M. A. Lysaght

The RMT method for many-electron atomic systems in intense short-pulse laser light

We describe a new ab initio method for solving the time-dependent Schrödinger equation for multi-electron atomic systems exposed to intense short-pulse laser light. We call the method the R-matrix with time-dependence (RMT) method. Our starting point is a finite-difference numerical integrator (HELIUM), which has proved successful at describing few-electron atoms and atomic ions in strong laser fields with high accuracy. By exploiting the R-matrix division-of-space concept, we bring together a numerical method most appropriate to the multi-electron finite inner region (R-matrix basis set) and a different numerical method most appropriate to the one-electron outer region (finite difference). In order to exploit massively parallel supercomputers efficiently, we time-propagate the wavefunction in both regions by employing Arnoldi methods, originally developed for HELIUM.

Signatures of collective electron dynamics in the angular distributions of electrons ejected during ultrashort laser pulse interactions with C

We use the time-dependent R-matrix approach to investigate an ultrashort pump–probe scheme to observe collective electron dynamics in C+ driven by the repulsion of two equivalent p electrons. By studying the two-dimensional momentum distributions of the ejected electron as a function of the time-delay between an ultrashort pump pulse and an ionizing ultrashort probe pulse it is possible to track the collective dynamics inside the C+ ion in the time domain.

Interference between competing pathways in atomic harmonic generation.

We investigate the influence of the autoionizing 3s3p(6)nℓ resonances on the fifth harmonic generated by 200-240 nm laser fields interacting with Ar. To determine the influence of a multielectron response we develop the capability within time-dependent R-matrix theory to determine the harmonic spectra generated. The fifth harmonic is affected by interference between the response of a 3s electron and the response of a 3p electron, as demonstrated by the asymmetric profiles in the harmonic yields as functions of wavelength.

Ab Initio Methods for Few- and Many-Electron Atomic Systems in Intense Short-Pulse Laser Light

We describe how we have developed an ab initio method for solving the time-dependent Schrodinger equation for multielectron atomic systems exposed to intense short-pulse laser light. Our starting point for this development is to take over the algorithms and numerical methods employed in the HELIUM code we formerly developed and which has proved highly successful at describing few-electron atoms and atomic ions in strong laser fields. We describe how we have extended the underlying methods of HELIUM to describe multielectron systems exposed to intense short-pulse laser light. We achieve this extension through exploiting the powerful R-matrix division-of-space concept to bring together a numerical method (basis set) most appropriate to the multielectron finite inner region and a different numerical method (finite difference) most appropriate to the one-electron outer region. In order for the method to exploit massively parallel supercomputers efficiently, we time-propagate the wave function in both regions by employing schemes based on the Arnoldi method, long employed in HELIUM.

Choice of dipole operator gauge in time-dependent R-matrix theory

We investigate multi-photon ionization of helium using the time-dependent R-matrix method in order to assess the best choice of gauge for the description of the laser field when the system under investigation is a multi-electron system. Ionization probabilities are obtained using the length gauge and the velocity gauge and various He basis sets, when a minimum of three or four photons need to be absorbed to achieve ionization. The probabilities are found to converge for both gauges as the number of orbitals used in the basis set increases, but they are more consistent in the length gauge. Ionization probabilities can be compared to those derived from other theoretical calculations. Agreement is within 10% when ionization requires absorption of at least three photons, but the differences increase to 20–50% when absorption of four photons is required. Analysis of the multi-photon matrix elements provides further evidence for better consistency in the length gauge than the velocity gauge when high-lying states are excluded from the calculations, which is, at present, unavoidable for a multi-electron system.

Ultrafast correlated-electron dynamics and atomic structure

We use the Time Dependent R-Matrix (TDRM) method to investigate the ultrafast dynamics of the 2s2p2 electron configuration in the C+ ion for M=0 and M=1 alignments by calculating ionization probabilities as a function of the delay time between laser pulses in an ultrafast pump-probe scheme.

The RMT method for describing many-electron atoms in intense short laser pulses

We describe how we have extended the underlying methods of the HELIUM code to describe multi-electron systems exposed to intense short-pulse laser light. We achieve this extension through exploiting the powerful R-matrix division-of-space concept to bring together a numerical method (basis set) most appropriate to the multi-electron finite inner region and a different numerical method (finite difference) most appropriate to the one-electron outer region. In order for the method to exploit massively parallel supercomputers efficiently, we time-propagate the wave function in both regions by employing schemes based on the Arnoldi method, long employed in HELIUM.

Multi-electron Dynamics using Time Dependent R-Matrix theory

We demonstrate the versatility and capability of the Time Dependent R-Matrix (TDRM) Theory for describing multi-electron dynamics in general atomic targets by applying it to multi-photon ionization of Argon and Aluminium.

Time delay between photoemission from the 2p and 2s subshells of Neon atoms

The R-Matrix incorporating Time (RMT) method is a new ab initio method for solving the time-dependent Schrodinger equation for multi-electron atomic systems exposed to intense short-pulse laser light. We have employed the RMT method to investigate the delay in the photoemission of an electron liberated from a 2p orbital in a neon atom with respect to one released from a 2s orbital. Using attosecond streaking methods, an experimental group measured this time delay to be twenty one attoseconds. We report RMT calculations of this time delay and demonstrate that such precise phase-sensitive information can be calculated using the new multi-electron RMT method.

Influence of excitation pulse length on a correlated two-electron wave packet

We employ the time-dependent R-matrix approach to investigate how ultrafast dynamics in the excited 2s2p2 configuration of C+ is affected by the length of the excitation pulse. The excitation pulse is tuned closer to the 2D state than to the 2S state. We demonstrate that, by varying the excitation pulse length, we can choose an excitation of the 2s2p2 configuration which is dominated by the 2D state or an excitation which is dominated by a breathing motion between the uncoupled state and the singlet states.