41 research outputs found
Attosecond transient absorption of a bound wave packet coupled to a smooth continuum
We investigate the possibility to use transient absorption of a coherent
bound electron wave packet in hydrogen as an attosecond pulse characterization
technique. In recent work we have shown that photoionization of such a coherent
bound electron wave packet opens up for pulse characterization with
unprecedented temporal accuracy --- independent of the atomic structure ---
with maximal photoemission at all kinetic energies given a wave packet with
zero relative phase [Pabst and Dahlstr\"om, Phys. Rev. A, 94, 13411 (2016)].
Here, we perform numerical propagation of the time-dependent Schr\"odinger
equation and analytical calculations based on perturbation theory to show that
the energy-resolved maximal absorption of photons from the attosecond pulse
does not uniquely occur at zero relative phase of the initial wave packet.
Instead, maximal absorption occurs at different relative wave packet phases,
distributed as a non-monotonous function with a smooth shift across
the central photon energy (given a Fourier-limited Gaussian pulse). Similar
results are found also in helium. Our finding is surprising because it implies
that the energy-resolved photoelectrons are not mapped one-to-one with the
energy-resolved absorbed photons of the attosecond pulse.Comment: 10 pages, 8 figues, submitted as part of a Special Issue on Emerging
Attosecond Technologies in Journal of Optic
Attosecond dynamics of light-induced resonant hole transfer in high-order-harmonic generation
We present a study of high-order-harmonic generation (HHG) assisted by
extreme ultraviolet (XUV) attosecond pulses, which can lead to the excitation
of inner-shell electrons and the generation of a second HHG plateau. With the
treatment of a one-dimensional model of krypton, based on time-dependent
configuration interaction singles (TDCIS) of an effective two-electron system,
we show that the XUV-assisted HHG spectrum reveals the duration of the
semiclassical electron trajectories. The results are interpreted by the
strong-field approximation (SFA) and the importance of the hole transfer during
the tunneling process is emphasized. Finally, coherent population transfer
between the inner and outer holes with attosecond pulse trains is discussed.Comment: 13 pages, 8 figure
Attosecond photoionization dynamics with stimulated core-valence transitions
We investigate ionization of neon atoms by an isolated attosecond pump pulse
in the presence of two coherent extreme ultraviolet or x-ray probe fields. The
probe fields are tuned to a core-valence transition in the residual ion and
induce spectral shearing of the photoelectron distributions. We show that the
photoelectron-ion coincidence signal contains an interference pattern that
depends on the temporal structure of the attosecond pump pulse and the
stimulated core-valence transition. Many-body perturbation theory is used to
compute "atomic response times" for the processes and we find strikingly
different behavior for stimulation to the outer-core hole (2p - 2s) and
stimulation to the inner-core hole (2p - 1s). The response time of the
inner-core transition is found to be comparable to that of state-of-the-art
laser-based characterization techniques for attosecond pulses.Comment: 12 pages, 5 figure
Multiphoton interaction phase shifts in attosecond science
Ab initio simulations of a range of interferometric experiments are used to
identify a strong dependence on multiphoton phase shifts in above-threshold
ionization. A simple rule of thumb for interaction phase shifts is derived to
explain both the conservation of photoelectron yield and its absolute
CEP-dependence. For instance, it is found that interferometric above-threshold
ionization experiments are shifted by relative to RABBIT experiments,
and that there is no RABBIT-term in a laser-assisted photoionization experiment
with odd and even harmonics. Thus, our work helps to resolve the issues of
interpretation of quantum dynamics in attosecond and free-electron laser
sciences
Photoelectron signature of dressed-atom stabilization in intense XUV field
Non-perturbative resonant multiphoton ionization is studied using the
resolvent operator technique. Scaling parameters for effective two-level
Hamiltonians are computed for hydrogen and helium atoms to provide a
quantitative description of Rabi oscillations at XUV wavelengths, which were
recently observed using a seeded Free-Electron Laser [S. Nandi et al., Nature
608, 488-493 (2022)]. The resulting photoelectron spectra exhibit a range of
Autler-Townes doublets, which are studied for different intensities, detunings
and interaction times. We identify a photoelectron signature that originates
from stabilization against ionization of helium atoms interacting with intense
circularly polarized XUV light. Thus, our work shows how it is possible to test
the prediction of dressed-atom stabilization by Beers and Armstrong [B. L.
Beers and L. Armstrong, Phys. Rev. A 12, 2447 (1975)], without the demanding
requirement of atomic saturation in the time domain.Comment: 14 pages, 6 figures, 3 tables; accepted versio
Gauge-invariant absorption of light from a coherent superposition of states
Absorption and emission of light is studied theoretically for excited atoms
in coherent superposition of states subjected to isolated attosecond pulses in
the extreme ultraviolet range. A gauge invariant formulation of transient
absorption theory is motivated using the energy operator from Yang's gauge
theory. The interaction, which simultaneously couples both bound and continuum
states, is simulated by solving the time dependent Schr\"odinger equation for
hydrogen and neon atoms. A strong dependence on the angular momentum and the
relative phase of the states in the superposition is observed. Perturbation
theory is used to disentangle the fundamental absorption processes and a rule
is established to interpret the complex absorption behaviour. It is found that
non-resonant transitions are the source of asymmetry in energy and phase, while
resonant transitions to the continuum contribute symmetrically to absorption of
light from coherent superpositions of states
Pulse analysis by delayed absorption from a coherently excited atom
In this tutorial we provide a short review of attosecond pulse
characterization techniques and a pedagogical account of a recently proposed
method called Pulse Analysis by Delayed Absorption (PANDA) [Pabst and
Dahlstr\"om, Phys. Rev. A, 94, 13411 (2016)]. We discuss possible
implementations of PANDA in alkali atoms using either principal quantum number
wave packets or spin-orbit wave packets. The main merit of the PANDA method is
that it can be used as a pulse characterization method that is free from atomic
latency effects, such as scattering phase shifts and long-lived atomic
resonances. Finally, we propose that combining the PANDA method with
angle-resolved photoelectron detection should allow for experimental
measurements of attosecond delays in photoionization from bound wave packets on
the order of tens of attoseconds.Comment: 12 pages, 7 figure
Frustrated tunneling dynamics in ultrashort laser pulses
We study a model for frustrated tunneling ionization using ultrashort laser
pulses. The model is based on the strong field approximation and it employs the
saddle point approximation to predict quasiclassical trajectories that are
captured on Rydberg states. We present a classification of the saddle-point
solutions and explore their behavior as functions of angular momentum of the
final state, as well as the carrier--envelope phase (CEP) of the laser pulse.
We compare the final state population computed by the model to results obtained
by numerical propagation of the time-dependent Schr\"odinger equation (TDSE)
for the hydrogen atom. While we find qualitative agreement in the CEP
dependence of the populations in principal quantum numbers, , the
populations to individual angular momentum channels, , are found to be
inconsistent between model and TDSE. Thus, our results show that improvements
of the quasiclassical trajectories are in order for a quantitative model of
frustrated tunneling ionizaiton