TS
Transition state
TSH
Trajectory surface hopping
TXAS
Transient X-ray absorption spectroscopy
XANES
X-Ray absorption nearedge structure
XCH
Excited core hole
XDQC
X-Ray double-quantum-coherence
XES
X-Ray emission spectroscopy
XFEL
X-Ray free electron laser
XPS
X-Ray photoelectron spectroscopy
1 Introduction
In linear spectroscopy experiments, the incident light field interacts with the studied
system only once and is relatively weak compared to the intrinsic interaction
potential of the system. The signal can be considered as the linear response of the
system to the perturbation of the light field. Linear spectroscopy provides useful
information about the atomic and electronic structure of the system. Nonlinear
spectroscopy techniques provide more detailed information. Nonlinear spectroscopy [1] employs multiple light fields to probe the correlations between different
spectral features. Many controlling factors, such as the frequencies, wavevectors,
and polarizations of the light fields and the time delays between them, can be varied
so that detection of the correlation between certain spectroscopy features and their
dynamics is possible.
Nonlinear optical spectroscopy became feasible soon after the invention of the
laser in the 1960s. With the development of laser technology, pulse durations were
reduced from picoseconds (1970s) to femtoseconds (1980s) [2], and now to
attoseconds [3]. Nonlinear infrared and optical spectroscopy techniques have
proved to be very successful for studying various excited state couplings and
dynamics in molecules and materials [4, 5]. Nonlinear spectroscopy techniques in
the X-ray regime made possible by new X-ray free electron lasers (XFEL) and high
harmonic generation (HHG) sources provide a unique window into the motions of
electrons, holes, and excitons in molecules and materials. Because of their broad
bandwidth (about 10 eV for a 100-attosecond pulse), X-ray pulses can create
coherent superpositions of many excited states localized at the target atoms. In
analogy to how optical pulses manipulate molecular vibrations, attosecond X-ray
pulses triggering and probing valence excited state dynamics have been considered
recently [6] and explored experimentally [7–10]. Sequences of coherent broadband
X-ray pulses can reveal the dynamics of nuclei and electrons in molecules with
attosecond temporal, and nanometer spatial resolution.
X-Ray pulses can be used in various ways:
• Off-resonant diffraction detects the charge density. This technique can be
extended to multiple dimensions to provide multipoint correlations of the charge
density [11, 12].
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Y. Zhang et al.
Transition state
TSH
Trajectory surface hopping
TXAS
Transient X-ray absorption spectroscopy
XANES
X-Ray absorption nearedge structure
XCH
Excited core hole
XDQC
X-Ray double-quantum-coherence
XES
X-Ray emission spectroscopy
XFEL
X-Ray free electron laser
XPS
X-Ray photoelectron spectroscopy
1 Introduction
In linear spectroscopy experiments, the incident light field interacts with the studied
system only once and is relatively weak compared to the intrinsic interaction
potential of the system. The signal can be considered as the linear response of the
system to the perturbation of the light field. Linear spectroscopy provides useful
information about the atomic and electronic structure of the system. Nonlinear
spectroscopy techniques provide more detailed information. Nonlinear spectroscopy [1] employs multiple light fields to probe the correlations between different
spectral features. Many controlling factors, such as the frequencies, wavevectors,
and polarizations of the light fields and the time delays between them, can be varied
so that detection of the correlation between certain spectroscopy features and their
dynamics is possible.
Nonlinear optical spectroscopy became feasible soon after the invention of the
laser in the 1960s. With the development of laser technology, pulse durations were
reduced from picoseconds (1970s) to femtoseconds (1980s) [2], and now to
attoseconds [3]. Nonlinear infrared and optical spectroscopy techniques have
proved to be very successful for studying various excited state couplings and
dynamics in molecules and materials [4, 5]. Nonlinear spectroscopy techniques in
the X-ray regime made possible by new X-ray free electron lasers (XFEL) and high
harmonic generation (HHG) sources provide a unique window into the motions of
electrons, holes, and excitons in molecules and materials. Because of their broad
bandwidth (about 10 eV for a 100-attosecond pulse), X-ray pulses can create
coherent superpositions of many excited states localized at the target atoms. In
analogy to how optical pulses manipulate molecular vibrations, attosecond X-ray
pulses triggering and probing valence excited state dynamics have been considered
recently [6] and explored experimentally [7–10]. Sequences of coherent broadband
X-ray pulses can reveal the dynamics of nuclei and electrons in molecules with
attosecond temporal, and nanometer spatial resolution.
X-Ray pulses can be used in various ways:
• Off-resonant diffraction detects the charge density. This technique can be
extended to multiple dimensions to provide multipoint correlations of the charge
density [11, 12].
276
Y. Zhang et al.