorption resonances in multilayer gas cells

We have studied theoretically nontrivial specific features of sub-Doppler resonances of saturated absorption in a multilayer gas cell with a rarefied gas medium, which is a compact analog of a large number of plane-parallel atomic beams. Spatially separated saturating and probing monochromatic laser beams that co- and counterpropagate (at the same frequency) in this cell have been considered. In this situation, the action of the light radiation of the saturating beam on the probing beam is determined by optically pumped atoms that fly between the beams under conditions that the longitudinal components of their velocities experience selection due to a specific geometry of the multilayer cell. Such a selection leads to a narrowing of the sub-Doppler resonance in the absorption of the probing beam and decreases the difference between the structures of this resonance for the cases of co- and counterpropagation of the saturating radiation. We have found that, in the considered multilayer cell, the effective width of the sub-Doppler resonance can be smaller (by a factor of about 1.5) than the extremely narrow characteristic width of the well-known Lamb dip in spectroscopy of saturated absorption in the standard gas cell. Results of this study can be used in atomic spectroscopy of ultrahigh resolution and for the laser-frequency stabilization.     

On the possibility of using parametric X-ray radiation to study anisotropy of a crystal mosaic structure

Parametric X-ray radiation (PXR) of relativistic electrons moving in a mosaic crystal is considered. A strong dependence of the PXR angular spectral distribution on the mosaic structure is shown. The effect of mosaic structure on the PXR spectrum strongly increases in the longitudinal plane specified by the radiating electron velocity vector and the averaged reciprocal lattice vector and decreases in the transverse plane with a decrease in the angle of incidence of the radiating electron onto a crystal plane, which allows one to study the anisotropy of mosaic structure of crystal planes.     

Photoprocesses with the sub-Doppler spectral selectivity excited by the monochromatic optical pulse in a thin gas cell

The method of sub-Doppler spectroscopy is theoretically elaborated, which is based on the specific dynamics of a number of optically excited atomic particles (atoms or molecules) of a rarefied gas medium in a thin cell after the action of the resonance pulse of the monochromatic radiation. Corresponding calculations are carried out on the basis of density matrix equations for the resonance optical transition between Zeeman degenerate ground and excited quantum levels of particles in case of the linear polarization of the laser pulse at its normal incidence on the cell. The situation is considered when the radiative lifetime of the excited level is much more than the characteristic transit time of particles between nearest plane-parallel walls of the cell. Then the distribution of a number of excited particles versus the pulse frequency detuning narrows in the process of particles collisions with walls of the cell after action of the laser pulse. The factor of such a narrowing (in comparison with the Doppler broadening of the spectral line of the resonance transition) may be more than the ratio of the characteristic transverse size of the thin gas cell to its inner thickness. We discuss possible use of given sub-Doppler resonances (of the number of excited particles) in the high-resolution spectroscopy and also in high-selective processes of photo-ionization and photo-dissociation, especially, for isotope (or isomer) separation and detection of rare (in particular single) atoms or molecules of a gas medium.     

On a possible structure in the proton elastic form factor

We investigate in the proton form factor the possibility to have a zero in the space-like region. As a consequence thee-p differential cross-section would have a dip and a second maximum which are accessible to experiment. If the existence of such structure would be revealed it would be very important for understanding the composite nature of the proton.     

Energy spectrum of fast atoms in a dark cathode space

A method is proposed for solving the kinetic equation for fast atoms moving in a proper gas. We find the Green's function which permits computation of the energy spectrum of fast atoms for an arbitrary source function in the volume of a proper gas or an amorphous body having plane boundaries. The energy spectrum of fast atoms is computed in a dark cathode space. The solution obtained is used to compute the contribution of fast atoms to atomization of the cathode surface.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 62–67, August, 1987.     

On condensation in the free-boson gas and the spectrum of the Laplacian

We prove the convergence of the thermodynamic functions of a free boson gas for ad-dimensional (d=3,4,...) van Hove sequence of convex regions. The thermodynamic functions behave singularly at a critical densityρ _c which is independent of the geometrical details of the sequence. We are led to define a second critical densityρ _m depending on the geometrical details of the sequence. For densities betweenρ _c andρ _m none of the single particle states is macroscopically occupied. We derive a sufficient condition on the sequence such thatρ _m =ρ _c .     

Coherent population trapping in a gas of excited atoms

Coherent population trapping in a gas discharge was studied for transitions between excited levels of neon atoms. Resonances corresponding to the arising of coherent population trapping in the Λ and V schemes for Zeeman sublevels of the lower and upper working states were observed in the presence of a longitudinal magnetic field. The effect of nonlinear polarization plane rotation under coherent population trapping conditions was studied. The possibility of using the results obtained in this work for diagnostics of local magnetic fields and other plasma parameters in gas discharges was considered.     

Nonclassical paths in the recurrence spectrum of diamagnetic atoms

Using time-independent scattering matrices, we study how the effects of nonclassical paths on the recurrence spectra of diamagnetic atoms can be extracted from purely quantal calculations. This study reveals an intimate relationship between two types of nonclassical paths: exotic ghost orbits and diffractive orbits. This relationship proves to be a previously unrecognized reason for the success of semiclassical theories, such as closed-orbit theory, and permits a comprehensive reformulation of the semiclassical theory that elucidates its convergence properties.     

Finite element approximation of nematic liquid crystal flows using a saddle-point structure

In this work, we propose finite element schemes for the numerical approximation of nematic liquid crystal flows, based on a saddle-point formulation of the director vector sub-problem. It introduces a Lagrange multiplier that allows to enforce the sphere condition. In this setting, we can consider the limit problem (without penalty) and the penalized problem (using a Ginzburg–Landau penalty function) in a unified way. Further, the resulting schemes have a stable behavior with respect to the value of the penalty parameter, a key difference with respect to the existing schemes. Two different methods have been considered for the time integration. First, we have considered an implicit algorithm that is unconditionally stable and energy preserving. The linearization of the problem at every time step value can be performed using a quasi-Newton method that allows to decouple fluid velocity and director vector computations for every tangent problem. Then, we have designed a linear semi-implicit algorithm (i.e. it does not involve nonlinear iterations) and proved that it is unconditionally stable, verifying a discrete energy inequality. Finally, some numerical simulations are provided.     

Noise spectrum of a gas plume

The spectrum of a “gas plume” formed by a marine hydrocarbon seep is described. The noise spectral density has maxima at frequencies corresponding to the natural (collective) oscillations of the bubble cloud, which are related to the size of the gas plume and to the mean volume gas content in the medium. The spatial distribution of noise in depth near a gas plume is highly nonuniform and modal in nature. The nonuniformity is localized in the horizontal plane, and the localization radius is the smaller the higher the mode number is.     

The spectrum of a gas laser

A He-Nc laser operating at 6328 Å produces beats between the modes for a linear cavity; these are related to the field distribution in the cross section of the beam. The power distribution in the modes is examined.