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We demonstrate a quantum stroboscope based on a sequence of identical attosecond pulses that are used to release electrons into a strong infrared (IR) laser field exactly once per laser cycle. The resulting electron momentum distributions are recorded as a function of time delay between the IR laser and the attosecond pulse train using a velocity map imaging spectrometer. Because our train of attosecond pulses creates a train of identical electron wave packets, a single ionization event can be studied stroboscopically. This technique has enabled us to image the coherent electron scattering that takes place when the IR field is sufficiently strong to reverse the initial direction of the electron motion causing it to rescatter from its parent ion.
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Coherent Electron Scattering Captured by an
Attosecond Quantum Stroboscope
J. Mauritsson
, P. Johnsson
, E. Gustafsson
, M. Swoboda
, T. Ruchon
, A. L’Huillier
& K. J. Schafer
Department of Physics, Lund University, P. O. Box 118, SE-221 00 Lund, Sweden
Department of Physics and Astronomy, Louisiana State University, Baton Rouge,
Louisiana 70803-4001, USA
The basic properties of atoms, molecules and solids are governed by electron
dynamics which take place on extremely short time scales. To measure and control
these dynamics therefore requires ultrafast sources of radiation combined with
efficient detection techniques. The generation of extreme ultraviolet (XUV)
attosecond (1 as = 10
s) pulses
has, for the first time, made direct
measurements of electron dynamics possible. Nevertheless, while various
applications of attosecond pulses have been demonstrated experimentally
, no one
has yet captured or controlled the full three dimensional motion of an electron on
an attosecond time scale. Here we demonstrate an attosecond quantum
stroboscope capable of guiding and imaging electron motion on a sub-femtosecond
(1 fs = 10
s) time scale. It is based on a sequence of identical attosecond pulses
which are synchronized with a guiding laser field. The pulse to pulse separation in
the train is tailored to exactly match an optical cycle of the laser field and the
electron momentum distributions are detected with a velocity map imaging
spectrometer (VMIS)
. This technique has enabled us to guide ionized electrons
back to their parent ion and image the scattering event. We envision that coherent
electron scattering from atoms, molecules and surfaces captured by the attosecond
quantum stroboscope will complement more traditional scattering techniques
since it provides high temporal as well as spatial resolution.
Pioneering experiments with femtosecond infrared (IR) laser fields have
demonstrated that temporally localized electron wave packets (EWPs) can be used to
study molecular structures and dynamics
. In these experiments the EWPs are
generated through tunnel ionization twice per optical cycle near the peaks in the laser’s
oscillating electric field. They are subsequently accelerated by the same laser field and
may be driven back to their parent ion for further interaction. If, for example, aligned
molecules are used as targets their orbitals can be characterized from the resulting
harmonic emission. The basic sequence of events, which is the essence of strong field
physics, is very versatile and leads to many different phenomena
. The only control
knob in these experiments, however, is typically the laser intensity, which must be quite
high in order that there be a reasonable probability of tunneling through the Coulomb
barrier. Further control of the electron dynamics requires that the creation and
acceleration of the EWPs are decoupled; and this is not possible using tunneling
ionization since the same laser field governs both events. Decoupling can be achieved
using XUV attosecond pulses to create temporally localized EWPs through single
photon ionization at a well defined phase of a synchronized IR field. These attosecond
EWPs are distinctly different from tunnel ionization EWPs: they are born at the centre
of the potential well with a non-zero velocity and their subsequent dynamics can be
controlled by choosing the phase and amplitude of a synchronized IR field
appropriately. In particular, the laser field needed to drive these EWPs back to the
potential is usually far weaker than the laser field needed to form tunnel EWPs, leading
to much less distortion of the electronic or nuclear properties to be studied.
