Laser Polarization Axis Research Articles

Attosecond imaging of XUV-induced atomic photoemission and Auger decay in strong laser fields

Velocity-map imaging has been employed to study the photoemission in Ne and N4,5OO Auger decay in Xe induced by an isolated 85 eV extreme ultraviolet (XUV) pulse in the presence of a strong few-cycle near-infrared (NIR) laser field. Full three-dimensional momentum information about the released electrons was obtained. The NIR and XUV pulse parameters were extracted from the measured Ne streaking traces using a FROG CRAB retrieval algorithm. The attosecond measurements of the Auger decay in Xe show pronounced broadening of the Auger lines corresponding to the formation of sidebands. The temporal evolution of the sideband signals and their asymmetry along the laser polarization axis exhibit oscillations similar to those known from attosecond streaking measurements. The experimental results are in good agreement with model calculations based on an analytical solution of the Schrödinger equation within the strong field approximation.

Breakup of the alignedH2molecule by xuv laser pulses: A time-dependent treatment in prolate spheroidal coordinates

We have carried out calculations of the triple-differential cross section for one-photon double ionization of molecular hydrogen for a central photon energy of $75$~eV, using a fully {\it ab initio}, nonperturbative approach to solve the time-dependent \Schro equation in prolate spheroidal coordinates. The spatial coordinates $\xi$ and $\eta$ are discretized in a finite-element discrete-variable representation. The wave packet of the laser-driven two-electron system is propagated in time through an effective short iterative Lanczos method to simulate the double ionization of the hydrogen molecule. For both symmetric and asymmetric energy sharing, the present results agree to a satisfactory level with most earlier predictions for the absolute magnitude and the shape of the angular distributions. A notable exception, however, concerns the predictions of the recent time-independent calculations based on the exterior complex scaling method in prolate spheroidal coordinates [Phys.~Rev.~A~{\bf 82}, 023423 (2010)]. Extensive tests of the numerical implementation were performed, including the effect of truncating the Neumann expansion for the dielectronic interaction on the description of the initial bound state and the predicted cross sections. We observe that the dominant escape mode of the two photoelectrons dramatically depends upon the energy sharing. In the parallel geometry, when the ejected electrons are collected along the direction of the laser polarization axis, back-to-back escape is the dominant channel for strongly asymmetric energy sharing, while it is completely forbidden if the two electrons share the excess energy equally.

Single-shot carrier-envelope-phase-tagged ion-momentum imaging of nonsequential double ionization of argon in intense 4-fs laser fields

Single-shot carrier-envelope-phase (CEP) tagging is combined with a reaction mircoscope (REMI) to investigate CEP-dependent processes in atoms. Excellent experimental stability and data acquisition longevity are achieved. Using this approach, we study the CEP effects for nonsequential double ionization of argon in 4-fs laser fields at 750 nm and an intensity of $1.6\ifmmode\times\else\texttimes\fi{}{10}^{14}$ W/cm${}^{2}$. The Ar${}^{2+}$ ionization yield shows a pronounced CEP dependence which compares well with recent theoretical predictions employing quantitative rescattering theory [S. Micheau et al., Phys. Rev. A 79, 013417 (2009)]. Furthermore, we find strong CEP influences on the Ar${}^{2+}$ momentum spectra along the laser polarization axis.

