arXiv subjects

Olga Smirnova

Publications and source records attributed to Olga Smirnova.

72 records · Page 4Linked to original sources

arXiv2016

Attosecond control of spin polarization in electron-ion recollision driven by intense tailored fields

We show that electrons recolliding with the ionic core upon tunnel ionization of noble gas atoms driven by a strong circularly polarized laser field in combination with a counter-rotating second harmonic are spin polarized and that their degree of polarization depends strongly on the recollision time. Spin polarization arises as a consequence of (1) entanglement between the recolliding electron and the ion, and (2) sensitivity of ionization to the sense of electron rotation in the initial state. We demonstrate that one can engineer the degree of spin polarization as a function of time by tuning the relative intensities of the counter-rotating fields, opening the door for attosecond control of spin-resolved dynamics.

physics.atom-ph↗

arXiv2016

Coulomb time delays in high harmonic generation

Measuring the time it takes to remove an electron from an atom or molecule during photoionization using newly developed attosecond spectroscopies has been a focus of many recent experiments. However, the outcome of such measurement depends on measurement protocols and specific observables available in each particular experiment. One of such protocols relies on high harmonic generation. First, we derive rigorous and general expressions for ionization and recombination times in high harmonic generation experiments. We show that these times are different from, but related to ionization times measured in photo-electron spectroscopy, i.e. using attosecond streak camera, RABBITT and atto-clock methods. Second, we use the Analytical R-Matrix theory (ARM) to calculate these times and compare them with experimental values.

physics.atom-ph↗

arXiv2015

Opportunities for detecting ring currents using the attoclock set-up

Strong field ionization by circularly polarized laser fields from initial states with internal orbital momentum has interesting propensity rule: electrons counter-rotating with respect to the laser field can be liberated more easily than co-rotating electrons [Barth and Smirnova PRA 84, 063415, 2011}]. Here we show that application of few-cycle IR pulses allows one to use this propensity rule to detect ring currents associated with such quantum states, by observing angular shifts of the ejected electrons. Such shifts present the main observable of the attoclock method. We use time-dependent Analytical $R$-Matrix (A$R$M) theory to show that the attoclock measured angular shifts of an electron originating from two counter-rotating orbitals ($p^{+}$ and $p^{-}$) are noticeably different. Our work opens new opportunities for detecting ring currents excited in atoms and molecules, using the attoclock set-up.

physics.atom-ph↗

arXiv2015

Spin-Orbit Larmor Clock for Ionisation Times in One-Photon and Strong-Field Regimes

Photoionisation is a process where absorption of one or several photons liberates an electron and creates a hole in a quantum system, such as an atom or a molecule. Is it faster to remove an electron using one or many photons, and how to define this time? Here we introduce a clock that allows us to define ionisation time for both one-photon and many-photon ionisation regimes. The clock uses the interaction of the electron or hole spin with the magnetic field created by their orbital motion, known as the spin-orbit interaction. The angle of spin precession in the magnetic field records time. We use the combination of analytical theory and ab-initio calculations to show how ionisation delay depends on the number of absorbed photons, how it appears in the experiment and what electron dynamics it signifies. In particular, we apply our method to calculate the derived time delays in tunnelling regime of strong-field ionisation.

quant-ph↗

arXiv2015

Opportunities for chiral discrimination using high harmonic generation in tailored laser fields

Chiral discrimination with high harmonic generation (cHHG method) has been introduced in the recent work by R. Cireasa et al ( Nat. Phys. 11, 654 - 658, 2015). In its original implementation, the cHHG method works by detecting high harmonic emission from randomly oriented ensemble of chiral molecules driven by elliptically polarized field, as a function of ellipticity. Here we discuss future perspectives in the development of this novel method, the ways of increasing chiral dichroism using tailored laser pulses, new detection schemes involving high harmonic phase measurements, and concentration-independent approaches. Using the example of the epoxypropane molecule C$_3$H$_6$O (also known as 1,2-propylene oxide), we show theoretically that application of two-color counter-rotating elliptically polarized laser fields yields an order of magnitude enhancement of chiral dichroism compared to single color elliptical fields. We also describe how one can introduce a new functionality to cHHG: concentration-independent measurement of the enatiomeric excess in a mixture of randomly oriented left-handed and right-handed molecules. Finally, for arbitrary configurations of laser fields, we connect the observables of the cHHG method to the amplitude and phase of chiral response, providing a basis for reconstructing wide range of chiral dynamics from cHHG measurements, with femtosecond to sub-femtosecond temporal resolution.

