Table of contents

The spectral and spatial characteristics of the Xe(L) amplifier at λ ∼ 2.9 Å determine an optimum for the scaling of the peak power with channel length. The Xe³¹⁺ and Xe³²⁺ (3d → 2p) transition arrays represent two identical spectral optima for amplification, a property stemming from the extremum of spectral components (3245) characteristic of their electron configurations. Adroit matching of the spatial distribution of the intensity characteristic of the propagating 248 nm pulse dynamically generating the self-trapped plasma channel with the intensity required to excite selectively and efficiently the Xe³¹⁺ and Xe³²⁺ arrays can also simultaneously maximize the spatial volume of the excitation. The net outcome of this double maximization is an amplifying channel for the optimal transitions that possesses high gain (∼100 cm⁻¹), low losses (<10⁻¹cm⁻¹) and a diameter of 15−20 µm, a size sufficient to produce an x-ray pulse energy of ∼50−100 mJ from a channel having an average xenon density of ∼10²⁰ cm⁻³ and a length of 1 cm. Since previous studies have experimentally demonstrated the ability to produce a saturated bandwidth of ∼60 eV, a magnitude sufficient to support a pulse duration of ∼30 as, peak powers P_x ≫ 1 PW are clearly within the scaling limits of the Xe(L) system. The corresponding peak brightness scaling limit is accordingly bounded from below by P_x/λ² ≅ 10³⁰ W cm⁻² sr⁻¹.

We present a gauge-invariant formulation of the so-called strong-field KFR approximations in the 'velocity' and 'length' gauges and demonstrate their equivalence in all orders. The theory thus overcomes a longstanding discrepancy between the strong-field velocity and the length-gauge approximations for non-perturbative processes in intense laser fields.

We review the development of the time-dependent close-coupling method to study atomic and molecular few body dynamics. Applications include electron and photon collisions with atoms, molecules, and their ions.

Temporary negative ion resonances above the ionization threshold of zinc atoms have been observed in angular differential electron excitation functions and photon excitation functions of the lowest optically forbidden ³P state. These resonances involve the opening of the 3d-shell or double excitation from the outermost 4s-shell and provide a sensitive test for channel coupling and electron correlation effects.

Electron-impact ionization cross sections for the 1s2s¹S and 1s2s³S metastable states of Li⁺ are calculated using both perturbative distorted-wave and non-perturbative R-matrix with pseudo-states and time-dependent close-coupling methods. Term-resolved distorted-wave calculations are found to be approximately 15% above term-resolved R-matrix with pseudostates calculations. On the other hand, configuration-average time-dependent close-coupling calculations are found to be in excellent agreement with the configuration-average R-matrix with pseudostates calculations. The non-perturbative R-matrix and close-coupling calculations provide a benchmark for experimental studies of electron-impact ionization of metastable states along the He isoelectronic sequence.

The spectral-angular distribution of polarizational bremsstrahlung from relativistic electrons crossing a polycrystalline target is studied theoretically. The effect of substantial growth in the emission spectral density is predicted for photons emitted in the backward direction relative to emitting electron velocity.

We present non-perturbative time-dependent calculations of single and double ionization of helium, under XUV radiation of photon energy ranging from 40 to 45 eV, through the direct propagation of the time-dependent Schrödinger equation. The time-dependent wavefunction of the atom under the field is expanded in terms of correlated multichannel states normalized with incoming-wave boundary conditions. In addition to presenting a new non-perturbative approach to the three-body problem, in a fully correlated scheme, capable of providing in the same calculation photoelectron energy and angularly resolved spectra, as well as cross sections through the lowest non-vanishing order transition amplitude, we also present a detailed comparison of the values of certain key quantities that have been obtained through a variety of other methods. The degree of agreement we find, while lending credence to the approach and its versatility, also highlights the remaining open questions in this novel context of double ionization.

We propose a scheme for generation of highly non-classical entangled field states of the type in the cavity QED scenario. The scheme utilizes an atomic analogue of the Mach–Zehnder interferometer having a quantized field in the high-Q cavity containing a superposition of zero and one photon that acts as the first beam splitter. The probability for production of the desired states may approach a value close to unity with high fidelity under prevailing experimental conditions.

Rydberg states with the principal quantum number n = 25–32, below the υ⁺ = 0 ionization limit, are excited by double resonance via the υ' = 0, N' = 0 and υ' = 0, N' = 2 rovibrational states of the A²Σ⁺ state. In the presence of dc electric fields in the range 0–120 V cm⁻¹, new resonances and hydrogenic manifolds are observed. The experimental spectra are simulated using a matrix-diagonalization approach.