Decoupling laser-driven dynamics from ionization is at the heart of the attosecond
quantum stroboscope, a device to guide and image electron motion on a sub-
femtosecond time scale, which is illustrated in Fig. 1a, b. The stroboscope technique is
based on a sequence of identical attosecond pulses that are used to release electrons into
a moderately strong laser field exactly once per laser cycle. Just as a conventional
stroboscope can be used to freeze the beating of a hummingbird’s wings, revealing
details that would normally be blurred, we use the quantum stroboscope to record the
electron momentum distribution from a single ionization event, free from interference
effects due to multiple ionization events. Operationally, the quantum stroboscope works
because ionized electrons receive a momentum impulse from the IR field along its
polarization direction (the vertical in Fig. 1). Since this impulse depends on the
magnitude and direction of the laser field at the moment of ionization
, each phase of
the oscillating laser field yields a unique final momentum distribution. If ionization
occurs over the whole IR cycle, or even at as few as two times during the cycle, the
distribution will be smeared out and show interference fringes that depend on the
different ionization times
. When the attosecond pulse periodicity matches the optical
cycle, the XUV pulses create identical EWPs that add up coherently, with the result that
the properties of an individual EWP can be studied stroboscopically. In addition, the
temporal localization of the EWPs within the optical period of the IR field allows for
precise guiding of the wave packets after their birth.
In a first experimental demonstration of the quantum stroboscope we use pulses
with a 300 as duration and a central energy of 24 eV to ionize argon in the presence of a
guiding IR laser field with an intensity of W/cm
. The attosecond pulse train
(APT) was generated from a two colour laser field consisting of the IR field and its
second harmonic to ensure that the XUV pulses were separated by one full optical
. Four stroboscopic images taken at different XUV-IR delays are presented in Fig.
1 (c). The clear up/down-asymmetry in the momentum distributions confirms that each
image corresponds to ionization at one particular phase of the IR field, so that the total
momentum is shifted up or down in the direction of polarization of the IR field. These
results illustrate two essential aspects of stroboscopic imaging. First, we can freeze the
periodically varying momentum distribution at a single phase of the IR field, and then
capture the entire time-dependent distribution by varying the XUV-IR delay. Second,
the measured signal is larger than what we would measure with a single pulse. In a
quantum stroboscope this effect is enhanced because a train of N pulses yields fringes
that are N
times brighter than the signal from an isolated pulse, owing to the coherence
of the process (
N in our experiment). This has allowed us to obtain full three
dimensional images of an attosecond EWP for the first time. An additional feature is
that the position of the stationary interference fringes depends only on the IR intensity.
The quantum stroboscope is therefore self-calibrating since the only unknown
parameter, the IR intensity, can be read directly from the interference pattern.
In the experimental results presented in Fig. 1 coherent electron scattering was not
observed since the intensity of the guiding laser field was not high enough to direct the
EWPs back to the ion core. To estimate the field intensity needed for coherent electron
scattering, simple classical arguments can be used. The momentum at time t of an
electron born in the field )sin()(
= at time t
with initial momentum p
is equal
. (1)
It is the sum of a drift momentum (which is a combination of the initial
momentum obtained from the XUV ionization and of that gained in the laser field) and
an oscillating term describing wiggle motion in the field. Introducing the wiggle energy
and the initial kinetic energy , where I
the ionization energy, the condition for the final (drift) momentum to be zero or
opposite to the initial momentum can be formulated as
IEmpW == 2/
. When 1
the momentum transferred by the field to the electrons is such that only electrons that
are born exactly at times when
E(t)=0 will return to the ion since the net transfer of
momentum from the laser field is maximized for these times. For smaller
-values the
momentum transfer is larger, and returns are possible also for other initial times. We can
tune the interaction (the energy of the returning electrons, the number of times they pass
the core and the field strength at the moment of re-collision) by varying the delay
between the attosecond pulse and the laser field. For an 800 nm laser wavelength, 1
can be obtained by using, for example, W/cm
and W=1.2 eV. This intensity
is an order of magnitude smaller than that needed for tunnel ionization.
To understand what we can expect from experiments that guide ionized electrons
to rescatter off the ion core, we have performed a series of calculations in helium by
numerically integrating the time-dependent Schrödinger equation (TDSE). The
momentum distributions obtained at the XUV-IR delay leading to maximum
momentum transfer from the guiding field to the electrons are shown in Fig. 2 for four
different IR intensities. The white circle in each panel denotes the range of momenta
expected if there is no rescattering from the ion. We clearly see the onset of scattering
values near one ( 2.1
). At this intensity the low energy fraction of the EWPs
has scattered off the atomic potential while the high energy portion remains unaffected.
With decreasing
values a larger portion of the EWP scatters off the potential, which
increases the scattering signal.
