electron is excited first, the N1s excitation which corresponds to peak B heavily
affects the following O1s excitation which corresponds to peak C (a strong feature
connecting peak B and C in the NONO spectrum). By comparing XDQC signals of
different pulse orders, we can understand the subtle many-body correlations
between core excitations. Generally, X-ray four-wave mixing signals including
the XDQC signals can measure double core excited states directly, thus providing
new experimental tests for the accuracy of the electronic structure methods and
offering a way to visualize projections of the complicated many-electron wave
functions [25, 26].
4.4 Ionization, Photoelectron Signals and Resonances
X-Ray photons with energies higher than the ionization threshold can excite the
system to a metastable resonance state, and then this resonance decays to a cationic
species and a leaving electron. In spectroscopy, either the cationic species or the
leaving electron can be detected, both providing us with windows into the electronic structure of the parent neutral system.
In addition to excitation, molecular ionization is another way to trigger impulsively rich electronic and nuclear dynamics such as geometry relaxation, molecular
dissociation [329], charge migration [330–334], and radiation emission [335]. Ionization has advantages over excitation in pumping the system because there is no
selection rule to restrict which ionization is allowed, and there is usually a lower
number of energetically accessible cationic states compared to the large number of
high energy exited neutral states in excitation experiments. Moreover, after ionization, we have the freedom to detect the cationic states or the ejected electron,
which can provide more information about the parent neutral species. X-Ray
ionization and fragmentation have been used to probe transient molecular structures
during a photoinduced chemical reaction process [336]. Recently we studied the
cationic states of the amino acid glycine prepared by a sudden N1s core ionization
produced by an attosecond X-ray pulse [337]. The created superposition of cationic
states is probed by 2D transient X-ray absorption (TXAS) and 3D ASRS. Our
simulated ASRS results reveal the complex coupling of the valence and core
excited states of the cation.
X-Ray photoelectron spectroscopy (XPS) [338] is a powerful technique for
probing the chemical compositions and electronic states of molecular systems
and materials. By measuring the kinetic energy of the ejected photoelectrons, the
electronic energy levels of the ionized species can be determined. The photoelectrons also carry momenta. Additional information about the initial and final
electronic wavefunctions can be obtained by measuring photoelectron angular
distributions (PAD) [339–341]. TRPES [342, 343] is also used to monitor the
electronic structure changes during a chemical reaction.
Nonlinear Spectroscopy of Core and Valence Excitations Using Short X-Ray. . .
331
affects the following O1s excitation which corresponds to peak C (a strong feature
connecting peak B and C in the NONO spectrum). By comparing XDQC signals of
different pulse orders, we can understand the subtle many-body correlations
between core excitations. Generally, X-ray four-wave mixing signals including
the XDQC signals can measure double core excited states directly, thus providing
new experimental tests for the accuracy of the electronic structure methods and
offering a way to visualize projections of the complicated many-electron wave
functions [25, 26].
4.4 Ionization, Photoelectron Signals and Resonances
X-Ray photons with energies higher than the ionization threshold can excite the
system to a metastable resonance state, and then this resonance decays to a cationic
species and a leaving electron. In spectroscopy, either the cationic species or the
leaving electron can be detected, both providing us with windows into the electronic structure of the parent neutral system.
In addition to excitation, molecular ionization is another way to trigger impulsively rich electronic and nuclear dynamics such as geometry relaxation, molecular
dissociation [329], charge migration [330–334], and radiation emission [335]. Ionization has advantages over excitation in pumping the system because there is no
selection rule to restrict which ionization is allowed, and there is usually a lower
number of energetically accessible cationic states compared to the large number of
high energy exited neutral states in excitation experiments. Moreover, after ionization, we have the freedom to detect the cationic states or the ejected electron,
which can provide more information about the parent neutral species. X-Ray
ionization and fragmentation have been used to probe transient molecular structures
during a photoinduced chemical reaction process [336]. Recently we studied the
cationic states of the amino acid glycine prepared by a sudden N1s core ionization
produced by an attosecond X-ray pulse [337]. The created superposition of cationic
states is probed by 2D transient X-ray absorption (TXAS) and 3D ASRS. Our
simulated ASRS results reveal the complex coupling of the valence and core
excited states of the cation.
X-Ray photoelectron spectroscopy (XPS) [338] is a powerful technique for
probing the chemical compositions and electronic states of molecular systems
and materials. By measuring the kinetic energy of the ejected photoelectrons, the
electronic energy levels of the ionized species can be determined. The photoelectrons also carry momenta. Additional information about the initial and final
electronic wavefunctions can be obtained by measuring photoelectron angular
distributions (PAD) [339–341]. TRPES [342, 343] is also used to monitor the
electronic structure changes during a chemical reaction.
Nonlinear Spectroscopy of Core and Valence Excitations Using Short X-Ray. . .
331