22 research outputs found
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High-order harmonic generation by polyatomic molecules
We present a theory of high-order harmonic generation by arbitrary polyatomic molecules based on the molecular strong-field approximation (MSFA) in the framework of the S-matrix theory. A polyatomic molecule is modeled by an (N + 1)-particle system, which consists of N heavy atomic (ionic) centers and an electron. We derived various versions (with or without the dressing of the initial and/or final molecular state) of the MSFA. The general expression for the T-matrix element takes a simple form for neutral polyatomic molecules. We show the existence of the interference minima in the harmonic spectrum and explain these minima as a multiple-slit type of interference. This is illustrated by numerical examples for the nitrous oxide (N2O) molecule exposed to strong linearly polarized laser field
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Electron Rescattering in a Bicircular Laser Field
We investigate high-order above-threshold ionization (HATI) of krypton atoms by a bicircular laser field, which consists of two coplanar co- or counter-rotating circularly polarized fields of frequencies rw and sw. We show that the photoelectron spectra in the HATI process, presented in the momentum plane, exhibit the same discrete rotational symmetry as the driving field. We also analyze HATI spectra for various combinations of the intensities of two field components for co- and counter-rotating fields. We find that the appearance of high-energy plateau for the counter-rotating case is vary sensitive to the laser intensity ratio, while the plateau is always absent for the co-rotating bicircular field
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Atomic processes in bicircular fields
We investigate laser-assisted electron-ion recombination (LAR), high-order harmonic generation (HHG) and above-threshold ionization (ATI) of argon atoms by a bicircular laser field, which consists of two coplanar counter-rotating circularly polarized fields of frequencies rω and sω. The energy of soft x rays generated in the LAR process is analyzed as a function of the incident electron angle and numerical results of direct recombination of electrons with Ar+ ions are presented. We also present the results of HHG by a bicircular field and confirm the selection rules derived earlier for inert-gas atoms in a p ground state. We show that the photoelectron spectra in the ATI process, presented in the momentum plane, as well as the LAR spectra exhibit the same discrete rotational symmetry as the applied field
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Strong-Field Ionization of Linear Molecules by a Bichromatic Elliptically Polarized Laser Field with Coplanar Counterrotating or Corotating Components of Different Frequencies
We investigate strong-field ionization of linear molecules by a two-color laser field of frequencies rω and sω having coplanar counterrotating or corotating elliptically polarized components (ω is the fundamental laser field frequency and r and s are integers). Using the improved molecular strong-field approximation we analyze direct above-threshold ionization (ATI) and high-order ATI (HATI) spectra. More precisely, reflection and rotational symmetries of these spectra for linear molecules aligned in the laser-field polarization plane are considered. The reflection symmetries for particular molecular orientations, known to be valid for a bicircular field (this is the field with circularly polarized counterrotating components), are valid also for arbitrary component ellipticities. However, specific rotational symmetries that are satisfied for HATI by a bicircular field, are violated for an arbitrary elliptically polarized field with counterrotating components. For the corotating case and the N2 molecule we analyze molecular-orientation-dependent interferences and plateau structures for various ellipticities
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Imaging of carrier-envelope phase effects in above-threshold ionization with intense few-cycle laser fields
Sub-femtosecond control of the electron emission in above-threshold ionization of the rare gases Ar, Xe and Kr in intense few-cycle laser fields is reported with full angular resolution. Experimental data that were obtained with the velocity-map imaging technique are compared to simulations using the strong-field approximation (SFA) and full time-dependent Schrödinger equation (TDSE) calculations. We find a pronounced asymmetry in both the energy and angular distributions of the electron emission that critically depends on the carrier-envelope phase (CEP) of the laser field. The potential use of imaging techniques as a tool for single-shot detection of the CEP is discussed. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft
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Imaging of carrier-envelope phase effects in above-threshold ionization with intense few-cycle laser fields
Sub-femtosecond control of the electron emission in above-threshold ionization of the rare gases Ar, Xe and Kr in intense few-cycle laser fields is reported with full angular resolution. Experimental data that were obtained with the velocity-map imaging technique are compared to simulations using the strong-field approximation (SFA) and full time-dependent Schrödinger equation (TDSE) calculations. We find a pronounced asymmetry in both the energy and angular distributions of the electron emission that critically depends on the carrier-envelope phase (CEP) of the laser field. The potential use of imaging techniques as a tool for single-shot detection of the CEP is discussed. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft
Energy- and angle-resolved photoelectron spectra of above-threshold ionization and detachment
We analyze strong-laser-field induced above-threshold
ionization and above-threshold detachment processes within the
strong-field approximation. Energy and angular distribution of
photoelectrons are presented for two cases: (1) high-order
above-threshold ionization of electrons from neutral atoms by a
linearly polarized laser field and (2) above-threshold detachment of
electrons from negative ions by a bicircular laser field. In
particular, for the case (1) we compare the results obtained using a
long pulse with the flat-top spatial intensity distribution in the
laser focus with the results obtained using the Gaussian
distribution. We also analyze the intensity-dependent
resonant-like enhancements. We show that the position and shape of these enhancements strongly
depend on the atomic ground state parity. The bicircular field of
the case (2) consists of two counter-rotating circularly polarized
fields having the frequencies rω and sω with integer
r and s. The properties of the photoelectron spectra generated
by such bicircular field are illustrated using an
ω–4ω example. The spectra obtained are explained in
terms of the interference of partial detachment amplitudes which
correspond to different complex solutions of the saddle-point
equations
Ionization by a strong orthogonal two-color laser field: a quantum-orbit-theory approach
A theory of high-order above-threshold ionization (HATI) of atoms is presented and applied to the ionization of inert gases by an orthogonal two-color (OTC) field. The transition matrix element is derived within the strong-field approximation and calculated by numerical integration and the saddle-point method. The atomic bound state is expressed as an expansion in terms of the Slater-type orbitals as well as an asymptotic wave function. The energy and momentum distributions of the HATI electrons are calculated for the – and – OTC fields. A detailed analysis of the saddle-point solutions and their contribution to the overall differential ionization rate for these two cases is presented. Optimal values of the OTC field parameters for which the ionization rate is maximal in the high-energy region of the photoelectron spectra are found. Special attention is devoted to the symmetry consideration of the photoelectron momentum distributions. All these results are analyzed in detail using the quantum-orbit formalism, which gives a better physical insight into the HATI process
Atoms and Molecules in a Strong Laser Field
We investigate high-order above-threshold ionization of diatomic molecules and their companion atoms by linearly polarized strong laser field using improved molecular (atomic) strong-field approximation. The most noticeable feature of the molecular spectra is the existence of minima that are absent in atomic case. We have derived an analytical formula for their position which is independent of the molecular symmetry