[en] Coincident measurement of the Auger electron and fragmentation momenta emitted after carbon core-level photoionization of acetylene has yielded new understanding of how the dication fragments. Ab initio calculations and experimental data, including body-frame Auger angular distributions, are used to identify the parent electronic states and together yield a comprehensive map of the dissociation pathways which include surface crossings and barriers to direct dissociation. The Auger angular distributions show evidence of core-hole localization

[en] The ionization and charge separation processes of nanoplasmas created by resonant excitation of atomic clusters in intense soft x-ray pulses have been investigated. Through irradiation with femtosecond pulses from the FLASH free electron laser (FEL) at λ = 13.7 nm and power densities exceeding 10¹⁴ W cm^-2 the clusters are highly ionized with transient atomic charge states up to 9+. Variation of the cluster composition from pristine to doped and core-shell systems allows tracking of the spatial origin and charge states of the fragments yielding insight into the nanoplasma dynamics. The data give evidence for efficient charge redistribution processes leading to a Coulomb explosion of the cluster outer part and recombination of the nanoplasma core. The experiments show qualitatively different processes for (soft) x-ray produced nanoplasmas from the optical (IR) strong-field regime where the clusters disintegrate completely in a Coulomb explosion. (fast track communication)

[en] We discuss the dynamics of an open two-mode Bose-Hubbard system subject to phase noise and particle dissipation. Starting from the full many-body dynamics described by a master equation the mean-field limit is derived resulting in an effective non-Hermitian (discrete) Gross-Pitaevskii equation which has been introduced only phenomenologically up to now. The familiar mean-field phase-space structure is substantially altered by the dissipation. In particular, the character of the fixed points shows an abrupt transition from elliptic or hyperbolic to attractive or repulsive, respectively. This reflects the metastable behaviour of the corresponding many-body system which surprisingly also leads to a significant increase of the purity of the condensate. A comparison of the mean-field approximation to simulations of the full master equation using the Monte Carlo wavefunction method shows an excellent agreement for wide parameter ranges. (fast track communication)

[en] The electron impact excitation of the Li₂ molecule was studied for the first time by measuring the autoionizing spectra of lithium vapour near the 1s-core excitation threshold. The broad line at 51.4 eV ejected-electron energy was observed in spectra at impact energies below 60 eV. It was tentatively assigned to the decay of the (1s → 2σ_u)³Σ_u⁺ lowest triplet autoionizing state of Li₂ with the excitation threshold of 56.5 ± 0.1 eV. The (1s → 2σ_u)¹Σ_u⁺ and (1s → 2σ_g)¹Σ_u¹ singlet autoionizing states were also observed in spectra at impact energies above 70 eV. The excitation cross-sections for the observed molecular states were estimated to be of the order of magnitude of 10^-17-10^-16 cm² at impact energies ≤80 eV. These values are on average two orders of magnitude larger than the cross-sections for the 1s2snl lowest atomic autoionizing states

[en] The mobility of C⁺ in He gas has been measured using a very low temperature drift tube mass spectrometer at 77 and 4.3 K. Ion mobility separation between the ground-state C⁺ (²P^o) and the metastable C⁺ (⁴P) ions has been observed at the effective temperature T_eff from 550 to 1500 K. The calculation of the mobility based on the classical momentum transfer theory has been carried out using the interaction potential provided by the multi-reference electron correlation method. The calculated mobility of C⁺ qualitatively agrees with the measurement, confirming the accuracy of the interaction potential between the metastable ion and He

[en] We describe elastic electron scattering data at high momentum transfer (between ∼20 and ∼40 au) from methane and Xe. Under these conditions there is a significant recoil energy transferred to the target and electrons scattered elastically from methane are separated into two peaks: one due to electrons scattered from carbon, and one due to electrons scattered from hydrogen. The separation of these peaks is within a few per cent identical to what is expected for scattering from isolated C and H atoms. The peak due to electrons scattered from C, is again shifted compared to the peak of electrons scattered from Xe. The Xe, C and H peaks all have clearly different widths. The C and H peak areas are compared. Their relative intensity shows no substantial deviation (<10%) from what is expected based on either simple Rutherford cross sections, or state-of-the-art elastic scattering calculations. The latter observation is in strong contrast to electron scattering results from a gaseous equimolar H₂-D₂ mixture and from electron and neutron scattering results from polymers at similar momentum transfer

[en] Single-impulse three-dimensional magnetic focusing of vertically launched cold atoms has been observed. Four different configurations of the lens were used to vary the relative radial and axial focusing properties. Compact focused clouds of ⁸⁵Rb were seen for all four configurations. It is shown that an atom-optical ray matrix approach for describing the lensing action is insufficient. Numerical simulation using a full approximation to the lens's magnetic field shows very good agreement with the radial focusing properties of the lens. However, the axial (vertical direction) focusing properties are less well described and the reasons for this are discussed

[en] We theoretically investigate the high-order harmonic generation (HHG) when a helium ion model is irradiated by a two-colour laser field, which is synthesized by a 5 fs pulse at 800 nm and a 10 fs pulse at 394 nm. The harmonic spectrum reveals a double-plateau structure and an extreme ultraviolet supercontinuum spectrum in the lower second plateau can be superposed to produce attosecond pulses. By increasing the intensity of 10 fs pulse at 394 nm, the cutoff of the second plateau is extended effectively and the harmonics in the second plateau become in phase. Consequently, an isolated 63 as pulse is observed. These results can be explained in terms of the time-frequency characteristics of attosecond pulses and a semiclassic three-step model

[en] We propose and simulate an attosecond pump-probe scheme applied to an ensemble of non-aligned (or partially aligned) diatomic molecules. Non-dissociative dynamics initiated by an ultraviolet pump pulse are probed by photoionizing the molecule with an attosecond extreme-ultraviolet probe pulse. Photoelectron spectra recorded for different delays between the pulses exhibit signatures of streaking and sub-femtosecond quantum beating. If more than one vibrational state is excited, the nuclear motion modulates the visibility of quantum interference effects

[en] Accurate normalized differential cross-sections for elastic electron scattering from H₂ and N₂ are reported. These measurements are obtained using the relative flow method with helium as the standard (calibrating) gas, except that a thin aperture source of gas is used, which obviates the need to know the gas kinetic molecular diameters of either the calibration gas (He) or the unknown gas (H₂ or N₂) whose differential cross-sections need to be determined. Our present measurements are found to be in very good agreement with past differential elastic electron scattering measurements using the relative flow with tube collimating sources and independently vindicate the application of the conventional relative flow method for 'hard-sphere' gases. They also resolve some observed differences between the differential cross-sections in the low incident energy region. They also provide a good test of our relative flow method for two well-known targets