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We have investigated the main features of ultrashort laser ablation of solid matter by using laser pulses of different durations, ranging from fs to ps timescales, and wavelengths, in the visible-near infrared spectral range. The analysis has been carried out on Si and Ni in terms of the two main characteristics of the ablation process: vacuum expansion of the ablated material and generation of nanoparticles of the target material. Fast photography and optical emission spectroscopy were used to characterize the plume expansion dynamics, while atomic force microscopy analysis of less than one layer deposits was employed to analyse the size distribution of the produced nanoparticles. Our analysis indicates that the properties characterizing the plume expansion in vacuum as well as the size distribution of the nanoparticles produced with laser pulses in the range of 100 fs–1 ps are almost independent of the specific material properties and laser pulse characteristics, thus representing general features of the process in these conditions.

We report a calculation of the harmonic emission from a one-electron heteronuclear nonsymmetric molecule (HeH²⁺) interacting with a few-cycle laser pulse linearly polarized along the molecular axis. We find that a 180° rotation of the molecule (or equivalently a 180° change in the carrier-envelope phase) leads to substantial changes in the harmonic emission of the molecule. Phase-dependent plateaux and cutoffs appear in the harmonic spectrum as a consequence of the phase-dependent electric field of few-cycle pulses. Asymmetries in the intensity of harmonics result from the phase dependence of ionization rates in nonsymmetric molecules, and from the fact that depending on the molecular orientation, the ionized electron wavepacket can be Coulomb focused as it visits the proton twice before the recollision leading to harmonic generation.

The B-spline R-matrix with pseudostates approach has been used to calculate excitation cross sections for the forbidden 2s²2p³⁴S^o–²D^o, ²P^o, 2s²2p³²D^o–²P^o and resonance 2s²2p³⁴S^o–2s²2p²3s⁴P, 2s2p⁴⁴P, 2s²2p²4s⁴P and 2s²2p²3d⁴P transitions in atomic nitrogen for incident electron energies from threshold to 120 eV. The excitation of these transitions gives rise to prominent lines in the spectra of solar and planetary atmospheres. The 24 spectroscopic bound and autoionizing states together with 15 pseudostates are included in the close-coupling expansion. The pseudostates are chosen to approximate the loss of flux into the infinite number of bound and continuum states that are dipole coupled with ground configuration terms. The contribution of the ionization continuum is significant for resonance transitions. An accurate description of target wavefunctions has been obtained on the basis of non-orthogonal spectroscopic and pseudo-orbitals to adequately account for the correlation corrections and interactions. A comparison of calculated cross sections with the measured absolute direct excitation cross sections is presented. A good agreement with measured integral cross sections is noted except at 5 eV for the forbidden transition and 30 eV for the resonance transitions.

The x-ray spectra, corresponding to the electric quadrupole transitions L₁–M_4,5, have been calculated for all lanthanides. An investigation of the influence of an open 4f^N shell on the main characteristics of spectra has been performed. Due to the validity of the ground term approximation, the concentration of lines on the high-energy side of maxima takes place. When the 4f^N shell is filled partially (N < 7), the natural width of L₁–M₄ maximum exceeds the width of L₁–M₅ maximum; for the almost filled shell the opposite result is obtained. It is explained by the change of total momentum of the ground level of 4f^N shell from the minimum value at N ⩽ 2l + 1 to the maximum value at N > 2l + 1.

Absolute differential elastic and vibrational excitation cross sections have been measured for N₂ in the scattering angle ranges starting between 0° and 20° and extending to 180°, at energies between 0.8 and 5 eV. The results agree with many previous measurements, in particular for angles around 90°, but discrepancies were found in some cases for angles close to 0° and 180°. Integral and momentum-transfer cross sections have been derived and compared with previous beam and swarm measurements. The procedures for determining the instrumental response function and for assuring optimal beam overlap over wide ranges of angles and energies are discussed.

We have produced ensembles of cold ¹⁶O⁺₂, ⁴⁰Ar⁺, ¹²C¹⁶O⁺₂, and various isotopes of barium ions (¹³⁵Ba⁺, ¹³⁶Ba⁺ and ¹³⁷Ba⁺) via sympathetic cooling with laser-cooled ¹³⁸Ba⁺ in a linear radiofrequency trap. The sympathetically cooled species were embedded in the centre of large ¹³⁸Ba⁺ Coulomb crystals containing up to 2000 ions and were identified by motional resonance excitation. Crystals with molecular fractions exceeding 70% were obtained. The observed multi-species crystal structures agree well with results from molecular dynamics simulations. The simulations were also used to deduce an upper limit for the translational temperature of the molecular ions, as low as 20 mK.

For many-electron systems, the electron repulsion energy density w(θ₁₂) is introduced to study the distribution of the electron repulsion energy W = ⟨r⁻¹₁₂⟩ as a function of the interelectronic angle θ₁₂. It is shown that in general the electron repulsion energy density w(θ₁₂) is expressed by a linear combination of Legendre polynomials P_m(cos θ₁₂). Explicit formulae for the expansion coefficients are presented for many-electron atoms described by determinantal wavefunctions. The results are applied to the ground-state helium atom at the Hartree–Fock and correlated levels.

