666 research outputs found
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.Comment: 10 pages, 2 figure
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
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.Comment: 42 pages, 3 figure
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 and orbitals.Comment: 46 pages, 5 figures, Appendix A-
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.Comment: 11 pages, 3 figure
Propensity rules in photoelectron circular dichroism in chiral molecules II: General picture
Photoelectron circular dichroism results from one-photon ionization of chiral
molecules by circularly polarized light and manifests itself in
forward-backward asymmetry of electron emission in the direction orthogonal to
the light polarization plane. To expose the physical mechanism responsible for
asymmetric electron ejection, we first establish a rigorous relation between
the responses of unaligned and partially or perfectly aligned molecules. Next,
we identify a propensity field, which is responsible for the chiral response in
the electric-dipole approximation, i.e. a chiral response without magnetic
interactions. We find that this propensity field, up to notations, is
equivalent to the Berry curvature in a two-band solid. The propensity field
directly encodes optical propensity rules, extending our conclusions regarding
the role of propensity rules in defining the sign of forward-backward asymmetry
from the specific case of chiral hydrogen to generic chiral systems. Optical
propensity rules underlie the chiral response in photoelectron circular
dichroism. The enantiosensitive flux of the propensity field through the sphere
in momentum space determines the forward-backward asymmetry in unaligned
molecules and suggests a geometrical origin of the chiral response. This flux
has opposite sign for opposite enantiomers and vanishes for achiral molecules.Comment: Added Fig. 4, Added Table 1, shortened section
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 CHO (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
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 -Matrix (AM) theory to show that the attoclock measured
angular shifts of an electron originating from two counter-rotating orbitals
( and ) are noticeably different. Our work opens new
opportunities for detecting ring currents excited in atoms and molecules, using
the attoclock set-up
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?Comment: 19 pages, 9 figure
General theory of photoexcitation induced photoelectron circular dichroism
The photoionization of chiral molecules prepared in a coherent superposition
of excited states can give access to the underlying chiral coherent dynamics in
a procedure known as photoexcitation induced photoelectron circular dichroism
(PXECD). This exclusive dependence on coherence can also be seen in a different
part of the angular spectrum, where it is not contingent on the chirality of
the molecule, thus allowing extension of PXECD's sensitivity to tracking
coherence to non-chiral molecules. Here we present a general theory of PXECD
based on angular momentum algebra and derive explicit expressions for all
pertinent asymmetry parameters which arise for arbitrary polarisation of pump
and probe pulses. The theory is developed in a way that clearly and simply
separates chiral and non-chiral contributions to the photoelectron angular
distribution, and also demonstrates how PXECD and PECD-type contributions,
which may be distinguished by whether pump or ionizing probe enables chiral
response, are mixed when arbitrary polarization is used
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