Temperature Dependence of the Lumirhodopsin I−Lumirhodopsin II Equilibrium

Time-resolved absorbance measurements, over a spectral range from 300 to 700 nm, were made at delays from 1 micros to 2 ms after photoexcitation of bovine rhodopsin in hypotonically washed membrane suspensions over a range of temperature from 10 to 35 degrees C. The purpose was to better understand the reversibility of the Lumi I-Lumi II process that immediately precedes Schiff base deprotonation in the activation of rhodopsin under physiological conditions. To prevent artifacts due to rotation of rhodopsin and its photoproducts in the membrane, probe light in the time-resolved absorbance studies was polarized at the magic angle (54.7 degrees) relative to the excitation laser polarization axis. The difference spectrum associated with the Lumi I to Lumi II reaction was found to have larger amplitude at 10 degrees C compared to higher temperatures, suggesting that a significant back-reaction exists for this process and that an equilibrated mixture forms. The equilibrium favors Lumi I entropically, and van't Hoff plot curvature shows the reaction enthalpy depends on temperature. The results suggest that Lumi II changes its interaction with the membrane in a temperature-dependent way, possibly binding a membrane lipid more strongly at lower temperatures (compared to its precursor). To elucidate the origin of the time-resolved absorbance changes, linear dichroism measurements were also made at 20 degrees C. The time constant for protein rotation in the membrane was found to be identical to the time constant for the Lumi I-Lumi II process, which is consistent with a common microscopic origin. We conclude that Lumi II (the last protonated Schiff base photointermediate under physiological conditions) is the first photointermediate whose properties depend on the protein-lipid environment.

Orbital symmetry and interference effects in molecular high-order harmonic generation

We investigate harmonic generation from $\text{H}_{2}{}^{+}$ molecules driven by intense few-cycle laser pulses whose linearly polarization axis makes an arbitrary angle $\ensuremath{\chi}$ with respect to the molecular axis. The $\text{H}_{2}{}^{+}$ molecule is considered initially in various orbitals with nodal planes. It is found that a strong enhancement of high-order harmonics (HOHs) occurs when the laser polarization axis overlaps with major axes of electron distribution in the active orbital, while broad suppression of HOHs occurs when the laser polarization axis is parallel to a nodal plane of the active molecular orbital. We show that this harmonic suppression is enhanced by destructive interferences when the nodal and the laser polarization axes are parallel to the internuclear axis, which leads to a shortening of the harmonic cutoff. It follows that the orientation dependence of HOHs intensities mimics the electronic density in active orbitals through the angular dependence of ionization and recombination processes.

Explanation for the smoothness of the phase in molecular high-order harmonic generation

We analyze the orientation dependence of harmonic amplitudes and phases from laser driven ${\text{H}}_{2}^{+}$. We use the Lewenstein model, with and without employing the saddle-point approximation for the summation over electron momenta. This means that the direction of the electron motion is not necessarily restricted to the laser polarization axis in contrast to previous implementations. The model predicts smooth phase jumps by almost $\ensuremath{\pi}$ in the orientation dependence. This demonstrates that the smoothness can be explained without Coulomb effects, but these may be relevant for the size of the phase jumps observed in recent experiments.

High-order above-threshold ionization beyond the first-order Born approximation

In the improved strong-field approximation, which describes high-order above-threshold ionization (HATI), the rescattering of the ionized electron on the parent ion is described within the first-order Born approximation. The low-frequency approximation for laser-assisted scattering goes beyond the first Born approximation. In the present paper, we derive the low-frequency approximation for HATI. The rescattering amplitude in the first Born approximation is replaced by the exact scattering amplitude calculated on the energy shell. Our numerical results for the angle-resolved HATI energy spectra show that the difference between the improved strong-field approximation and the low-frequency approximation is significant for scattering away from the laser polarization axis. In the context of quantum-orbit theory and the uniform approximation, we also show that on the back-rescattering ridge, the rescattering $T$-matrix element can be factorized into the product of the incoming flux and the elastic-scattering cross section so that the latter can be extracted from the angle- and energy-resolved HATI spectra.

Robust carrier-envelope phase retrieval of few-cycle laser pulses from high-energy photoelectron spectra in the above-threshold ionization of atoms

The two-dimensional momentum distributions of high-energy photoelectrons generated in the above-threshold ionization of atoms by intense laser pulses are analysed as resulting from the elastic backscattering of laser-induced returning electrons. Using this quantitative rescattering theory, we show that the returning electron wave packets are entirely determined by the properties of the laser pulses, including the absolute value of the carrier-envelope phase (CEP). We then present a robust method to precisely retrieve the absolute phase of few-cycle pulses by analysing separately the CEP dependence of the electron spectra emitted on the left and on the right directions along the laser polarization axis. In addition, our approach offers the possibility of measuring the laser peak intensity and pulse duration with an estimated error of a few per cent.

