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ABSTRACT: We study high-order harmonic generation (HHG) spectra by a focused truncated Bessel beam versus a Gaussian beam, including macroscopic propagation of the fundamental laser and harmonic fields in the gas medium, with the single-atom-induced dipole response calculated from the recently developed quantitative rescattering theory. We first simulate the HHG spectra of Ar by an 8-fs and 780-nm short laser pulse, reported by Wörner et al. [ Phys. Rev. Lett. 102 103901 (2009)], assuming the incident beam is a truncated Bessel beam or a Gaussian beam. Both simulations fail to reproduce the observed wide and deep Cooper minimum in the HHG spectra. However, we are able to reproduce the HHG spectra of Ar generated by few-cycle 1800-nm near-infrared lasers, reported recently by Shiner et al. [ Nature Phys. 7 464 (2011)]. We also provide a systematic phase-mismatch analysis in the gas jet to examine the spatial growth of harmonics for tight-focusing versus loose-focusing lasers. The dependence of phase mismatch on the gas-jet position and gas pressure is investigated. Finally, we check the divergence of the harmonic beam generated by a tight-focusing versus loose-focusing truncated Bessel beam. These studies show that for a typical gas-jet experiment, whether the spatial mode is a truncated Bessel beam or a Gaussian beam is important only when the laser beam is tightly focused.
Phys. Rev. A. 03/2012; 85(3).
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ABSTRACT: We report measurements and theoretical simulations of high-order harmonic
generation (HHG) in aligned N$_2$ molecules using a 1200-nm intense laser field
when the generating pulse is perpendicular to the aligning one. With increasing
laser intensity, the minimum in the HHG spectra first shifts its position and
then disappears. Theoretical simulations including the macroscopic propagation
effects in the medium reproduce these observations and the disappearance of the
minimum is attributed to the additional contribution of HHG from inner
orbitals. We also predict that the well-known shape resonance in the
photoionization spectra of N$_2$ should exist in the HHG spectra. It is most
clearly seen when the generating laser is parallel to the aligning one, and
disappears gradually as the angle between the two lasers increases. No clear
evidence of this shape resonance has been reported so far when using lasers
with different wavelengths. Further experimentation is needed to draw
conclusions.
10/2011;
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ABSTRACT: We study the effect of gas pressure on the generation of high-order harmonics
where harmonics due to individual atoms are calculated using the recently
developed quantitative rescattering theory, and the propagation of the laser
and harmonics in the medium is calculated by solving the Maxwell's wave
equation. We illustrate that the simulated spectra are very sensitive to the
laser focusing conditions at high laser intensity and high pressure since the
fundamental laser field is severely reshaped during the propagation. By
comparing the simulated results with several experiments we show that the
pressure dependence can be qualitatively explained. The lack of quantitative
agreement is tentatively attributed to the failure of the complete knowledge of
the experimental conditions.
07/2011;
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ABSTRACT: We report theoretical calculations of high-order harmonic generation (HHG) of
Xe with the inclusion of multi-electron effects and macroscopic propagation of
the fundamental and harmonic fields in an ionizing medium. By using the
time-frequency analysis we show that the reshaping of the fundamental laser
field is responsible for the continuum structure in the HHG spectra. We further
suggest a method for obtaining an isolated attosecond pulse (IAP) by using a
filter centered on axis to select the harmonics in the far field with different
divergence. We also discuss the carrier-envelope-phase dependence of an IAP and
the possibility to optimize the yield of the IAP. With the intense few-cycle
mid-infrared lasers, this offers a possible method for generating isolated
attosecond pulses.
07/2011;
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ABSTRACT: We calculate high-harmonic generation (HHG) by intense infrared lasers in atoms and molecules with the inclusion of macroscopic propagation of the harmonics in the gas medium. We show that the observed experimental spectra can be accurately reproduced theoretically despite the sensitivities of the HHG spectra to the experimental conditions. We further demonstrate that the simulated (or experimental) HHG spectra can be factored out as a product of a 'macroscopic wave packet' and the photo-recombination transition dipole moment where the former depends on the laser properties and the experimental conditions, while the latter is the property of the target only. The factorization makes it possible to extract target structure from experimental HHG spectra, and for ultrafast dynamic imaging of transient molecules.
Journal of Physics B Atomic Molecular and Optical Physics 04/2011; 44(9):095601. · 1.88 Impact Factor
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ABSTRACT: We report theoretical calculations on the effect of the multiple orbital
contribution in high-order harmonic generation (HHG) from aligned CO$_2$ with
inclusion of macroscopic propagation of harmonic fields in the medium. Our
results show very good agreements with recent experiments for the dynamics of
the minimum in HHG spectra as laser intensity or alignment angle changes.
Calculations are carried out to check how the position of the minimum in HHG
spectra depends on the degrees of molecular alignment, laser focusing
conditions, and the effects of alignment-dependent ionization rates of the
different molecular orbitals. These analyses help to explain why the minima
observed in different experiments may vary.