To experimentally access the regime of coherent electron scattering we change the
target gas to helium, so that W=1.2 eV, and we increase the IR intensity to
. Four experimental momentum distributions recorded at different XUV-IR
delays are presented in Fig. 3. When the XUV-IR timing is set to maximize the
momentum transfer from the IR field in the upwards (panel 1) or downwards (panel 3)
directions we see a clear signature of re-scattering, manifested by a significant increase
of low-energy electrons in the direction opposite to the momentum transfer from the IR
102.1 =I
. The experimental results are compared with theoretical calculations in Figure 4
and the agreement is excellent with all the substructures well reproduced. We believe
that this is the first evidence for coherent electron scattering of attosecond EWPs
created by single photon ionization.
In this letter we have demonstrated an attosecond quantum stroboscope capable of
imaging the electron momentum distribution resulting from a single ionization event.
We have also used it to guide ionized electrons back to their parent ions and to image
the coherent electron scattering. The basic technique we have demonstrated is very
versatile and may be altered in a number of potentially useful ways. For example, the
guiding field could be a replica of the two-colour driving field we used to make the
attosecond pulses. This would provide additional control over the return time and
energy of the electrons. Using a longer wavelength driving laser would lengthen the
time between attosecond pulses, allowing more time for internal dynamics initiated by
the launch of the EWP to develop before being probed by the returning electron. We
envision that controlled, coherent scattering such as we have demonstrated will enable
time resolved measurements with very high spatial resolution in atoms and molecules or
at surfaces.
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Acknowledgments: This research was supported by a Marie Curie Intra-European Fellowship, the Marie
Curie Research Training Networks XTRA, the Marie Curie Early Stage Training Site MAXLAS, the
Integrated Initiative of Infrastructure LASERLAB-EUROPE within the 6th European Community
Framework Programme, the Knut and Alice Wallenberg Foundation, the Crafoord foundation, the
Swedish Research Council and the National Science Foundation.
Author Information The authors declare no competing financial interests. Correspondence and requests
for materials should be addressed to J.M. (
Figure 1: Principle of the attosecond quantum stroboscope. An attosecond pulse train
containing N pulses is used to ionize the target atoms once per cycle of an IR laser field. This
periodicity ensures that the created EWPs are identical and that they add up coherently on the
detector leading to an enhanced signal that is modulated by interferences fringes (a,b). The
bright fringes are increased by a factor N
compared to the signal that would be obtained with
only one pulse. The IR field is used to guide the electrons and the momentum transfer can be
controlled by simply varying the XUV-IR delay. When the EWPs are created at the maxima of
the IR electric field (a) the net transfer of momentum is zero and the resulting momentum
distribution is symmetric relative to the plane perpendicular to the laser polarization. When the
EWPs instead are created at the zero-crossings of the IR electric field (b), the momentum
distribution is shifted by the field up- or down-wards along the direction of the laser polarization.
In our experiment, the final momentum distributions of the EWPs are detected using a VMIS.
Experimental results obtained in Ar at four different XUV-IR delays are shown in (c), panels 1-4
correspond to the delays t
=-π/2ω, 0, π/2ω, π/ω respectively for an IR intensity of W/cm
The delay-dependent momentum distribution is imprinted on a set of concentric rings which are
due to the coherent addition of many EWPs. These rings, which are evenly spaced one IR
photon apart in energy, show a decreasing spacing when plotted as a function of momentum.
The positions of the rings do not shift as a function of XUV-IR delay, which shows that we are
imaging a train of identical EWPs spaced one IR cycle apart in time.
Figure 2: Theoretical illustration of coherent scattering. Theoretical results obtained by
integrating the TDSE are shown for IR intensities ranging from zero to W/cm
for the
XUV-IR delay which corresponds to the maximum momentum transfer. The momentum transfer
from the field to the electrons is upward in the figure. The white circles are positioned at the
highest energy electrons in the field free case (panel 1) and shifted by the amount of momentum
added by the IR field in the other panels. If no post-ionization interaction between the electron
and the atom occurs the momentum distributions would remain within the circles. This is the
case in panel 2, in which the IR field is too weak to drive the electron back to the atomic
potential, but not in panel 3 and 4 where electrons appear outside the circles in the downward
direction. Two features are highlighted in panel three, which is calculated at an intermediate
intensity where
= 1.2, by white arrows: I) electrons that have scattered off the core appearing
outside the white circle in the downward direction and II) interference minima in the momentum
distribution. The interference minima occur in the region where the rescattered and direct
electrons overlap.