Strong-field approximation for ionization of a diatomic molecule by a strong laser field. II. The role of electron rescattering off the molecular centers

The strong-field approximation for ionization of diatomic molecules by a strong laser field [D. B. Milo\ifmmode \check{s}\else \v{s}\fi{}evi\ifmmode \acute{c}\else \'{c}\fi{}, Phys. Rev. A 74, 063404 (2006)] is generalized to include rescattering of the ionized electron wave packet off the molecular centers (the electron's parent ion or the second atom). There are four rescattering contributions to the ionization rate, which are responsible for the high-energy plateau in the electron spectra and which interfere in a complicated manner. The spectra are even more complicated due to the different symmetry properties of the atomic orbitals of which a particular molecular orbital consists. Nevertheless, a comparatively simple condition emerges for the destructive interference of all these contributions, which yields a curve in the $({E}_{{\mathbf{p}}_{f}},\ensuremath{\theta})$ plane. Here $\ensuremath{\theta}$ is the electron emission angle and ${E}_{{\mathbf{p}}_{f}}$ is the electron kinetic energy. The resulting suppression of the rescattering plateau can be strong and affect a large area of the $({E}_{{\mathbf{p}}_{f}},\ensuremath{\theta})$ plane, depending on the orientation of the molecule. We illustrate this using the examples of the $3{\ensuremath{\sigma}}_{g}$ molecular orbital of ${\mathrm{N}}_{2}$ and the $1{\ensuremath{\pi}}_{g}$ molecular orbital of ${\mathrm{O}}_{2}$ for various orientations of these molecules with respect to the laser polarization axis. For ${\mathrm{N}}_{2}$, for perpendicular orientation and the equilibrium internuclear distance ${R}_{0}$, we find that the minima of the ionization rate form the curve ${E}_{{\mathbf{p}}_{f}}\phantom{\rule{0.2em}{0ex}}{\mathrm{cos}}^{2}\phantom{\rule{0.2em}{0ex}}\ensuremath{\theta}={\ensuremath{\pi}}^{2}∕(2{R}_{0}^{2})$ in the $({E}_{{\mathbf{p}}_{f}},\ensuremath{\theta})$ plane. For ${\mathrm{O}}_{2}$ the rescattering plateau is absent for perpendicular orientation.

Attosecond pulse carrier-envelope phase effects on ionized electron momentum and energy distributions: roles of frequency, intensity and an additional IR pulse

The effects of the carrier-envelope phase (CEP) of a few-cycle attosecond pulse on ionized electron momentum and energy spectra are analyzed, both with and without an additional few-cycle IR pulse. In the absence of an IR pulse, the CEP-induced asymmetries in the ionized electron momentum distributions are shown to vary as the 3/2 power of the attosecond pulse intensity. These asymmetries are also found to satisfy an approximate scaling law involving the frequency and intensity of the attosecond pulse. In the presence of even a very weak IR pulse (having an intensity of the order of 1011–1012 W cm−2), the attosecond pulse CEP-induced asymmetries in the ionized electron momentum distributions are found to be significantly augmented. In addition, for higher IR laser intensities, we observe for low electron energies peaks separated by the IR photon energy in one electron momentum direction along the laser polarization axis; in the opposite direction, we find structured peaks that are spaced by twice the IR photon energy. Possible physical mechanisms for such asymmetric, low-energy structures in the ionized electron momentum distribution are proposed. Our results are based on single-active-electron solutions of the three-dimensional, time-dependent Schrödinger equation including atomic potentials appropriate for the H and He atoms.

