Irina Znakovskaya

Electron localization following attosecond molecular photoionization

For the past several decades, we have been able to directly probe the motion of atoms that is associated with chemical transformations and which occurs on the femtosecond (10−15-s) timescale. However, studying the inner workings of atoms and molecules on the electronic timescale has become possible only with the recent development of isolated attosecond (10−18-s) laser pulses. Such pulses have been used to investigate atomic photoexcitation and photoionization and electron dynamics in solids, and in molecules could help explore the prompt charge redistribution and localization that accompany photoexcitation processes. In recent work, the dissociative ionization of H2 and D2 was monitored on femtosecond timescales and controlled using few-cycle near-infrared laser pulses. Here we report a molecular attosecond pump–probe experiment based on that work: H2 and D2 are dissociatively ionized by a sequence comprising an isolated attosecond ultraviolet pulse and an intense few-cycle infrared pulse, and a localization of the electronic charge distribution within the molecule is measured that depends—with attosecond time resolution—on the delay between the pump and probe pulses. The localization occurs by means of two mechanisms, where the infrared laser influences the photoionization or the dissociation of the molecular ion. In the first case, charge localization arises from quantum mechanical interference involving autoionizing states and the laser-altered wavefunction of the departing electron. In the second case, charge localization arises owing to laser-driven population transfer between different electronic states of the molecular ion. These results establish attosecond pump–probe strategies as a powerful tool for investigating the complex molecular dynamics that result from the coupling between electronic and nuclear motions beyond the usual Born–Oppenheimer approximation.

Field-Free Orientation of CO Molecules by Femtosecond Two-Color Laser Fields

We report the first experimental observation of nonadiabatic field-free orientation of a heteronuclear diatomic molecule (CO) induced by an intense two-color (800 and 400 nm) femtosecond laser field. We monitor orientation by measuring fragment ion angular distributions after Coulomb explosion with an 800 nm pulse. The orientation of the molecules is controlled by the relative phase of the two-color field. The results are compared to quantum mechanical rigid rotor calculations. The demonstrated method can be applied to study molecular frame dynamics under field-free conditions in conjunction with a variety of spectroscopy methods, such as high-harmonic generation, electron diffraction, and molecular frame photoelectron emission.

Attosecond Electron Spectroscopy Using a Novel Interferometric Pump-Probe Technique

We present an interferometric pump-probe technique for the characterization of attosecond electron wave packets (WPs) that uses a free WP as a reference to measure a bound WP. We demonstrate our method by exciting helium atoms using an attosecond pulse (AP) with a bandwidth centered near the ionization threshold, thus creating both a bound and a free WP simultaneously. After a variable delay, the bound WP is ionized by a few-cycle infrared laser precisely synchronized to the original AP. By measuring the delay-dependent photoelectron spectrum we obtain an interferogram that contains both quantum beats as well as multipath interference. Analysis of the interferogram allows us to determine the bound WP components with a spectral resolution much better than the inverse of the AP duration.

Strong-field control of electron localisation during molecular dissociation

We demonstrate how the waveform of light can be used to control a molecular dissociation by the steering and localisation of electrons. Experimental results have been obtained for the dissociative ionisation of the homonuclear and heteronuclear hydrogen derivates D2 and HD. Asymmetric ejection of the ionic fragments reveals that light-driven electronic motion prior to dissociation localises the electron on one of the two ions in diatomic molecular ions in a controlled way. Extension of these results to electron transfer in complex molecules suggests a new paradigm for controlling photochemistry.

Carrier–envelope phase-tagged imaging of the controlled electron acceleration from SiO2 nanospheres in intense few-cycle laser fields

Waveform-controlled light fields offer the possibility of manipu- lating ultrafast electronic processes on sub-cycle timescales. The optical light- wave control of the collective electron motion in nanostructured materials is key to the design of electronic devices operating at up to petahertz frequencies. We have studied the directional control of the electron emission from 95nm 10 Authors to whom any correspondence should be addressed.

Dual frequency comb spectroscopy with a single laser

We demonstrate a simple scheme for dual frequency comb spectroscopy in which the second frequency comb is generated by propagating the primary pulse train through a dazzler. The two frequency combs are combined behind a Mach-Zehnder interferometer, and the optical spectrum is read out by an rf-spectrum analyzer. The method is applied to record the overtone absorption spectrum of C2H2 (acetylene) in the wavelength region around 1.03 μm. A spectrum with a resolution of 4  cm(-1) is obtained, which compares well with that from the HITRAN database. A simple method for improving the spectral resolution is demonstrated.

Sub-cycle electron control in the photoionization of xenon using a few-cycle laser pulse in the mid-infrared

Using few-cycle laser pulses generated by optical parametric chirped pulse amplification, sub-cycle light-wave control of electrons was achieved at a carrier wavelength of 2.1 μm. We demonstrate the sub-cycle light-wave control in the case of strong field ionization of xenon atoms. Angle-resolved spectra of electrons emitted in the photoionization process were recorded as a function of the carrier-envelope phase (CEP) using an electron imaging technique. We observed a clear CEP-dependent asymmetry in the electron momentum distribution.

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 Schr?dinger equation within the strong field approximation.

Transition between Mechanisms of Laser-Induced Field-Free Molecular Orientation

The transition between two distinct mechanisms for the laser-induced field-free orientation of CO molecules is observed via measurements of orientation revival times and subsequent comparison to theoretical calculations. In the first mechanism, which we find responsible for the orientation of CO up to peak intensities of 8 × 10(13) W/cm(2), the molecules are impulsively oriented through the hyperpolarizability interaction. At higher intensities, asymmetric depletion through orientation-selective ionization is the dominant orienting mechanism. In addition to the clear identification of the two regimes of orientation, we propose that careful measurements of the onset of the orientation depletion mechanism as a function of the laser intensity will provide a relatively simple route to calibrating absolute rates of nonperturbative strong-field molecular ionization.

Attosecond electron interferometry

The basic properties of atoms, molecules, and solids are governed by ultrafast electron dynamics. Attosecond pulses bear the promise to resolve these electronic dynamics on their natural time scale, the atomic unit of time, which is 24 attoseconds. The high frequency of the pulses, however, means that in most of the experiments performed so far the electrons that are excited by attosecond pulses are directly moved into the ionization continuum, where they rapidly disperse [1,2]. More interesting dynamics arise when electrons are excited into bound [3] or autoionizing states [4]. Here we present a method to determine the dynamics of a bound wave packet excited by an attosecond pulse, while - for the first time - keeping track of its spectral content with high precision. The key idea is that coincident with the creation of the bound wave packet, we also launch a broad continuum wave packet (Fig. 1). This free wave packet serves as a reference when, after a variable delay, the bound wave packet is ionized by a 7 fs infrared laser pulse, locked in phase with the bound wave packet. The interference fringes observed in the photoelectron spectrum enable precise determination of the bound electron wave packet. As in Ramsey spectroscopy, the spectral precision is here set not by the bandwidth of the excitation pulse, but by the delay between the pump and probe pulses as well as the experimental energy resolution of the photoelectron spectrometer used.