Hugo W van der Hart

Two-photon double ionization of He

We determine generalized cross sections for two-photon double ionization of He in the photon energy region between 40.7 and 47 eV where absorption of two photons can lead to non-sequential double ionization only. The present cross sections, obtained in R-matrix Floquet theory, agree with cross sections obtained from time-dependent calculations. By examining the ratio of two-photon double ionization to two-photon single ionization, we demonstrate that core excitation effects at an intensity of 1013 W cm−2 are relatively unimportant at 45 eV, but that they are significant at other photon energies.

Energy levels, wavefunction compositions and electric dipole transitions in neutral Ca

A configuration-interaction approach, based on the use of B-spline basis sets combined with a model potential including monoelectronic and dielectronic core polarization effects, is employed to calculate term energies and wavefunctions for neutral Ca. Results are reported for singlet and triplet bound states, and some quasi-bound states above the lowest ionization limit, with angular momentum up to L = 4. Comparison with experiment and with other theoretical results shows that this method yields the most accurate energy values for neutral Ca obtained to date. Wavefunction compositions, necessary for labelling the levels, and the effects of semi-empirical polarization potentials on the wavefunctions are discussed, as are some recent identifications of doubly-excited states. It is shown that taking into account dielectronic core polarization changes the energies of the lowest terms in Ca significantly, in general by a few hundred cm-1, the effect decreasing rapidly for the higher bound states. For Rydberg states with n7 the accuracy of the results is often better than a few cm-1. For series members (or perturbers) with a pronounced 3d character the error can reach 150 cm-1. The wavefunctions are used to calculate oscillator strengths and lifetimes for a number of terms and these are compared with existing measurements. The agreement is good but points to a need for improved measurements.

B-spline methods in R-matrix theory for scattering in two-electron systems

The use of B-spline basis sets in R-matrix theory for scattering processes has been investigated. In the present approach a B-spline basis is used for the description of the inner region, which is matched to the physical outgoing wavefunctions by the R-matrix. Using B-splines, continuum basis functions can be determined easily, while pseudostates can be included naturally. The accuracy for low-energy scattering processes is demonstrated by calculating inelastic scattering cross sections for colliding on H. Very good agreement with other calculations has been obtained. Further extensions of the codes to quasi two-electron systems and general atoms are discussed as well as the application to (multi) photoionization.

Time-resolved four-wave-mixing spectroscopy for inner-valence transitions

Noncollinear four-wave-mixing (FWM) techniques at near-infrared (NIR), visible, and ultraviolet frequencies have been widely used to map vibrational and electronic couplings, typically in complex molecules. However, correlations between spatially localized inner-valence transitions among different sites of a molecule in the extreme ultraviolet (XUV) spectral range have not been observed yet. As an experimental step toward this goal, we perform time-resolved FWM spectroscopy with femtosecond NIR and attosecond XUV pulses. The first two pulses (XUV-NIR) coincide in time and act as coherent excitation fields, while the third pulse (NIR) acts as a probe. As a first application, we show how coupling dynamics between odd- and even-parity, inner-valence excited states of neon can be revealed using a two-dimensional spectral representation. Experimentally obtained results are found to be in good agreement with ab initio time-dependent R-matrix calculations providing the full description of multielectron interactions, as well as few-level model simulations. Future applications of this method also include site-specific probing of electronic processes in molecules.

Double photoionization of excited He-like atoms

We have developed a B-spline based technique for evaluating double photoionization rates for He-like atomic systems. The accuracy of the approach is tested by comparing the results from this code with available theoretical and experimental results for double ionization of the 1s2 and 1s2s 1S states of He. We require significantly more angular momenta to obtain converged results for 1s2s 1S than for 1s2, which is ascribed to the larger spatial extent of the 1s2s 1S wavefunction. By extending the calculations to Li+, Be2+, C4+ and O6+, we show that no obvious 1/Z2 scaling law applies to the ratio between double and single photoionization of the 1s2s 1S state, in contrast to the 1s2 1S state.

Two- and three-photon ionization of He between 1013 and 1014 W cm-2

We have applied the R-matrix Floquet approach to study two- and three-photon ionization of He for intensities between 1013 and 1014 W cm−2. The non-perturbative influence of the laser field manifests itself in a number of ways: resonance positions shift as a function of intensity, and the widths of both bound-state and autoionizing resonances increase due to their increased photoionization. We demonstrate that even at relatively modest intensities the widths of some low-lying autoionizing states are dominated by photoionization.

Benchmark multiphoton ionization rates for He at 390 nm

We compare single-electron multiphoton ionization rates of He obtained by the R-matrix Floquet approach with those obtained by the numerical integration of the time-dependent Schr?dinger equation. The calculations are performed at a laser wavelength of 390 nm, a wavelength accessible to Ti:sapphire lasers via frequency doubling. For intensities between 1 ? 1014 W cm?2 and 2.5 ? 1014 W cm?2, we find general agreement between the two approaches within 10%. Over this range of intensities we further find our single-ionization rates typically two orders of magnitude greater than those predicted by the ADK model. The ionization rates are strongly enhanced by resonances. This resonance structure is strongly distorted by the laser field: the 1s3d 1D state lies below the 1s3s 1S state at the present intensities.

Single- and two-photon ionization of Sr

Using the R-matrix Floquet approach we have calculated single- and two-photon ionization cross sections for Sr within a model potential approach. The frequency ranges investigated range from 0.21 to 0.27 au and from 0.105 to 0.158 au. The results are in reasonable agreement with previous experimental and theoretical work. The two-photon cross sections are dominated by the 5s6p 1Po resonance. For the m = 0 level of this resonance, we obtain photoionization cross sections ranging typically between 5 and 20 Mb. A comparison with results for Ca shows good similarity for single-photon ionization, but significantly less agreement for two-photon ionization.

Multiphoton ionization cross sections of neon and?argon

We present the results of R-matrix Floquet calculations for generalized two- and three-photon ionization cross sections of neon and argon in the XUV region. Photoionization is strongly enhanced by singly- and doubly-excited resonances. We show that interference effects between different Rydberg states can strongly modify the photoionization cross sections. The observation of three- and four-photon resonances in the two- and three-photon spectra, respectively, demonstrates the influence from higher-order processes. The cross sections may be used to estimate the intensity attained by harmonic radiation in the XUV region.

Electron-impact ionization of He+: consequences for double ionization of He in strong laser fields

A B-spline-based approach has been developed for the determination of electron-impact excitation and ionization cross sections for e--He+ collisions. Good agreement is obtained with the convergent-close-coupling approach and with experiment. In order to assist the description of electron scattering processes occurring in strong laser fields, we separate the contributions from singlet states and from triplet states. Since the cross sections for 100% singlet scattering are significantly larger than those for 100% triplet scattering, one needs to take the symmetry of the initial state into account when obtaining estimates for double ionization in strong laser fields using a recollision model.