Figure 3: Experimental demonstration of coherent electron scattering. Experimental
results obtained in helium at an intensity of W/cm
at the same delays as in Fig. 1 (c)
are shown. The results are distinctively different from those taken in argon (Fig. 1). The
momentum distributions are more peaked along the laser polarization direction, since in helium
the excited EWP is entirely in an m=0 state, whereas in argon there was a mixture of m=0 and
m=1 and the latter has no amplitude along the polarization axis. This is a favourable condition to
observe electron scattering since the electrons along the polarization direction have the highest
probability to scatter off the potential. With this higher intensity more momentum is transferred to
the electrons and in combination with the lower initial energy some electrons return to the
atomic potential for further interaction.
Figure 4: Comparison between theory and experiment. We compare the experimental
results (right) with theoretical calculations (left) obtained for the same conditions. The excellent
agreement is the strongest evidence for coherent scattering effects in the experiment. All the
substructures well reproduced except for the innermost peak in the experiment, which most
likely is due to above threshold ionization of residual water in the experimental chamber and
therefore not included in the theoretical results.
Figure 1
Figure 2
No IR field
Momentum (a. u.)
Momentum (a. u.)
1 0.5 0 -0.5 -1
Figure 3
Momentum (a. u.)
Momentum (a. u.)
1 0.5 0 -0.5 -1
Theory Experiment
Figure 4
... The photoelectron momentum distribution (PMD) of atoms exposed in an intense laser field provides both temporal and structural information about ionizing electrons with unprecedented accuracies [11,12]. In the PMD, various patterns appear because of the interference between different photoelectron wave packets [13], e.g. the photoelectron hologram [14,15] formed by the direct ionization electron wave packet and the scattering electron wave packet. ...
... In general, the interference pattern in PMD is influenced by the pulse duration. In a multicycle laser field, electron wave packets are released from their parent ion repeatedly with every optical cycle, then both intracycle and intercycle interferences exist simultaneously and interplay in the whole interaction process [11,12]. Thus, it is hard to extract the electron scattering dynamics information from the PMD. ...
Full-text available
The electron scattering process has been investigated by analyzing the interference structure in the photoelectron momentum distribution (PMD) of a hydrogen atom exposed to a single-cycle linearly polarized near-infrared laser field, based on the numerical solution of the full-dimensional time-dependent Schrödinger equation and the Coulomb correlative classical trajectory simulation. The interference pattern in the PMD is closely related to the form of the ultrashort pulse which is dominated by the carrier-envelope phase. A fish-bone-like pattern appears in the PMD using the sine electric field and a spider-like pattern appears using the cosine electric field. These interference structures reflect the scattering process. It is found that the stripe density of the spider-like pattern is mainly dominated by the recollision time of scattering electron trajectories, i.e., the longer the recollision time, the greater the stripe density. Therefore, the photoelectron interference pattern can be used to understand the ionization and scattering processes, and identify these processes on the attosecond time scale.
... The system under scrutiny is thus controlled by varying the time delay between the two pulses. Such an approach has been successfully employed to manipulate benchmark systems such as the electronic charge distribution within a molecular target [7,8] or the photoelectron emission from atoms [9][10][11]. Despite these impressive proofs-ofprinciple, attosecond control of quantum phenomena in matter is still in its infancy, though, mainly because attosecond pulse shaping techniques are still developing. ...
... Manipulation of the angular emission pattern of photoelectrons generated by attosecond pulses has recently received considerable scientific interest owing to the fundamental role played by the photoionization process in nature. A non-exhaustive list of achievements includes the control of the photoelectron motion with a sufficiently strong IR laser field [9], the generation of an asymmetric emission along the laser polarization based on an orbital parity mix interference process [10], or the modification of the symmetric photoelectron angular distributions resulting from the asymmetric contributions of the IR absorption and emission mechanisms in the two-photon (XUV ± IR) ionization process [11]. Our experiment bears a similarity with the recent study reported by Cheng et al, where a sequence of three attosecond pulses combined with a relatively weak IR field is used as an efficient means for controlling the photoemission from atoms [33] through interferences between the temporally-delayed electron wavepackets. ...