With radiation in the region 104–170 nm from a synchrotron and dispersed with a grating monochromator at spectral resolution 0.02–0.03 nm, we measured absorption spectra of ¹²C¹⁶O in the gaseous phase at 303 K and in the solid phase at 10 K, and dispersed in solid argon at molar ratios Ar:CO = 10, 50 and 250 and at 10 K. We assign observed spectral features to transitions to electronic states A ¹Π, B ¹Σ⁺, C ¹Σ⁺ and E ¹Π from the ground state X ¹Σ⁺. Vibrational progressions are discernible for all these systems of CO in the gaseous phase, but for only the system A—X for CO in the pure solid phase of CO or a dispersion in solid argon; for all condensed phases, multiple series of features are deducible in this vibrational structure.

Atomic local model potentials, reproducing experimental energy levels, are built by a combination of the Rydberg–Klein–Rees (RKR) inversion method with the quantum defect theory (QDT). An efficient iterative procedure leads to the production of spectrum-equivalent potentials containing a number of free parameters. These parameters can be adjusted in order to improve the accuracy of other physical properties, calculated via the constructed potentials. The high efficiency of the method is demonstrated by calculating the lifetimes of singly excited states as well as the dipole and quadrupole polarizability and hyper-polarizability of the ground 1s²2s and first excited 1s²2p state of Lithium. The obtained value γ = 3390 au for the ground state hyper-polarizability is in excellent agreement with other elaborate theoretical calculations. The scalar hyper-polarizability of 1s²2p acquires a much higher value, γ₀ = 1.002 × 10⁷ au, while its tensor part is γ₂ = −0.621 × 10⁷ au.

Relativistic configuration interaction calculations are done for Kr ions Kr II and Kr III. Transition energies and transition probabilities for 122 transitions between Kr II 4p⁵J = 1/2, 3/2→ 1s 4p⁶, 1s 4p⁵np (n = 5–8) J = 1/2, 3/2, 5/2; 437 transitions between Kr III 4p⁴J = 0, 1, 2→ 1s 4p⁵, 1s 4p⁴np (n = 5–8) J = 0, 1, 2, 3 have been calculated. These data have been used by experiment to reproduce the absorption spectrum of Kr II and Kr III and found to be in good agreement with their measurement. Also, the K-edge energy of neutral Kr, Kr II and Kr III has been calculated. The first is within 0.94 eV of an existing experiment, while the last two are calculated for the first time.

Wave numbers and argon-pressure-induced shifts of mercury emission lines were measured using a UV/visible Fourier transform spectrometer (FTS). The observations were made with electrodeless lamps containing isotopically pure ¹⁹⁸Hg and argon buffer gas at pressures of 33 Pa (1/4 Torr), 400 Pa (3 Torr), 933 Pa (7 Torr) and 1333 Pa (10 Torr). Calibration of the FTS wave number scale was obtained from the four most prominent ¹⁹⁸Hg lines (6p ³P₂–7s ³S₁ at 546.2 nm, 6p ³P₁–7s ³S₁ at 436 nm, 6p ³P₀–7s ³S₁ at 404.8 nm and 6p ³P₂–6d ³D₃ at 365.1 nm), enabling measurements of wave numbers and argon pressure shifts of other UV and visible mercury transitions with high accuracy. Our measurements provide new data for the wave numbers and pressure-induced shifts of 20 mercury lines. The wave numbers of mercury lines emitted from the 400 Pa (3 Torr) lamp can be used as standards for wavelength calibration in inductively coupled plasma (ICP) spectrochemical analysis or in experiments where medium-resolution monochromators are used. The pressure-induced shifts of the ¹⁹⁸Hg emission lines are in reasonable agreement with theoretical predictions and could be of interest for validating calculations of mercury–argon interactions.

Photoelectron angular distributions (PADs) for O 1s, C 1s and S 2p_1/2, 2p_3/2 ionization of OCS molecules have been measured in shape resonance regions. These PAD results are compared with the results for O 1s and C 1s ionization of CO molecules, and multi-scattering Xα (MSXα) calculations. The mechanism of the PAD formation both for parallel and perpendicular transitions differs very significantly in these molecules and a step from a two-centre potential (CO) to a three-centre potential (OCS) plays a principal role in electron scattering and the formation of the resulting PAD. For parallel transitions, it is found that for the S 2p and O 1s ionization the photoelectrons are emitted preferentially in a hemisphere directed to the ionized S and O atom, respectively. In OCS O 1s ionization, the S–C fragment plays the role of a strong 'scatterer' for photoelectrons, and in the shape resonance region most intensities of the PADs are concentrated on the region directed to the O atom. The MSXα calculations for perpendicular transitions reproduce the experimental data, but not so well as in the case of parallel transitions. The results of PAD, calculated with different ℓ_max on different atomic centres, reveal the important role of the d (ℓ = 2) partial wave for the S atom in the partial wave decompositions of photoelectron wavefunctions.

An exchange energy functional is proposed and tested for obtaining a class of excited-state energies using density functional formalism. The functional is the excited-state counterpart of the local-density approximation functional for the ground state. It takes care of the state dependence of the energy functional and leads to highly accurate excitation energies.