Ab initio, nonperturbative calculations of laser-induced continuum structure in helium

Published versio

Transparency of a short laser pulse via decay interference in a closed V-type system

Published versio

Propagation and nonlinear generation dynamics in a coherently prepared four-level system

Published versio

Photoionization By Ultrashort Pulses In The High-intensity Limit

The ionization of one-electron model atoms interacting with an intense few-cycle laser pulse is calculated by integrating numerically the time-dependent Schrödinger equation. If the pulse is sufficiently intense and short, the electron behaves as if it were not interacting with the binding potential. The probability that the atom is ionized or is excited to a given bound state then depends only on the extent to which the electronic wave packet spreads during the pulse; in particular, it does not depend on the frequency or on the peak intensity. Calculations at 800 nm wavelength for a two-dimensional model of a circular state suggest, however, that radiation pressure is likely to prevent the observation of the effect in a real atom. At intensities below 1016 W/cm2, however, the main effect of the magnetic field component of the incident pulse is to redistribute the population among bound states, not to increase photoionization.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Resonant 2-photon Ionization of Atoms Via 2 Coupled Rydberg Series

We present non-perturbative ab initio calculations of resonant two-photon ionization in which the ground state of a complex atom is resonantly coupled by a laser field to two Rydberg states, each of which belongs to a different Rydberg series. As an example, we have studied the resonant two-photon ionization of neon via the 5s and 4d Rydberg states using the R-matrix-Floquet theory. We find that for weak fields, where the Rydberg states are not overlapping, interference effects are already present which lead to a suppression of ionization for a particular laser frequency. For strong fields, the field induced coupling between the two Rydberg states can strongly modify the weak-field behaviour. We give a qualitative explanation of these phenomena by considering the dressed states of a simple three-level model atom coupled to two continua. Finally we show, by varying the intensity, how the Floquet eigenstates corresponding to the ground state and the two Rydberg states of neon are adiabatically connected to the field-free states

Josephson spectroscopy of a dilute Bose-Einstein condensate in a double-well potential

The dynamics of a Bose-Einstein condensate in a double-well potential are analysed in terms of transitions between energy eigenstates. By solving the time-dependent and time-independent Gross-Pitaevskii equation in one dimension, we identify tunnelling resonances associated with level crossings, and determine the critical velocity that characterises the resonance. We test the validity of a non-linear two-state model, and show that for the experimentally interesting case, where the critical velocity is large, the influence of higher-lying states is important

Three-step Processes With Relativistic Ions

A new accelerator complex will be built over the next years at GSI-Darmstadt, capable of accelerating multicharged ions to Lorentz factors up to about 30. It will be possible to make the relativistic ions interact with ultraintense laser pulses. At high intensities, three-step recollision processes, such as high-order harmonic generation and high-order ATI, are normally severely suppressed by the magnetic field component of the laser field. However, owing to the boosts in relative frequency and intensity that can be achieved by directing the laser against the ion beams, and as shown by calculations based on a Coulomb-corrected nondipole strong field approximation, there is a range of intensities, Lorentz factors, and ion charges for which suppression is not severe even for ponderomotive energies largely exceeding 10 keV.SCOPUS: cp.jinfo:eu-repo/semantics/publishe

Singly, doubly and triply resonant multiphoton processes involving autoionizing states in magnesium

Using the R-matrix Floquet theory we have carried out non-perturbative, ab initio one- and two-colour calculations of the multiphoton ionization of magnesium with the laser frequencies chosen such that the initial state of the atom is resonantly coupled with autoionizing resonances of the atom. Good agreement is obtained with previous calculations in the low-intensity regimes. The single-photon ionization from the 3s3p(1)P(o) excited state of magnesium has been studied in the vicinity of the 3p(2) S-1(e) autoionizing resonance at non-perturbative laser intensities. Laser-induced degenerate states (LIDS) are observed for modest laser intensities. By adding a second laser which resonantly couples the 3p2 S-1(e) and 3p3d P-1(o) autoionizing levels, we show that, due to the,small width of the 3p3d P-1(o) state, LIDS occur between this state and the 3s3p P-1(o) state at intensities of the first laser below 10(10) W cm(-2). We next investigate the case in which the first laser induces a resonant two-photon coupling between the ground state and the 3p2 S-1(e) autoionizing state, while the second laser again resonantly couples the respective 3p2 S-1(e) and 3p3d( 1)P(o) autoionizing states. At weak intensities, our calculations compare favourably with recent experimental data and calculations. We show that when the intensity of the first laser is increased, the effect of an additional autoionizing state, the 4s5s S-1(e) state, becomes significant. This state is coupled to the 3p3d P-1(o) autoionizing level by one photon, inducing a triply resonant processes. We show that LIDS occur among the three autoionizing levels and we discuss their effect on the decay rate of the ground state. We consider dressed two- and three-level atoms which can be used to model the results of our calculations