03/2011;
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ABSTRACT: We investigate high-order-harmonic generation of isotropically distributed gas-phase H2O molecules exposed to an intense laser field. The induced dipole of each individual molecule by the laser field is first calculated using the recently developed quantitative rescattering theory. In a thin medium, harmonic spectra generated coherently from all the molecules are then calculated by solving Maxwell’s equation of propagation. By using accurate transition dipoles of H2O, we show that the harmonics in the lower plateau region are quite different from models that employ the simpler strong-field approximation. We also examine the magnitude and phase of the harmonics and their dependence on laser focusing conditions.
Phys. Rev. A. 03/2011; 83(3).
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ABSTRACT: In the strong field molecular tunnelling ionization theory (Tong X M 2002 Phys. Rev. A 66 033402), the ionization rate depends on structure parameters of molecules which can be extracted from molecular wavefunctions in the asymptotic region. By using molecular orbitals obtained from standard quantum chemistry packages, we extract these parameters for several selected nonlinear polyatomic molecules. We show that the symmetry properties of the molecular orbital are reflected vividly in the angle-dependent tunnelling ionization rates. The structure parameters for 17 nonlinear molecules have been calculated and tabulated for future applications.
Journal of Physics B Atomic Molecular and Optical Physics 01/2011; 44(3):035601. · 1.88 Impact Factor
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ABSTRACT: We report theoretical calculations of high harmonic generation (HHG) by
intense infrared lasers in atomic and molecular targets taking into account the
macroscopic propagation of both fundamental and harmonic fields. On the
examples of Ar and N$_2$, we demonstrate that these {\it ab initio}
calculations are capable of accurately reproducing available experimental
results with isotropic and aligned target media. We further present detailed
analysis of HHG intensity and phase, under various experimental conditions, in
particular, as the wavelength of the driving laser changes. Most importantly,
our results strongly support the factorization of HHG at the macroscopic level
into a product of a returning electron wave packet and the photorecombination
transition dipole, under typical experimental conditions. This implies that the
single-atom/molecule structure information can be retrieved from experimentally
measured HHG spectra.
12/2010;
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ABSTRACT: We study the effect of one-electron model potentials on the tunneling ionization rates of molecules in strong fields. By including electron correlation using the modified Leeuwen-Baerends (LB α) model, the binding energies of outer shells of molecules are significantly improved. However, we show that the tunneling ionization rates from the LB α do not differ much from the earlier calculations [ Phys. Rev. A 81 033423 (2010)], in which the local correlation potential was neglected.
Phys. Rev. A. 09/2010; 82(3).
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ABSTRACT: We calculate photoelectron angular distributions (PADs) resulting from single-photon (43 eV) ionization of molecules that have been transiently aligned with a short laser pulse. The total ionization cross sections of N2 and CO2 vs the time delay between the aligning laser pulse and the soft x-ray photon are calculated and compared to experimental results reported by I. Thomann et al. J. Phys. Chem. A 112 9382 (2008). We present the PADs from these aligned molecules in the laboratory frame which can be compared directly with future experiments from aligned N2 and CO2. The alignment dependence of single-photon ionization, multiphoton ionization, and high-order harmonic generation are also analyzed.
Phys. Rev. A. 03/2010; 81(3).
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ABSTRACT: In the strong field molecular tunneling ionization theory of Tong et al. [Phys. Rev. A 66, 033402 (2002)], the ionization rate depends on the asymptotic wavefunction of the molecular orbital from which the electron is removed. The orbital wavefunctions obtained from standard quantum chemistry packages in general are not good enough in the asymptotic region. Here we construct a one-electron model potential for several linear molecules using density functional theory (DFT). We show that the asymptotic wavefunction can be improved with an iteration method and after one iteration accurate asymptotic wavefunctions and structure parameters are determined. With the new parameters we examine the alignment-dependent tunneling ionization probabilities for several molecules and compare with other calculations and with recent measurements, including ionization from inner molecular orbitals.
01/2010;
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ABSTRACT: We investigate high-order harmonic generation (HHG) in a thin macroscopic medium by solving Maxwell’s equation using microscopic single-atom induced dipole moment calculated from the recently developed quantitative rescattering (QRS) theory. We show that macroscopic HHG yields calculated from QRS compared well with those obtained from solving the single-atom time-dependent Schrödinger equation but with great saving of computer time. We also show that macroscopic HHG can be expressed as a product of a “macroscopic wave packet” and the photorecombination cross section of the target gas. The latter enables us to extract target structure from the experimentally measured HHG spectra, thus paves the way to use few-cycle infrared lasers for time-resolved chemical imaging of transient molecules with few-femtosecond temporal resolution.
Phys. Rev. A. 05/2009; 79(5).
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ABSTRACT: We analyzed the discrepancy of the angular dependence of strong-field ionization for CO2 among the different theoretical calculations and experiments. Using a more accurate ground-state wave function of CO2 in the asymptotic region, we showed that the accuracy in the earlier tunneling ionization theory of Tong et al. Phys. Rev. A 66 033402 (2002)] is much improved. We also concluded that the angular dependence deduced from the experiment of Pavičić et al. Phys. Rev. Lett. 98 243001 (2007)] appears to be too narrowly distributed.
Phys. Rev. A. 80(5).