Correlated Two-Electron Momentum Spectra for Strong-Field Nonsequential Double Ionization of He at 800 nm

We report on a kinematically complete experiment on nonsequential double ionization of He by 25 fs 800 nm laser pulses at 1.5 PW/cm;{2}. The suppression of the recollision-induced excitation at this high intensity allows us to address in a clean way direct (e,2e) ionization by the recolliding electron. In contrast with earlier experimental results, but in agreement with various theoretical predictions, the two-electron momentum distributions along the laser polarization axis exhibit a pronounced V-shaped structure, which can be explained by the role of Coulomb repulsion and typical (e,2e) kinematics.

Plasmon-Enhanced Electron Acceleration in Intense Laser Metal-Cluster Interactions

We have measured the energy and angular-resolved electron emission from medium-sized silver clusters (N approximately 500-2000) exposed to dual laser pulses of moderate intensity (I approximately (10(13-14) W/cm2). When the second pulse excites the plasmon resonantly, we observe enhanced emission along the laser polarization axis. The asymmetry of the electron spectrum is strongly increasing with electron energy. Semiclassical simulations reveal the following mechanism: Electrons bound in highly excited states can leave, return to, and traverse the cluster. Those electrons that return at zero plasmon deflection and traverse the cluster during a favorable plasmon half-cycle can experience maximum acceleration by the evolving polarization field. As a result of these constraints energetic electrons are emitted in direction of the laser polarization axis in subcycle bursts.

Dynamics of orientations in an ensemble of Na7+ clusters

We investigate from a theoretical perspective a possible pump and probe scenario for analyzing the rotational motion of free deformed metal clusters considering as test case an ensemble of Na7+ clusters. To that end, we develop a simple classical model for the orientational dynamics and rate equations for the ionizing effect of laser pulses. The model parameters are determined microscopically by time-dependent density functional theory for the cluster electrons and a detailed description of the cluster ions by local pseudo-potentials. We find that pump and probe analysis can very well resolve the rotational motion provided that the laser parameters are properly tuned to the Me surface plasmon of the given sample. Moreover, we see that the laser pulses burn a hole into the distribution of orientations. Stroboscopic repetition of pulses will allow to produce an ensemble clusters oriented in a direction perpendicular to the laser polarization axis.

Three-dimensional time-profile analysis of high-order harmonic generation in molecules: Nuclear interferences inH2+

We describe a numerical method used previously [Phys. Rev. A 70, 011404(R) (2004)] for solving the three-dimensional time-dependent Schr\"odinger equation for ${\mathrm{H}}_{2}^{+}$ (with fixed nuclei) in interaction with an intense, arbitrary oriented laser pulse. In this approach, we use the prolate spheroidal coordinate system, and expand the time-dependent wave function in a complex basis of Laguerre polynomials and Legendre functions. Our results indicate that ionization, excitation, and harmonic generation are strongly influenced by the orientation of the molecular axis with respect to the laser polarization axis. We evaluate the contribution of each nucleus to harmonic generation, as this permits a quantitative and nonambiguous assessment of interference effects as a function of molecular orientation. A time-profile analysis, using a Gabor transform of the harmonic spectrum around certain harmonics, shows that every half-cycle high-order harmonics are emitted by each nucleus when the electron wave packet returns for a recollision with the molecular core, thus confirming the strong field recollision model in molecules. In general, each nucleus emits both odd and even harmonics, but even harmonics are destroyed by interferences between contributions of each nucleus. These interferences are shown to be maximum at certain harmonic orders as a function of molecular orientation. A comparison of acceleration and dipole formulations of the harmonic emission process is made in order to assess the use of high-order harmonic generation for electron wave-function imaging.

