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We describe an ab initio nonperturbative time-dependent R-matrix theory for ultrafast atomic processes. This theory enables investigations of the interaction of few-femtosecond and -attosecond pulse lasers with complex multielectron atoms and atomic ions. A derivation and analysis of the basic equations are given, which propagate the atomic wave function in the presence of the laser field forward in time in the internal and external R-matrix regions. To verify the accuracy of the approach, we investigate two-photon ionization of Ne irradiated by an intense laser pulse and compare current results with those obtained using the R-matrix Floquet method and an alternative time-dependent method. We also verify the capability of the current approach by applying it to the study of two-dimensional momentum distributions of electrons ejected from Ne due to irradiation by a sequence of 2 as light pulses in the presence of a 780 nm laser field.

The R-matrix incorporating time (RMT) method is a method developed recently for solving the time-dependent Schrödinger equation for multielectron atomic systems exposed to intense short-pulse laser light. We have employed the RMT method to investigate the time 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 following absorption of an attosecond xuv pulse. Time delays due to xuv pulses in the range 76-105 eV are presented. For an xuv pulse at the experimentally relevant energy of 105.2 eV, we calculate the time delay to be 10.2 ± 1.3 attoseconds (as), somewhat larger than estimated by other theoretical calculations, but still a factor of 2 smaller than experiment. We repeated the calculation for a photon energy of 89.8 eV with a larger basis set capable of modeling correlated-electron dynamics within the neon atom and the residual Ne + ion. A time delay of 14.5 ± 1.5 as was observed, compared to a 16.7 ± 1.5 as result using a single-configuration representation of the residual Ne + ion.

We have developed an ab initio R-matrix-based approach for the time-dependent description of the multielectron response of complex atoms irradiated by intense ultrashort laser pulses. Ionization rates for Ar irradiated by intense 390 nm laser light are in excellent agreement with the most accurate rates available. The present approach shows how complex atomic structure affects the atomic response in the time domain. The approach extends the range of intensities, for which accurate ionization rates can be obtained for complex multielectron atoms, and the amount of atomic structure that can be included at high intensities, compared to the R-matrix Floquet approach. Because of these advantages, the approach is highly promising for the ab initio investigation of the multielectron response to intense ultrashort light pulses.