Ultrafast Atomic Dynamics in Arbitrary Light Fields
This thesis reports the development, implementation and application of advanced theoretical techniques for simulating multielectron atoms and atomic ions in light fields of arbitrary complexity. An ab initio R-matrix with time-dependence (RMT) theory for ultrafast atomic processes in arbitrarily polarised laser fields is described. The theory is applicable to multielectron atoms and atomic ions subject to ultrashort laser pulses with any given ellipticity, and generalises previous time-dependent R-matrix techniques restricted to linearly polarised fields. The accuracy and predictive capabilities of this approach are demonstrated through investigations of two-photon ionisation in He, and of single-photon detachment from F- in circularly polarised, extreme-ultraviolet (XUV) light fields. Furthermore, a non-dipole variant of the RMT method is proposed, which incorporates the lowest-order non-dipole corrections (electric quadrupole and magnetic dipole) to the laser-atom interaction Hamiltonian. This progress enables the modelling of ultrafast electron dynamics in complex atoms and ions to order 1/c, and expands the predictive scope of the theory to a range of laser intensities and wavelengths in which relativistic-kinematic corrections are negligible, but the laser magnetic field is significant. Following this, a range of numerical studies are presented concerning the ultrafast dynamics of laser-driven atomic ions. The RMT method is used to investigate the spectroscopic potential of XUV-initiated high-harmonic generation in Ar+, and to characterise electron rotational asymmetry in few-photon detachment from F-, initiated by circularly polarised laser fields of visible and near-infrared wavelengths. Finally, a semi-analytical density-matrix model, predicated on a Keldysh-type theory, is developed to rationalise recent experimental observations concerning orbital alignment and quantum beats in C, Si and Ge atoms, produced by strong-field detachment from their anions. We confirm a fundamental criterion for coherent spin-orbit wavepacket generation in this process, and demonstrate that its violation may account for the observed suppression of quantum beats in polarisation-anisotropy measurements.
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