Intense femtosecond laser field-induced Coulomb fragmentation of C 2H 4

The fragments from Coulomb explosion of C 2H 4 irradiated by 130fs laser pulses at 810 nm wavelength and intensity around 10 15 W/cm 2 are studied by using a time-of-flight mass spectrometer. The angular distributions of atomic ions exhibit anisotropy with the maximum along the laser polarization axis and the minimum perpendicular to it. The similar full width values at half maximum of angular distributions for C m+ ( m = 1–3) ions imply a geometric alignment. On the other side, the singly and doubly charged molecular ions exhibit a markedly isotopic distribution. The dependence of the doubly charged parent ion (C 2H 4) 2+ on laser intensity indicates a nonsequential ionization mechanism.

Electron and nuclear dynamics of a molecular ion in an intense laser field

The dynamics of a two-dimensional $\mathrm{H}_{2}^{+}$ molecule interacting with a strong laser pulse beyond the usual approximation of fixed nuclei is presented. The motion of the nuclei is studied by using the classical Newton laws while the electron is described with a full quantal treatment. The axis of the molecule, initially not aligned to the laser field, performs a long-period pendular motion around the laser polarization axis. Vibrational degrees of freedom are seen to be excited. The radiation emitted by the oscillating charges presents variations that are synchronous to the pendular motion. The possibility of monitoring the motion of the molecule through the emitted radiation is discussed.

Scattering of low-energy electrons by helium in a CO2 laser field

The scattering of low-energy electrons by helium in the presence of a CO2 laser field is studied using the R-matrix Floquet theory. Two target states are retained in the calculations, the 1S ground state and a 1Po pseudostate optimized to describe the polarizability of the target. Differential cross sections for elastic scattering with net absorption/emission of up to two photons are calculated for collision energies between 1 eV and 8 eV and over a range of intensities from 107 W cm−2 up to 108 W cm−2. Various geometries are considered, including those in which the incident electron beam is not parallel to the laser polarization axis. The results are compared with those of Wallbank and Holmes (2001 Can. J. Phys. 79 1237), and are found to be several orders of magnitude smaller.

Enantiomeric purification of nonpolarized racemic mixtures using coherent light

Angular momentum constraints for coherently controlling chiral purification of racemic mixtures using the dipole-electric field interaction are examined in detail for two different scenarios. First shown is that achieving enantiomeric control in our earlier scheme [M. Shapiro, E. Frishman, and P. Brumer, Phys. Rev. Lett. 84, 1669 (2000)], using parallel laser pulses, requires that the system be M-polarized, where M is the projection of the total angular momentum along the axis of laser polarization. An alternate scenario is then introduced that allows chiral control in an unpolarized racemic mixture by using three mutually perpendicular linearly polarized light fields. Analytic expressions for the enantiomeric excess in both cases are derived and computational results are presented.

Field ionization and Coulomb explosion of CO in an intense femtosecond laserfield

The field ionization and Coulomb explosion of diatomic carbon monoxideinteracting with intense femtosecond laser pulses was systematically investigatedby using a time-of-flight mass spectrometer. The kinetic energy release (KER) in aCoulomb explosion causes the atomic ions to exhibit a double-peak structure. Thevalues of KER calculated using a two-step model were found to agree well withthe measured ones. The distributions of atomic ions are highly anisotropic, havinga maximum along the laser polarization axis and a minimum perpendicular toit. In contrast to the atomic ions, the parent ion CO+exhibits a strong isotropic angular distribution. Suppression of ionization wasobserved for circularly polarized pulses, indicating that the dynamic alignment isresponsible for the observed anisotropic angular distribution of the atomic ions.

Production of energetic electrons in the process of photodetachment of F-.

An imaging technique is used to record an energy and angle resolved spectrum of electrons produced by photodetachment of F- in a strong infrared laser pulse. The spectrum involves contributions from more than 23 excess photon detachment channels. Its higher energy part extends beyond the classical cutoff value, and it appears as a pronounced plateau localized within a small angle along the laser polarization axis. A Keldysh-like theory is able to qualitatively reproduce the spectrum without taking into account the rescattering mechanism. The role of the parity of the initial bound state is discussed.