| Summary: | This thesis focuses on the application of a Keldysh-type approach (KTA) using the adiabatic saddle-point method to describe multiphoton detachment of negative ions by few-cycle mid-infrared laser pulses. The physical phenomena investigated in this thesis include direct photodetachment, orbital alignment of the residual atom and high-energy photoelectron rescattering. Photoelectron ejection spectra for the fluorine anion are computed in the multiphoton and tunnelling regimes for a range of laser parameters, and are analysed in relation to wave packet interferences and above-threshold channel closures. The KTA results demonstrate good agreement with full numerical calculations obtained using R-matrix theory with time-dependence (RMT). This study is developed to consider orbital alignment in coherently excited atomic fine-structure manifolds produced by short-pulse-detachment of half-filled shell carbon, silicon and germanium anions. Under the assumption of an instantaneous pulse, simple analytical formulae are derived which describe the quantum beats in agreement with pump-probe experiments. Subsequently, density matrix calculations are performed using the KTA for the doublet spin-orbit states of halogen atoms (F, Cl, Br). The degree of coherence is shown to be a near universal function of the scaled laser pulse duration for each atom, which provides a clear description of the effect of decoherence in heavier systems. Finally, the KTA is extended to study electron rescattering in strong-field detachment of negative ions. Based on the classical model, the saddle-point approximation reveals the recollisional trajectories which contribute to produce interference effects within the high-energy spectrum. The results obtained from this model reproduce the extended energy plateau for the F- anion in good quantitative agreement with RMT simulations, which demonstrates the efficiency and accuracy of KTA-type theories in describing high-energy processes from negative ions.
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