7 research outputs found
Sensitivity of nonlinear photoionization to resonance substructure in collective excitation
Collective behaviour is a characteristic feature in many-body systems, important for developments in fields such as magnetism, superconductivity, photonics and electronics. Recently, there has been increasing interest in the optically nonlinear response of collective excitations. Here we demonstrate how the nonlinear interaction of a many-body system with intense XUV radiation can be used as an effective probe for characterizing otherwise unresolved features of its collective response. Resonant photoionization of atomic xenon was chosen as a case study. The excellent agreement between experiment and theory strongly supports the prediction that two distinct poles underlie the giant dipole resonance. Our results pave the way towards a deeper understanding of collective behaviour in atoms, molecules and solid-state systems using nonlinear spectroscopic techniques enabled by modern short-wavelength light sources
Adiabaticity and diabaticity in strong-field ionization
If the photon energy is much less than the electron binding energy,
ionization of an atom by a strong optical field is often described in terms of
electron tunneling through the potential barrier resulting from the
superposition of the atomic potential and the potential associated with the
instantaneous electric component of the optical field. In the strict tunneling
regime, the electron response to the optical field is said to be adiabatic, and
nonadiabatic effects are assumed to be negligible. Here, we investigate to what
degree this terminology is consistent with a language based on the so-called
adiabatic representation. This representation is commonly used in various
fields of physics. For electronically bound states, the adiabatic
representation yields discrete potential energy curves that are connected by
nonadiabatic transitions. When applying the adiabatic representation to optical
strong-field ionization, a conceptual challenge is that the eigenstates of the
instantaneous Hamiltonian form a continuum; i.e., there are no discrete
adiabatic states. This difficulty can be overcome by applying an
analytic-continuation technique. In this way, we obtain a rigorous
classification of adiabatic states and a clear characterization of
(non)adiabatic and (non)diabatic ionization dynamics. Moreover, we distinguish
two different regimes within tunneling ionization and explain the dependence of
the ionization probability on the pulse envelope.Comment: 9 pages, 6 figure
State-resolved attosecond reversible and irreversible dynamics in strong optical fields
Strong-field ionization (SFI) is a key process for accessing real-time quantum dynamics of electrons on the attosecond timescale. The theoretical foundation of SFI was pioneered in the 1960s, and later refined by various analytical models. While asymptotic ionization rates predicted by these models have been tested to be in reasonable agreement for a wide range of laser parameters, predictions for SFI on the sub-laser-cycle timescale are either beyond the scope of the models or show strong qualitative deviations from full quantum-mechanical simulations. Here, using the unprecedented state specificity of attosecond transient absorption spectroscopy, we follow the real-time SFI process of the two valence spin–orbit states of xenon. The results reveal that the irreversible tunnelling contribution is accompanied by a reversible electronic population that exhibits an observable spin–orbit-dependent phase delay. A detailed theoretical analysis attributes this observation to transient ground-state polarization, an unexpected facet of SFI that cannot be captured by existing analytical models that focus exclusively on the production of asymptotic electron/ion yields