1,095 research outputs found
Polarization and ellipticity of high-order harmonics from aligned molecules generated by linearly polarized intense laser pulses
We present theoretical calculations for polarization and ellipticity of
high-order harmonics from aligned N, CO, and O molecules generated
by linearly polarized lasers. Within the rescattering model, the two
polarization amplitudes of the harmonics are determined by the
photo-recombination amplitudes for photons emitted parallel and perpendicular
to the direction of the {\em same} returning electron wave packet. Our results
show clear species-dependent polarization states, in excellent agreement with
experiments. We further note that the measured polarization ellipse of the
harmonic furnishes the needed parameters for a "complete" experiment in
molecules.Comment: 4 pages, 4 figure
Retrieval of interatomic separations of molecules from laser-induced high-order harmonic spectra
We illustrate an iterative method for retrieving the internuclear separations
of N, O and CO molecules using the high-order harmonics generated
from these molecules by intense infrared laser pulses. We show that accurate
results can be retrieved with a small set of harmonics and with one or few
alignment angles of the molecules. For linear molecules the internuclear
separations can also be retrieved from harmonics generated using isotropically
distributed molecules. By extracting the transition dipole moment from the
high-order harmonic spectra, we further demonstrated that it is preferable to
retrieve the interatomic separation iteratively by fitting the extracted dipole
moment. Our results show that time-resolved chemical imaging of molecules using
infrared laser pulses with femtosecond temporal resolutions is possible.Comment: 14 pages, 9 figure
Accurate retrieval of structural information from laser-induced photoelectron and high-harmonic spectra by few-cycle laser pulses
By analyzing ``exact'' theoretical results from solving the time-dependent
Schr\"odinger equation of atoms in few-cycle laser pulses, we established the
general conclusion that differential elastic scattering and photo-recombination
cross sections of the target ion with {\em free} electrons can be extracted
accurately from laser-generated high-energy electron momentum spectra and
high-order harmonic spectra, respectively. Since both electron scattering and
photoionization (the inverse of photo-recombination) are the conventional means
for interrogating the structure of atoms and molecules, this result shows that
existing few-cycle infrared lasers can be implemented for ultrafast imaging of
transient molecules with temporal resolution of a few femtoseconds.Comment: 4 pages, 4 figure
Generation of isolated attosecond pulses in the far field by spatial filtering with an intense few-cycle mid-infrared laser
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.Comment: 8 figure
Retrieval of electron-atom scattering cross sections from laser-induced electron rescattering of atomic negative ions in intense laser fields
We investigated the two-dimensional electron momentum distributions of atomic
negative ions in an intense laser field by solving the time-dependent
Schrodinger equation (TDSE) and using the first- and 2nd-order strong-field
approximations (SFA). We showed that photoelectron energy distributions and
low-energy photoelectron momentum spectra predicted from SFA are in reasonable
agreement with the solutions from the TDSE. More importantly, we showed that
accurate electron-atom elastic scattering cross sections can be retrieved
directly from high-energy electron momentum spectra of atomic negative ions in
the laser field. This opens up the possibility of measuring electron-atom and
electron-molecule scattering cross sections from the photodetachment of atomic
and molecular negative ions by intense short lasers, respectively, with
temporal resolutions in the order of femtoseconds.Comment: 6 papges, 5 figure
Analysis of effects of macroscopic propagation and multiple molecular orbitals on the minimum in high-order harmonic generation of aligned CO
We report theoretical calculations on the effect of the multiple orbital
contribution in high-order harmonic generation (HHG) from aligned CO 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.Comment: 7 figure
Probing molecular frame photoionization via laser generated high-order harmonics from aligned molecules
Present photoionization experiments cannot measure molecular frame
photoelectron angular distributions (MFPAD) from the outermost valence
electrons of molecules. We show that details of the MFPAD can be retrieved with
high-order harmonics generated by infrared lasers from aligned molecules. Using
accurately calculated photoionization transition dipole moments for
fixed-in-space molecules, we show that the dependence of the magnitude and
phase of the high-order harmonics on the alignment angle of the molecules
observed in recent experiments can be quantitatively reproduced. This result
provides the needed theoretical basis for ultrafast dynamic chemical imaging
using infrared laser pulses.Comment: 5 pages, 4 figure
Quantitative Rescattering Theory for high-order harmonic generation from molecules
The Quantitative Rescattering Theory (QRS) for high-order harmonic generation
(HHG) by intense laser pulses is presented. According to the QRS, HHG spectra
can be expressed as a product of a returning electron wave packet and the
photo-recombination differential cross section of the {\em laser-free}
continuum electron back to the initial bound state. We show that the shape of
the returning electron wave packet is determined mostly by the laser only. The
returning electron wave packets can be obtained from the strong-field
approximation or from the solution of the time-dependent Schr\"odinger equation
(TDSE) for a reference atom. The validity of the QRS is carefully examined by
checking against accurate results for both harmonic magnitude and phase from
the solution of the TDSE for atomic targets within the single active electron
approximation. Combining with accurate transition dipoles obtained from
state-of-the-art molecular photoionization calculations, we further show that
available experimental measurements for HHG from partially aligned molecules
can be explained by the QRS. Our results show that quantitative description of
the HHG from aligned molecules has become possible. Since infrared lasers of
pulse durations of a few femtoseconds are easily available in the laboratory,
they may be used for dynamic imaging of a transient molecule with femtosecond
temporal resolutions.Comment: 50 pages, 15 figure
Analysis of two-dimensional high-energy photoelectron momentum distributions in single ionization of atoms by intense laser pulses
We analyzed the two-dimensional (2D) electron momentum distributions of
high-energy photoelectrons of atoms in an intense laser field using the
second-order strong field approximation (SFA2). The SFA2 accounts for the
rescattering of the returning electron with the target ion to first order and
its validity is established by comparing with results obtained by solving the
time-dependent Schr\"{o}dinger equation (TDSE) for short pulses. By analyzing
the SFA2 theory, we confirmed that the yield along the back rescattered ridge
(BRR) in the 2D momentum spectra can be interpreted as due to the elastic
scattering in the backward directions by the returning electron wave packet.
The characteristics of the extracted electron wave packets for different laser
parameters are analyzed, including their dependence on the laser intensity and
pulse duration. For long pulses we also studied the wave packets from the first
and the later returns.Comment: 12 pages, 10 figure
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