Full-text available
In this work, we report on coherent control of electron dynamics in atoms via attosecond pulse-shaping. We show that the photoelectron emission from argon gas produced by absorption of an attosecond pulse train (APT) made of odd and even harmonics can be manipulated along the direction of polarization of the light by tuning the spectral components (amplitude and phase) of the pulse. In addition, we show that APTs produced with a two-color (400- plus 800 nm) femtosecond driving field exhibit high temporal tunability, which is optimized for an intensity ratio between the two colors in the range of 0.1 to 5%.
... A similar method, using an attosecond pulse train phase-locked to the infrared field [46], has already been used in experiments to manipulate and probe electron wave packets at low energies [47][48][49][50]. Depending on the delay between the pulse train and the infrared field, recollisions have been observed in photoelectron distributions [51] and in the emitted radiation [52,53]. The series of wave packets created by the pulse train makes the analysis more challenging. ...
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The control of low-energy electrons by carrier-envelope-phase-stable near-single-cycle THz pulses is demonstrated. A femtosecond laser pulse is used to create a temporally localized wave packet through multiphoton absorption at a well defined phase of a synchronized THz field. By recording the photoelectron momentum distributions as a function of the time delay, we observe signatures of various regimes of dynamics, ranging from recollision-free acceleration to coherent electron-ion scattering induced by the THz field. The measurements are confirmed by three-dimensional time-dependent Schrödinger equation simulations. A classical trajectory model allows us to identify scattering phenomena analogous to strong-field photoelectron holography and high-order above-threshold ionization.
... Additionally, trains of phase-locked attosecond pulses can be used in an interferometric scheme to provide simultaneous time and energy resolution [29] and intense attosecond x-ray pulse trains can improve nonlinear, sitespecific measurements. One way to produce pulse trains with greater fluxes is harmonic mixing with a seeded FEL, which has been shown to produce attosecond pulse trains with microjoule-level total pulse energies. ...
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We report the demonstration of optical compression of an electron beam and the production of controllable trains of femtosecond, soft x-ray pulses with the Linac Coherent Light Source (LCLS) free-electron laser (FEL). This is achieved by enhanced self-amplified spontaneous emission with a 2 μm laser and a dechirper device. Optical compression was achieved by modulating the energy of an electron beam with the laser and then compressing with a chicane, resulting in high current spikes on the beam which we observe to lase. A dechirper was then used to selectively control the lasing region of the electron beam. Field autocorrelation measurements indicate a train of pulses, and we find that the number of pulses within the train can be controlled (from 1 to 5 pulses) by varying the dechirper position and undulator taper. These results are a step toward attosecond spectroscopy with x-ray FELs as well as future FEL schemes relying on optical compression of an electron beam.
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We present a compact velocity-map imaging (VMI) spectrometer for photoelectron imaging at 100 MHz repetition rate. Ultrashort pulses from a near-infrared frequency comb laser are amplified in a polarization-insensitive passive femtosecond enhancement cavity. In the focus, multi-photon ionization (MPI) of gas-phase atoms is studied tomographically by rotating the laser polarization. We demonstrate the functioning of the VMI spectrometer by reconstructing photoelectron angular momentum distributions from xenon MPI. Our intra-cavity VMI setup collects electron-energy spectra at high rates, with the advantage of transferring the coherence of the cavity-stabilized femtosecond pulses to the electrons. In addition, the setup will allow studies of strong-field effects in nanometric tips.
Electron dynamics play a fundamental role in chemical dynamics and are the basic components of light-matter interaction. The natural timescale electronic motion in quantum systems is less than one femtosecond. Therefore the study of electron dynamics is closely tied to the development of attosecond light sources. To date these developments have been driven primarily by strong-field driven high harmonic generation based sources. The ability to probe electronic processes with atomic-site specificity is paramount to the continued investigation in this area, which points to the need for soft X-ray pulses with sub-femtosecond duration. The development of attosecond X-ray free-electron lasers marks the beginning of a new era of attosecond science, markedly increasing the peak power with respect to conventional sources. In this chapter we review the development of sub-femtosecond capabilities at the Linac Coherent Light Source and discuss some of their applications to the study of electron dynamics in molecules. We describe the use of this novel source to study electron dynamics with nonlinear X-ray spectroscopy, including attosecond pump/probe studies.