physics.optics↗

arXiv2015

Interpreting Attoclock Measurements of Tunnelling Times

Resolving in time the dynamics of light absorption by atoms and molecules, and the electronic rearrangement this induces, is among the most challenging goals of attosecond spectroscopy. The attoclock is an elegant approach to this problem, which encodes ionization times in the strong-field regime. However, the accurate reconstruction of these times from experimental data presents a formidable theoretical challenge. Here, we solve this problem by combining analytical theory with ab-initio numerical simulations. We apply our theory to numerical attoclock experiments on the hydrogen atom to extract ionization time delays and analyse their nature. Strong field ionization is often viewed as optical tunnelling through the barrier created by the field and the core potential. We show that, in the hydrogen atom, optical tunnelling is instantaneous. By calibrating the attoclock using the hydrogen atom, our method opens the way to identify possible delays associated with multielectron dynamics during strong-field ionization.

physics.atom-ph↗

arXiv2014

Hole dynamics and spin currents after ionization in strong circularly polarized laser fields

We apply the time-dependent analytical R-matrix theory to develop a movie of hole motion in a Kr atom upon ionization by strong circularly polarized field. We find rich hole dynamics, ranging from rotation to swinging motion. The motion of the hole depends on the final energy and the spin of the photoelectron and can be controlled by the laser frequency and intensity. Crucially, hole rotation is a purely non-adiabatic effect, completely missing in the framework of quasistatic (adiabatic) tunneling theories. We explore the possibility to use hole rotation as a clock for measuring ionization time. Analysing the relationship between the relative phases in different ionization channels we show that in the case of short-range electron-core interaction the hole is always initially aligned along the instantaneous direction of the laser field, signifying zero delays in ionization. Finally, we show that strong-field ionization in circular fields creates spin currents (i.e. different flow of spin-up and spin-down density in space) in the ions. This phenomenon is intimately related to the production of spin-polarized electrons in strong laser fields [Barth I and Smirnova O 2013 Phys. Rev. A 88 013401]. We demonstrate that rich spin dynamics of electrons and holes produced during strong field ionization can occur in typical experimental conditions and does not require relativistic intensities or strong magnetic fields.

physics.atom-ph↗

arXiv2014

Photoelectron angular distributions from aligned molecules using the R-matrix method

We present a new extension of the UKRmol electron-molecule scattering code suite, which allows one to compute ab initio photoionization and photorecombination amplitudes for complex molecules, resolved both on the molecular alignment (orientation) and the emission angle and energy of the photoelectron. We illustrate our approach using CO$_2$ as an example, and analyze the importance of multi-channel effects by performing our calculations at different, increasing levels of complexity. We benchmark our method by comparing the results of our calculations with experimental data and with theoretical calculations available in the literature.

physics.chem-ph↗

arXiv2013

CDENPROP: Transition matrix elements involving continuum states

Transition matrix elements between electronic states where one electron can be in the continuum are required for a wide range of applications of the molecular R-matrix method. These include photoionization, photorecombination and photodetachment; electron-molecule scattering and photon-induced processes in the presence of an external D.C. field, and time-dependent R-matrix approaches to study the effect of the exposure of molecules to strong laser fields. We present a new algorithm, implemented as a module (CDENPROP) in the UKRmol electron-molecule scattering code suite.

physics.chem-ph↗

arXiv2013

Time-resolving electron-core dynamics during strong field ionization in circularly polarized fields

