X-ray radiation toward relativistic electrons incident on a flat target

Features of X-ray radiation emitted toward the velocity vector of relativistic electrons incident on a flat target are discussed. The contribution of polarization bremsstrahlung (PB) considered as scattering of the intrinsic field of a fast charge by electrons of the medium is estimated taking into account its dispersion properties. Spectral-angular characteristics of coherent and incoherent PB are analyzed for unstructured and structured targets. Such PB feature not only different intensities, but also different angular dependences reaching a maximum near the velocity direction of a fast charge. It is shown that coherent PB emitted from the target surface layer is characterized by an extraordinary, i.e., inversely proportional to the squared frequency, intensity dependence.     

Zur Theorie der nichtstationären Resonanzfluoreszenz an bewegten Molekülen. II. Untersuchungen zur hochauflösenden Resonanzfluoreszenzspektroskopie

Non-stationary Theory of Resonance Fluorescence for Moving Molecules. II. Highly Resolved Fluorescence Spectroscopy With respect to tke Doppler-effect a formula for the spectral energy density of fluorescence radiation of gases (vapours) generated by resonance scattering of short laser pulses is derived. The spectral energy density consists of a coherent and an incoherent part which differs in a characteristic way. The coherent fluorescence radiation reproduces the spectrum of the laser light. Informations about the homogeneous line widths are obtained only from the incoherent spectra. The results of numerical calculations of highly resolved spectra in forward and backward directions under different conditions are discussed.     

Coherent and incoherent radiation from high-energy electron and the LPM effect in oriented single crystal

The process of radiation from high-energy electron in oriented single crystal is considered using the method which permits inseparable consideration of both coherent and incoherent mechanisms of photon emission. The total intensity of radiation is calculated. The theory, where the energy loss of projectile has to be taken into account, agrees quite satisfactory with available CERN data. It is shown that the influence of multiple scattering on radiation process is suppressed due to action of crystal field.     

Concept of formation length in radiation theory

The features of electromagnetic processes are considered which connected with finite size of space region in which final particles (photon, electron–positron pair) are formed. The longitudinal dimension of the region is known as the formation length. If some external agent is acting on an electron while traveling this distance the emission process can be disrupted. There are different agents: multiple scattering of projectile, polarization of a medium, action of external fields, etc. The theory of radiation under influence of the multiple scattering, the Landau–Pomeranchuk–Migdal (LPM) effect, is presented. The probability of radiation is calculated with an accuracy up to “next to leading logarithm” and with the Coulomb corrections taken into account. The integral characteristics of bremsstrahlung are given, it is shown that the effective radiation length increases due to the LPM effect at high energy. The LPM effect for pair creation is also presented. The multiple scattering influences also on radiative corrections in a medium (and an external field too) including the anomalous magnetic moment of an electron and the polarization tensor as well as coherent scattering of a photon in a Coulomb field. The polarization of a medium alters the radiation probability in soft part of spectrum. Specific features of radiation from a target of finite thickness include: the boundary photon emission, interference effects for thin target, multi-photon radiation. The theory predictions are compared with experimental data obtained at SLAC and CERN SPS. For electron–positron colliding beams following items are discussed: the separation of coherent and incoherent mechanisms of radiation, the beam-size effect in bremsstrahlung, coherent radiation and mechanisms of electron–positron creation.     

Polarization bremsstrahlung of a hydrogen-like ion in a single crystal

Polarization bremsstrahlung (PB) that arises when a fast hydrogen-like ion is scattered in a single crystal is investigated theoretically. Four types of the process are analyzed that are caused by virtual excitation of electrons of the target and of the incident particle (IP), as well as by a coherent and incoherent interaction between the IP and the single crystal. The spectral, angular, and velocity (of the IP) characteristics of PB are calculated with regard to the spectral function of a photodetector. Optimal observation conditions and regions in which different types of PB are dominant are determined, and the dependence of these regions on the charge of the IP nucleus and of the target atoms is revealed.     

Λ-型三能级单原子的辐射谱

采用时间演化算符方法，研究腔内单模光场中Λ－型三能级单原子的辐射谱，给出了辐射谱一般公式，并讨论在粒子数纯态、相干态和热辐射三种光场激励下的辐射频谱结构。     

Scattering of low-energy atoms by metallic surfaces

The problem of a neutral low-energy atom impinging on a well-defined metallic surface is approached from first principles. The solid and its potential energy of interaction with the incident atom is treated in the most general way, but under the following assumptions: (a) the conduction electrons interact adiabatically with the lattice ions and the gas atom; (b) no chemical reactions occur; (c) the one-phonon approximation is valid. The scattering amplitudes for surface and bulk mode excitations are obtained in terms of the dynamical properties of the metallic surface. Direct collisions of the incident atoms with the lattice ions are shown to give a negligible contribution to the scattering. The most important contribution comes from the interaction of the gas atom with the surface conduction electrons; the excitation of lattice vibrational modes occurs through the electron-phonon term of the Hamiltonian. The general expressions for the scattering amplitudes obtained show that the scattering is incoherent. With further assumptions one obtains a separation of the scattering amplitude into a coherent and incoherent part.     