The above-threshold ionization process of ammonia molecules induced by a femtosecond laser field at 800 nm is studied in the intensity range from 1.6 × 10 ¹³ to 5.7 × 10 ¹³ W/cm ² . Channel switching under different laser intensities is observed and identified in the photoelectron kinetic energy spectra of ammonia. Based on the photoelectron kinetic energy distributions and the photoelectron angular distributions, the characteristic peaks observed are exclusively assigned to the multiphoton resonance through certain intermediate states, followed by multiphoton above-threshold ionization.
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We consider an electron in an atom driven by an infrared (IR) elliptically polarized laser field after its ionization by an ultrashort extreme ultraviolet (XUV) pulse. We find that, regardless of the atom species and the laser ellipticity, there exists XUV parameters for which the electron returns to its parent ion after ionizing, i.e., undergoes a recollision. This shows that XUV pulses trigger efficiently recollisions in atoms regardless of the ellipticity of the IR field. The XUV parameters for which the electron undergoes a recollision are obtained by studying the location of recolliding periodic orbits (RPOs) in phase space. The RPOs and their linear stability are followed and analyzed as a function of the intensity and ellipticity of the IR field. We determine the relation between the RPOs identified here and the ones found in the literature and used to interpret other types of highly nonlinear phenomena for low elliptically and circularly polarized IR fields.
Ultrafast laser pulses generated at the attosecond timescale represent a unique tool to explore the fastest dynamics in matter. An accurate control of their properties, such as polarization, is fundamental to shape three-dimensional laser-driven dynamics. We introduce a technique to generate attosecond pulse trains whose polarization state varies from pulse to pulse. This is accomplished by driving high-harmonic generation with two time-delayed bichromatic counter-rotating fields with proper orbital angular momentum (OAM) content. Our simulations show that the evolution of the polarization state along the train can be controlled via OAM, pulse duration, and time delay of the driving fields. We, thus, introduce an additional control into structured attosecond pulses that provides an alternative route to explore ultrafast dynamics with potential applications in chiral and magnetic materials.
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High-intensity X-ray sources are essential diagnostic tools for science, technology and medicine. Such X-ray sources can be produced in laser-plasma accelerators, where electrons emit short-wavelength radiation due to their betatron oscillations in the plasma wake of a laser pulse. Contemporary available betatron radiation X-ray sources can deliver a collimated X-ray pulse of duration on the order of several femtoseconds from a source size of the order of several micrometres. In this paper we demonstrate, through particle-in-cell simulations, that the temporal resolution of such a source can be enhanced by an order of magnitude by a spatial modulation of the emitting relativistic electron bunch. The modulation is achieved by the interaction of the that electron bunch with a co-propagating laser beam which results in the generation of a train of equidistant sub-femtosecond X-ray pulses. The distance between the single pulses of a train is tuned by the wavelength of the modulation laser pulse. The modelled experimental setup is achievable with current technologies. Potential applications include stroboscopic sampling of ultrafast fundamental processes.
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Thesis (Ph. D.)--State University of New York at Stony Brook, 1999. Includes bibliographical references.
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A theoretical description of atomic photoionization by attosecond pulses in the presence of an intense laser pulse is presented. It is based on the numerical solving of the non-stationary Schrödinger equation which includes on an equal footing the realistic atomic potential and the electric fields of both pulses. The calculated energy spectra and angular distributions of photoelectrons are compared with those obtained using a simple approximate model based on the strong-field approximation. The agreement is excellent for large energy of photoelectrons. When the energy is small, the rescattering of electrons by the ionic core affects the cross section considerably making the strong-field approximation inadequate. Influence of the electron orbital polarization on the ionization cross section is investigated.
We present an iterative method for the extraction of velocity and angular distributions from two-dimensional (2D) ion/photoelectron imaging experiments. This method is based on the close relationship which exists between the initial 3D angular and velocity distribution and the measured 2D angular and radial distributions, and gives significantly better results than other inversion procedures which are commonly used today. Particularly, the procedure gets rid of the center-line noise which is one of the main artifacts in many current ion/photoelectron imaging experiments. © 2001 American Institute of Physics.