Electron-core interactions play a key role in strong-field ionization and the formation of photoelectron spectra. We analyse the temporal dynamics of strong field ionization associated with these interactions using the time-dependent analytical R-matrix (ARM) method, developed in our previous work [J. Kaushal and O. Smirnova, Phys. Rev. A 88, 013421 (2013)]. The approach is fully quantum but includes the concept of trajectories. However, the trajectories are not classical in the sense that they have both real and imaginary components all the way to the detector. We show that the imaginary parts of these trajectories, which are usually ignored, have a clear physical meaning and are crucial for the correct description of electron-core interactions after ionization. In particular, they give rise to electron deceleration, as well as dynamics associated with electron recapture and release. Our approach is analytical and time-dependent, and allows one to gain access to the electron energy distribution and ionization yield as a function of time. Thus we can also rigorously answer the question: when is ionization completed?

physics.atom-ph↗

arXiv2013

Non-adiabatic Coulomb effects in strong field ionisation in circularly polarised laser fields

We develop the recently proposed analytical R-matrix (ARM) method to encompass strong field ionisation by circularly polarised fields, for atoms with arbitrary binding potentials. Through ARM, the effect of the potential can now be included consistently both during and after ionisation, providing a complete picture for the effects of the long-range potential. We find that the Coulomb effects modify the ionisation dynamics in several ways, including modification of (i) the ionisation (exit) times, (ii) the initial conditions for the electron continuum dynamics, (iii) the "tunnelling angle", at which the electron "enters" the barrier, and (iv) the electron drift momentum. We derive analytical expressions for the Coulomb-corrected ionisation times, initial velocities, momentum shifts and ionisation rates in circularly polarised fields, for arbitrary angular momentum of the initial state. We also analyse how the non-adiabatic Coulomb effects modify (i) the calibration of the attoclock in the angular streaking method, and (ii) the ratio of ionisation rates from $p^{-}$ and $p^{+}$ orbitals.

physics.atom-ph↗

arXiv2013

Multidimensional high harmonic spectroscopy: A semi-classical perspective on measuring multielectron rearrangement upon ionization

High harmonic spectroscopy has the potential to combine attosecond temporal with sub-Angstrom spatial resolution of the early nuclear and multielectron dynamics in molecules. It involves strong field ionization of the molecule by the IR laser field followed by time-delayed recombination of the removed electron with the molecular ion. The time-delay is controlled on the attosecond time scale by the oscillation of the IR field and is mapped into the harmonic number, providing a movie of molecular dynamics between ionization and recombination. One of the challenges in the analysis of high harmonic signal stems from the fact that the complex dynamics of both ionization and recombination with their multiple observables are entangled in the harmonic signal. Disentangling this information requires multidimensional approach, capable of mapping ionization and recombination dynamics into different independent parameters. We suggest multidimensional high harmonic spectroscopy as a tool for characterizing of ionization and recombination processes separately allowing for simultaneous detection of both the ionization delays and sub-cycle ionization rates. Our method extends the capability of the two dimensional (2D) set-up suggested recently by Shafir et al on reconstructing ionization delays, while keeping the reconstruction procedure as simple as in the original proposal. The scheme is based on the optimization of the high harmonic signal in orthogonally polarized strong fundamental and relatively weak multicolour control fields.

physics.atom-ph↗

arXiv2013

Nonadiabatic tunneling in circularly polarized laser fields. III. Comparison with experiment

We compare results of the recent experiment by Herath et al [T. Herath, L. Yan, S. K. Lee, and W. Li, Phys. Rev. Lett. 109, 043004 (2012)] on strong-field nonadiabatic tunneling in circularly polarized laser fields with the original predictions of our theory [I. Barth and O. Smirnova, Phys. Rev A 84, 063415 (2011)] that stimulated these experiments. We show that the theory and experiment are in very good agreement. We also explain why the initial comparison performed by Herath et al [T. Herath, L. Yan, S. K. Lee, and W. Li, Phys. Rev. Lett. 109, 043004 (2012)] has suggested quantitative discrepancies with our theory. We confirm that these seeming discrepancies are removed with an accurate application of our theoretical model. We suggest an experiment for unique determination of the ionization preference of valence orbitals p+ or p-.