Electron transitions and the spectrum of radiation emitted by an atom interacting with an ultrashort electromagnetic pulse

Electron transitions and the spectrum of radiation emitted by an atom interacting with an ultrashort electromagnetic pulse are studied based on the sudden perturbation approximation. The excitation and ionization probabilities, the spectra of pulse reradiation by the atom, and the reradiation cross sections are calculated. It is concluded that the reradiation of ultrashort electromagnetic pulses by multielectron atoms is of a coherent nature.     

Opportunity to study the LPM effect in oriented crystal at GeV energy

The spectral distribution of electron-positron pair created by photon and the spectral distribution of photons radiated from high-energy electron in an oriented single crystal are calculated using the method which permits inseparable consideration both of the coherent and incoherent mechanisms of two relevant processes. The method includes the action of field of axis (or plane) as well as the multiple scattering of radiating electron or particles of the created pair (the Landau-Pomeranchuk-Migdal (LPM) effect). The influence of scattering on the coherent mechanism and the influence of field on the incoherent mechanism are analyzed. In tungsten, axis 〈111〉 for the pair creation process at temperature T=100 K the LPM effect attains 8% at photon energy 5 GeV and for the radiation process at T=293 K the LPM effect reaches 6% at electron energy 10 GeV.     

The role of incoherent scattering in laser-induced and laser-assisted processes

We use the S-matrix formalism in order to investigate the possible role of incoherent scattering in different processes in a strong laser field. In the cases of above-threshold ionization (ATI) of atoms and above-threshold detachment (ATD) of negative ions, we assume that the ionized (detached) electron may scatter off a target other than its parent ion (atom). We call such a process incoherent in order to distinguish it from the coherent rescattering of an ionized (detached) electron off its parent ion (atom) which occurs in high-order above-threshold ionization (HATI) and high-order above-threshold detachment (HATD) processes. The same line of reasoning can be applied to the laser-assisted processes, such as the laser-assisted electron-ion radiative recombination (LAR) and the laser-assisted electron-atom scattering (LAS). If the density of the ionic beam used in a LAR experiment is high, it is possible that the electron scatters off an ionic target and subsequently recombines with another ion. Analogously, the electron that scatters on an atom in the LAS process may scatter once more off another atom. We show that the contribution of these incoherent processes to the energy spectra can be higher than that of the corresponding coherent processes if the density of atomic (ionic) targets is high enough. The spectra of the incoherent processes have a cutoff-like behavior and the cutoff energies are higher than those of the corresponding coherent processes. These results are explained by semiclassical analysis.     

Coherent transition radiation from thin targets irradiated by high intensity laser pulses

A theoretical examination on coherent transition radiations (CTR) from the surface of thin solid density target irradiated by high intensity laser is presented. The theory is extended to consider the expansion dynamics of thin foils. The motion of target surfaces leads to the modulation on the temporal structure of micro bunches in the electron beam as well as the spectrum of CTR. The spectral shifts of radiation are owing to the enhancement of electron bunch separation and the relativistic Doppler effects. The radiation power distribution is strongly affected by the temporal coherence of electron beam structure, so thus the electron temperature and velocity dispersions. With these effects accounted for, the spectral properties of coherent transition radiation can provide insights into the expansion of thin foil targets irradiated by intense laser pulse as well as the fast electron transport through it.     

Photon,quantum and collective,effects from rydberg atoms in cavities

I review some theories of the interaction ofN Rydberg atoms interacting collectively with radiation in microwave cavities. The radiation may be incoherent (black body) radiation or it may be coherent. In the former case theories of the steady state inversion and of the superradiance from initially inverted atoms in low-Q cavities agree well with experimental observations. In the latter case in low-Q cavities ‘phase transitions’ of both first and second order types are predicted and should be observable by monitoring the output of an atomic beam by an atomic ionisation detector. The first order transition which occurs at opposite detunings of the cavity and atoms from the frequency of the coherent driving field is of “optically” bistable type but hysteresis is suppressed by quantum fluctuations which can be large in the cavity field close to the transition. I also review a theory of the spectra from single atoms in cavities ofarbitrary Q containing a few microwave photons. A transition from a single peaked Lorentzian spectrum at low-Q to a double-peaked spectrum forQ≃10⁶ is predicted and peaks representing one or more photon transitions of the Jaynes-Cummings model are also expected to be observable at these or largerQ values. The collective theories are all based onN atom Dicke type models driven by the coherent or incoherent field. Substantial squeezing of the fluorescent radiation field from these Dicke models is also predicted and may be observable with Rydberg atoms.     