The application of electrostatic lenses is demonstrated to give a substantial improvement of the two-dimensional (2D) ion/electron imaging technique. This combination of ion lens optics and 2D detection makes “velocity map imaging” possible, i.e., all particles with the same initial velocity vector are mapped onto the same point on the detector. Whereas the more common application of grid electrodes leads to transmission reduction, severe trajectory deflections and blurring due to the non-point source geometry, these problems are avoided with open lens electrodes. A three-plate assembly with aperture electrodes has been tested and its properties are compared with those of grid electrodes. The photodissociation processes occurring in molecular oxygen following the two-photon 3dπ(3Σ1g −)(v=2, N=2)←X(3Σg −) Rydberg excitation around 225 nm are presented here to show the improvement in spatial resolution in the ion and electron images. Simulated trajectory calculations show good agreement with experiment and support the appealing properties of this velocity mapping technique.
We present a theoretical analysis of how intense, few-cycle infrared laser pulses can be used to image the structure of small molecules with nearly 1 fs temporal and sub-Å spatial resolution. We identify and analyse several physical mechanisms responsible for the distortions of the diffraction image and describe a recipe for recovering an un-distorted image from angle and energy-resolved electron spectra. We also identify holographic patterns in the photoelectron spectra and discuss the requirements to enhancing the hologram resolution for imaging the scattering potential.
When a strong laser field ionizes atoms (or molecules), the electron wave packet that tunnels from the molecule moves under the influence of the strong field and can re-collide with its parent ion. The maximum re-collision electron kinetic energy depends on the laser wavelength. Timed by the laser field oscillations, the re-colliding electron interferes with the bound state wave function from which it tunneled. The oscillating dipole caused by the quantum interference produces attosecond optical pulses. Interference can characterize both interfering beams-their wavelength, phase and spatial structure. Thus, written on the attosecond pulse is an image of the bound state orbital and the wave function at the re-collision electron. In addition to interfering, the re-collision electron can elastically or inelastically scatter from its parent ion, diffracting from the ion, and exciting or even exploding it. We review attosecond technology while emphasizing the underlying electron-ion re-collision physics.
Ultrashort high harmonic extreme-ultraviolet pulses are applied for taking snapshots of the relaxation of atomic inner-shell vacancies. A novel gating mechanism utilizes the momentum exchange between electron wave packets and a laser field.
We investigate the interaction of a two-active electron system (Li-) with intense single-cycle and double half-cycle pulses. The “intensity” and “frequency” considered correspond to the “multiphoton above-barrier regime.” For the single-cycle pulse (SCP), the electric field changes sign once, allowing electron wave packets created during the first half cycle to recollide with the parent ion when driven back by the field. For the double half-cycle pulse (DHP), however, the electric field does not change sign, and electron wave packets created during the first half cycle are not driven back to the parent ion. We find that both single and double ionization are significantly larger for the SCP than for the DHP, thereby elucidating the role of the rescattering mechanism. On the other hand, doubly ionized electrons produced by a half-cycle pulse and a DHP are found to have angular distributions in which one electron is ejected in the direction of the pulse field, and the other in the opposite direction. This clear signature of electron correlations suggests that “shake-off,” “knockout,” and, possibly, “multiphoton-sharing” processes are alternative contributing mechanisms for double ionization in this regime.
The contrast and penetrating power afforded by soft X-rays when they interact with matter makes this form of radiation ideal for studying micrometre-sized objects(1,2). But although soft X-rays are useful for probing detail too fine for visible light microscopy in specimens too thick for electron microscopy, the highest-resolution applications of X-ray imaging have been traditionally limited to crystalline samples. Here we demonstrate imaging (at similar to 75 nm resolution) of a non-crystalline sample, consisting of an array of gold dots, by measuring the soft X-ray diffraction pattern from which an image can be reconstructed. The crystallographic phase problem(3)-the usually unavoidable loss of phase information in the diffraction intensity-is overcome by oversampling(4) the diffraction pattern, and the image is obtained using an iterative algorithm(5). Our X-ray microscopy technique requires no high-resolution X-ray optical elements or detectors. We believe that resolutions of 10-20 nm should be achievable; this would provide an imaging resolution about 100 times lower than that attainable with conventional X-ray crystallography, but our method is applicable to structures roughly 100 times larger. This latter feature may facilitate the imaging of small whole cells or large subcellular structures in cell biology.