1 Molecular Movies from Molecular Frame Photoelectron Angular
15
The use of mid-infrared strong-field ionization as a means of probing molecular
dynamics is a research field that is only now being first attempted in a number of laboratories. However, there exists already a highly relevant body of work concerned
with how HHG, beyond its use as a source of coherent XUV light, can be used to
study atomic and molecular structure and time-resolved dynamics, by measuring
the harmonic emission as a function of molecular alignment. These experiments,
which have come to be known as “orbital tomography” or “harmonic imaging” experiments, probe the molecular structure since they are sensitive to multi-particle
interference effects [47], and allow to re-construct the amplitude and phase of the
orbital from which the ionized electron was removed [48]. Of particular interest
have also been recent experimental and theoretical works showing that attosecond
time-scale electron dynamics in molecular ions can be probed [49], as well as experiments where the breaking of a molecular bond was followed by monitoring the
harmonic emission from the dissociating molecule as a function of time [50].
In the harmonic imaging experiments, the available observables are typically the
amplitude and phase of a limited number (typ. 5–10) of harmonics. Alternatively,
outcomes of the electron-ion re-collision that do not involve photon emission, but
where the electron elastically or inelastically scatters off the ion, can be measured.
Measurements of 2D photoelectron momentum distributions in principle provide a
very rich observable, since every distinguishable final momentum of the electron
(p z , p x ), where p z is the momentum along the polarization axis and p x the momentum orthogonal to it, may be viewed as an independent measurement. Scattering of
re-collision electrons from different constituent atoms within a molecule may lead
to diffraction patterns characterized by constructive and destructive interferences
that appear at specific final momenta [51, 52]. In addition, the interference between
scattered and non-scattered, laser-ionized electrons leads to holographic interferences that provide further opportunities for the retrieval of dynamical and structural
information.
The first experimental observation of the above-mentioned holographic interference structures was recently made in an experiment where metastable Xe atoms
were ionized using 7 µm radiation from the FELICE FEL at Rijnhuizen in the
Netherlands (see Fig. 1.6) [14]. 2D photoelectron momentum maps were measured
with the help of a velocity map imaging spectrometer that was integrated into the
FEL cavity. Under the influence of the FEL the outermost electron is pulled out
of the atom along the polarization axis and starts an oscillatory motion in the laser
field. The outer turning point of this oscillatory motion can be viewed as an electron
source (at a distance of about 20 Angströms from the atom!) from which electron
waves are emitted that reach the detector either with or without interacting with the
ion from which they are produced. In the former case we are justified in thinking of
the electron wave as a signal wave that encodes information about the ion, while in
the latter case we are justified in thinking of the electron wave as a reference wave.
In this sense, the experiment records a hologram that can in principle be used to
retrieve information about the atomic or molecular target from which the electron
was extracted.
15
The use of mid-infrared strong-field ionization as a means of probing molecular
dynamics is a research field that is only now being first attempted in a number of laboratories. However, there exists already a highly relevant body of work concerned
with how HHG, beyond its use as a source of coherent XUV light, can be used to
study atomic and molecular structure and time-resolved dynamics, by measuring
the harmonic emission as a function of molecular alignment. These experiments,
which have come to be known as “orbital tomography” or “harmonic imaging” experiments, probe the molecular structure since they are sensitive to multi-particle
interference effects [47], and allow to re-construct the amplitude and phase of the
orbital from which the ionized electron was removed [48]. Of particular interest
have also been recent experimental and theoretical works showing that attosecond
time-scale electron dynamics in molecular ions can be probed [49], as well as experiments where the breaking of a molecular bond was followed by monitoring the
harmonic emission from the dissociating molecule as a function of time [50].
In the harmonic imaging experiments, the available observables are typically the
amplitude and phase of a limited number (typ. 5–10) of harmonics. Alternatively,
outcomes of the electron-ion re-collision that do not involve photon emission, but
where the electron elastically or inelastically scatters off the ion, can be measured.
Measurements of 2D photoelectron momentum distributions in principle provide a
very rich observable, since every distinguishable final momentum of the electron
(p z , p x ), where p z is the momentum along the polarization axis and p x the momentum orthogonal to it, may be viewed as an independent measurement. Scattering of
re-collision electrons from different constituent atoms within a molecule may lead
to diffraction patterns characterized by constructive and destructive interferences
that appear at specific final momenta [51, 52]. In addition, the interference between
scattered and non-scattered, laser-ionized electrons leads to holographic interferences that provide further opportunities for the retrieval of dynamical and structural
information.
The first experimental observation of the above-mentioned holographic interference structures was recently made in an experiment where metastable Xe atoms
were ionized using 7 µm radiation from the FELICE FEL at Rijnhuizen in the
Netherlands (see Fig. 1.6) [14]. 2D photoelectron momentum maps were measured
with the help of a velocity map imaging spectrometer that was integrated into the
FEL cavity. Under the influence of the FEL the outermost electron is pulled out
of the atom along the polarization axis and starts an oscillatory motion in the laser
field. The outer turning point of this oscillatory motion can be viewed as an electron
source (at a distance of about 20 Angströms from the atom!) from which electron
waves are emitted that reach the detector either with or without interacting with the
ion from which they are produced. In the former case we are justified in thinking of
the electron wave as a signal wave that encodes information about the ion, while in
the latter case we are justified in thinking of the electron wave as a reference wave.
In this sense, the experiment records a hologram that can in principle be used to
retrieve information about the atomic or molecular target from which the electron
was extracted.