physics.atom-ph↗

arXiv2013

Multielectron High Harmonic Generation: simple man on a complex plane

From the famous classical "simple man" model to the recent multichannel model for polyatomic molecules, this tutorial will guide you through the several landmarks in our understanding of high harmonic generation and high harmonic spectroscopy. Our goal was to provide recipes and insights for modelling the harmonic response in various regimes, from the single active electron regime typical for noble gas atoms to the laser-driven attosecond hole dynamics in polyatomics. We have tried to pay particular attention to both simple recipes and their limitations, including the connection of real-valued classical and complex-valued quantum times and velocities. In addition to physical pictures, general approaches and their practical realizations, we have also tried to discuss some of the sticky technical issues, such as the possibility to factorize high harmonic response into the three steps of ionization, propagation and recombination and practical ways to treat strong field ionization, search for the saddle points of multi-dimensional integrals, deal with poles arising in the quantum-mechanical integrands, remove divergences appearing in approximate saddle point expressions, etc.

physics.atom-ph↗

arXiv2013

Spin polarized electrons produced by strong field ionization

We show that ionization of noble gas atoms by strong infrared circularly polarized laser field under standard exerimental conditions can yield electrons with up to 100% spin polarization in energy resolved measurements. Spin polarization arises due to the interplay of the electron-core entanglement and the sensitivity of ionization in circularly polarized fields to the sense of electron rotation in the initial state.

physics.atom-ph↗

arXiv2012

Nonadiabatic tunneling in circularly polarized laser fields: Derivation of formulas

We provide detailed analysis of strong field ionization of degenerate valence p orbitals by circularly polarized fields. Our analytical approach is conceptually equivalent to the Perelomov, Popov, and Terent'ev (PPT) theory and is virtually exact for short range potentials. After benchmarking our results against the PPT theory for s orbitals, we obtain the results for p orbitals. We also show that, as long as the dipole approximation is valid, both the PPT method and our results are gauge invariant, in contrast with widely used strong field approximation (SFA). Our main result, which has already been briefly outlined in [I. Barth and O. Smirnova, Phys. Rev. A 84, 063415 (2011)], is that strong field ionization preferentially removes electrons counter-rotating to the circularly polarized laser field. The result is illustrated using the example of Kr atom. Strong, up to one order of magnitude, sensitivity of strong field ionization to the sense of electron rotation in the initial state is one of the key signatures of non-adiabatic regime of strong field ionization.

physics.atom-ph↗

arXiv2011

Vectorial Phase Retrieval for Linear Characterization of Attosecond Pulses

The waveforms of attosecond pulses produced by high-harmonic generation carry information on the electronic structure and dynamics in atomic and molecular systems. Current methods for the temporal characterization of such pulses have limited sensitivity and impose significant experimental complexity. We propose a new linear and all-optical method inspired by widely-used multi-dimensional phase retrieval algorithms. Our new scheme is based on the spectral measurement of two attosecond sources and their interference. As an example, we focus on the case of spectral polarization measurements of attosecond pulses, relying on their most fundamental property -- being well confined in time. We demonstrate this method numerically reconstructing the temporal profiles of attosecond pulses generated from aligned $CO_2$ molecules.

physics.optics↗

arXiv2007

Reconstructing vibrational states in warm molecules using four-wave mixing with femtosecond laser pulses

We propose a method to reconstruct the vibrational quantum state of molecules excited by a general excitation laser pulse. Unlike existing methods, we do not require the molecules before excitation to be in a pure state, allowing us to treat the important case of initially thermally excited molecules. Even if only a single initial level is appreciably populated, initial levels with small populations can still give major contributions to the unknown vibrational state, making it essential to take them into account. In addition to the excitation pulse, the method uses two incident, short laser pulses in a non-co-linear geometry to create four-wave mixing in the molecules. The measurements used in the reconstruction are spectra of the outgoing four-wave mixing pulse at different time delays of the excitation laser pulse. An important point is that the method does not require detailed knowledge of molecular transition moments between excited states nor of any of the incoming laser pulses, but circumvents this requirement by using one or more calibration laser pulses in a separate experiment either before or after the main data are recorded. The only requirements for the calibration laser pulses are that the constant parts of their spectrums should together cover the spectral range of the excitation laser pulse, and the constant part of each should have sufficient spectral overlap with one other calibration pulse to populate two of the same levels. Finally, we discuss the extension of the reconstruction method in this paper to more general situations, hereby presenting the new idea of quantum state reconstruction through perturbations with calibration.

quant-ph↗