The numerical solution of the time-dependent Schrödinger equation for vibrating hydrogen molecular ions in many-cycle laser pulses shows that high-order harmonic generation is sensitive to laser-induced molecular vibrations. In particular, the odd harmonic lines in the emitted spectra are surrounded by additional regular peaks whose spacing is given by the vibrational frequency of the nuclei motion. Analytical theory relates these satellite peaks to the molecular vibrations in terms of an approximated effective potentials. These results are not affected by the dimensionality of the system.

By studying one-dimensional periodic structures with modulation of both linear and quadratic nonlinear susceptibilities, we obtain the stable propagation of a fundamental soliton pair with both bright and dark shapes and the stable propagation of a fundamental soliton series with bright shapes. Furthermore, we find that optical logic gate functions can be constructed by using these kinds of periodic structures and by setting different parameters. As examples, OR and XOR gates are presented.

The doubly excited ^1,3P^e meta-stable bound states and resonance states of helium embedded in weakly coupled plasmas are calculated using Slater orbital basis functions. The plasma effects are taken care of by using the screened Coulomb potential obtained from the Debye model. We have calculated the 2pnp ¹P^e (n = 3, 4) and 2pnp ³P^e (n = 2, 3, 4) meta-stable bound state energies of He in plasmas for various Debye lengths. In addition, a total of ten resonances (five each for ¹P^e and ³P^e states) below the N = 3 He⁺ thresholds has also been obtained by calculating the density of states using a stabilization method. For the ¹P^e states, this includes three members from the 3pnp (n = 4, 5, 6) series, and two members from the 3dnd (n = 4, 5) series. For the ³P^e state, this includes three members from the 3pnp (n = 3, 4, 5) series, and two members from the 3dnd (n = 3, 4) series. The resonance energies and widths for the ^1,3P^e resonances for different screening parameters below the He⁺(3S) threshold are reported along with the eigen-energies of the ^1,3P^e meta-stable bound states lying below the He⁺(2S) threshold.

The initially prepared coherent state coupled to a third-order nonlinear medium is responsible for stimulated Raman and spontaneous Raman processes. By exploiting the analytical solutions of field operators for various modes, we investigate the quantum statistical properties for pure and two modes of the input coherent light responsible for stimulated Raman and spontaneous Raman processes. For spontaneous Raman processes the photons are unbunched for the pure and two-mode cases. By a suitable choice of phase, we obtain the bunching and antibunching of photons for stimulated Raman processes.

The universal three-body dynamics in ultra-cold binary Fermi and Fermi–Bose mixtures is studied. Two identical fermions of mass m and a particle of mass m₁ with the zero-range two-body interaction in states of total angular momentum L = 1 are considered. Using the boundary condition model for the s-wave interaction of different particles, both eigenvalue and scattering problems are treated by solving hyper-radial equations, whose terms are derived analytically. The dependences of the three-body binding energies on the mass-ratio m/m₁ for the positive two-body scattering length are calculated; it is shown that the ground and excited states arise at m/m₁ ⩾ λ₁ ≈ 8.172 60 and m/m₁ ⩾ λ₂ ≈ 12.917 43, respectively. For m/m₁ ≲ λ₁ and m/m₁ ≲ λ₂, the relevant bound states turn to narrow resonances, whose positions and widths are calculated. The 2 + 1 elastic scattering and the three-body recombination near the three-body threshold are studied, and it is shown that a two-hump structure in the mass-ratio dependences of the cross sections is connected with the rise of the bound states.

The orientation parameter O₁₀ for the 4p⁴(¹D)5p²P_3/2 state of KrII populated in resonant Auger transitions from 3d⁹5p/6p photo-excited states was measured at BESSY II for photon energies between 90.8 eV and 92.9 eV. The (¹D)5p²P_3/2 satellite was populated in the Raman regime, i.e. with a bandwidth of the exciting photons (10 meV) smaller than the natural width of the 3d⁹5p/6p states. Partial cross sections determined from the measurement of the orientation O₁₀ and alignment A₂₀ are compared with results of calculations where the non-resonant part of the photoionization cross section is computed within the sudden approximation. The electron partial wave analysis as the main intention of this investigation shows that this approach yields partial cross sections for the s_1/2 (d_5/2) waves that are larger (smaller) than the measured ones.

We show that the trichromatic manipulation of the absorption spectrum leads to sub-half-wavelength atom localization. In particular, a three-level atom in the Λ configuration is considered, in which one transition is coupled by a trichromatic field with one sideband component being a standing-wave field while the other transition is probed by a weak monochromatic field. By varying the sum of relative phases of the sideband components of the trichromatic field to the central component, the atom is localized in either of the two half-wavelength regions with 50% detecting probability when the absorption spectrum is measured.

We comment on the accuracy of the results of Taioli and Tennyson in their paper 'A wave packet method for treating nuclear dynamics on complex potentials' (2006 J. Phys. B: At. Mol. Opt. Phys.39 4379).