High harmonic generation via continuum wave-packet interference

High-order harmonic generation (HHG) is investigated theoretically in the over-the-barrier ionization regime revealing the strong signature of interference between two separately ionized and separately propagating free wave packets of a single electron. The interference leads to the emission of coherent light at a photon energy corresponding to the kinetic-energy difference of the two recolliding electron quantum paths, thus complementary to the well-known classical three-step picture of HHG. As will be shown by time-frequency analysis of the emitted radiation, the process entirely dominates the coherent HHG emission after the atomic ground state has been depleted by a strong field. Moreover, it can be isolated from the continuum-bound harmonics via phase matching.     

Experimental Investigation of the Polarization Bremsstrahlung from Relativistic Electrons in Amorphous and Polycrystalline Media

The polarization bremsstrahlung (PBS) generated by relativistic charged particles in a dense medium represents a new and important problem in the radiation physics of charged particles. In the present paper, the results of experimental investigations of PBS conducted with the linear electron accelerator developed at the Scientific Research Institute for Nuclear Physics at Moscow State University (SRINP MSU) are presented; the specific features of spectral and angular PBS distribution in an amorphous and polycrystalline target and directions for their further study are discussed.     

Dynamics of coherent and incoherent emission from an artificial atom in a 1D space

We study dynamics of a two-level superconducting quantum system, analogous to a natural atom in an open space, by measuring the evolution of its coherent and incoherent emission. The emitted waves containing full information about the states of the artificial atom are efficiently collected due to strong atom-transmission-line coupling. This allows us to do simultaneous measurements of all the quantum state projections and perform a full characterization of the system. We derive coherence times and extract the two-time correlation function from the dynamics of the coherent emission.     

Investigation on non-LTE radiation emitted from a laser-irradiated Au disk

1 Introduction The laser-target coupling physics is a key topic in indirect-driven inertial confinement fusion (ICF) and X-ray application research[1―3]. When intense laser light irradiates the solid target, the plasmas are produced rapidly on the surface of the target. The laser en-ergy is mainly absorbed by inverse bremsstrahlung absorption, and a coronal region with high-temperature and low-density plasma is formed. Electron thermal conduction proc-ess transfers energy into over-dense re…     

Classical theory of non-degenerate sum-frequency generation by incoherent nonlinear optical mixing of coherent and chaotic radiations II. Coherent and chaotic input radiations — Efficiency and spectral distribution

Sum-frequency generation by incoherent nonlinear optical mixing of one coherent and one chaotic, mutually uncorrelated input radiations in a dispersive medium is treated in this paper. The efficiency of the process is calculated in the second approximation of the iterative method. It is shown that for perfect phase matching or small phase mismatch of interacting waves and small spectral width of chaotic input radiation the efficiency of incoherent sum-frequency generation can be enhanced compared with the coherent interaction due to the difference between group velocities of sub-frequency radiations. On the other hand, for greater spectral width the efficiency of the process decreases with increasing spectral width of chaotic input radiation. In the case of considerable phase mismatch the efficiency of sum-frequency generation first decreases, but then increases with increasing spectral width of chaotic input radiation. The spectral distribution of the resulting sum-frequency radiation is calculated in the first approximation of the iterative method. There is a general tendency to narrowing the spectral distribution of generated radiation in the course of the process. Moreover, when there is phase mismatch present, a spectral shift of the maximum of generated radiation towards blue or red region, according to the signs of the phase mismatch and the typical dispersion coefficient, appears in the later phases of the sum-frequency generation.This work was partially supported by Research Project C.P.B.P. 01. 07.     

Highly monochromatic fluorescence via simulation of vacuum-induced coherence

It's shown that the incoherent fluorescence spectrum from a three-level Λ atom with orthogonal atomic transition dipole moments is comprised of a single ultrasharp line by simulating the vacuum-induced coherence (VIC), and the line is on resonance with the driving field. The physical interpretation of the spectral characteristics is given in terms of dressed states.     

Signatures of nonlocality in the first-order coherence of scattered light

The spatial coherence of an atomic wavepacket can be detected in scattered photons, even when the center-of-mass motion is in the quantum coherent superposition of two distant, nonoverlapping wavepackets. Spatial coherence manifests itself in the power spectrum of the emitted photons, whose spectral components can exhibit interference fringes as a function of the emission angle. The contrast and the phase of this interference pattern provide information about the quantum state of the center of mass of the scattering atom.     

Spectroscopic diagnostics of the laser erosion plasma of an AgGaS<Subscript>2</Subscript> polycrystalline target

Results are presented from experimental studies of the radiation emitted from a plasma produced in vacuum after irradiating a polycrystalline target by 1.06-μm laser radiation with an intensity of (3–5)×10⁸ W/cm². Plasma radiation from regions located at distances of 1 and 7 mm from the target is analyzed. It is shown that the main contribution to the plasma radiation in the 220–600 nm spectral range is made by transitions from the excited states of single-charged Ag⁺ and S⁺ ions. The atomic component of plasma radiation is represented by intense spectral lines corresponding to transitions from the Rydberg states of Ag and Ga atoms, whereas no resonance lines of these atoms are observed.