71 research outputs found
Stochastic stimulated electronic x-ray Raman spectroscopy
Resonant inelastic x-ray scattering (RIXS) is a well-established tool for
studying electronic, nuclear and collective dynamics of excited atoms,
molecules and solids. An extension of this powerful method to a time-resolved
probe technique at x-ray free electron lasers (XFELs) to ultimately unravel
ultrafast chemical and structural changes on a femtosecond time scale is often
challenging, due to the small signal rate in conventional implementations at
XFELs that rely on the usage of a monochromator set up to select a small
frequency band of the broadband, spectrally incoherent XFEL radiation. Here, we
suggest an alternative approach, based on stochastic spectroscopy, that uses
the full bandwidth of the incoming XFEL pulses. Our proposed method is relying
on stimulated resonant inelastic x-ray scattering, where in addition to a pump
pulse that resonantly excites the system a probe pulse on a specific electronic
inelastic transition is provided, that serves as seed in the stimulated
scattering process. The limited spectral coherence of the XFEL radiation
defines the energy resolution in this process and stimulated RIXS spectra of
high resolution can be obtained by covariance analysis of the transmitted
spectra. We present a detailed feasibility study and predict signal strengths
for realistic XFEL parameters for the CO molecule resonantly pumped at the
O1s-{\pi}* transition. Our theoretical model describes the evolution of the
spectral and temporal characteristics of the transmitted x-ray radiation, by
solving the equation of motion for the electronic and vibrational degrees of
freedom of the system self consistently with the propagation by Maxwell's
equations
Stimulated resonant inelastic x-ray scattering with chirped, broadband pulses
We present an approach for initiating and tracing ultra-fast electron
dynamics in core-excited atoms, molecules and solids. The approach is based on
stimulated resonant inelastic x-ray scattering induced by a single, chirped,
broadband XUV/x-ray pulse. A first interaction with this pulse prepares a
core-excited state wave packet by resonant core-excitation. A second
interaction with the pulse at a later time induces the transition to
valence-excited states which is associated with stimulated emission. The
preparation of the core-excited wave packet and the transition from the
core-excited states to the valence-excited states occur at distinct
chirp-dependent times. As a consequence, the stimulated emission carries
information about the time evolution of the core-excited state wave packet
Quantum-beat Auger spectroscopy
The concept of nonlinear quantum-beat pump-probe Auger spectroscopy is
introduced by discussing a relatively simple four-level model system. We
consider a coherent wave packet involving two low-lying states that was
prepared by an appropriate pump pulse. This wave packet is subsequently probed
by a weak, time-delayed probe pulse with nearly resonant coupling to a
core-excited state of the atomic or molecular system. The resonant Auger
spectra are then studied as a function of the duration of the probe pulse and
the time delay. With a bandwidth of the probe pulse approaching the energy
spread of the wave packet, the Auger yields and spectra show quantum beats as a
function of pump-probe delay. An analytic theory for the quantum-beat Auger
spectroscopy will be presented, which allows for the reconstruction of the wave
packet by analyzing the delaydependent Auger spectra. The possibility of
extending this method to a more complex manifold of electronic and vibrational
energy levels is also discussed.Comment: 13 papees,6 figure
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
Theoretical Study of Molecular Electronic and Rotational Coherences by High-Harmonic Generation
The detection of electron motion and electronic wavepacket dynamics is one of
the core goals of attosecond science. Recently, choosing the nitric oxide (NO)
molecule as an example, we have introduced and demonstrated a new experimental
approach to measure coupled valence electronic and rotational wavepackets using
high-harmonic generation (HHG) spectroscopy [Kraus et al., Phys. Rev. Lett.
111, 243005 (2013)]. A short outline of the theory to describe the combination
of the pump and HHG probe process was published together with an extensive
discussion of experimental results [Baykusheva et al., Faraday Discuss 171, 113
(2014)]. The comparison of theory and experiment showed good agreement on a
quantitative level. Here, we present the generalized theory in detail, which is
based on a generalized density matrix approach that describes the pump process
and the subsequent probing of the wavepackets by a semiclassical quantitative
rescattering approach. An in-depth analysis of the different Raman scattering
contributions to the creation of the coupled rotational and electronic
spin-orbit wavepackets is made. We present results for parallel and
perpendicular linear polarizations of the pump and probe laser pulses.
Furthermore, an analysis of the combined rotational-electronic density matrix
in terms of irreducible components is presented, that facilitates
interpretation of the results.Comment: 14 figure
- …