1,602 research outputs found
On the recombination in high-order harmonic generation in molecules
We show that the dependence of high-order harmonic generation (HHG) on the
molecular orientation can be understood within a theoretical treatment that
does not involve the strong field of the laser. The results for H_2 show
excellent agreement with time-dependent strong field calculations for model
molecules, and this motivates a prediction for the orientation dependence of
HHG from the N_2 3s_g valence orbital. For both molecules, we find that the
polarization of recombination photons is influenced by the molecular
orientation. The variations are particularly pronounced for the N_2 valence
orbital, which can be explained by the presence of atomic p-orbitals.Comment: 6 pages 7 figure
Carrier-wave Rabi flopping signatures in high-order harmonic generation for alkali atoms
We present the first theoretical investigation of carrier-wave Rabi flopping
in real atoms by employing numerical simulations of high-order harmonic
generation (HHG) in alkali species. Given the short HHG cutoff, related to the
low saturation intensity, we concentrate on the features of the third harmonic
of sodium (Na) and potassium (K) atoms. For pulse areas of 2 and Na atoms,
a characteristic unique peak appears, which, after analyzing the ground state
population, we correlate with the conventional Rabi flopping. On the other
hand, for larger pulse areas, carrier-wave Rabi flopping occurs, and is
associated with a more complex structure in the third harmonic. These new
characteristics observed in K atoms indicate the breakdown of the area theorem,
as was already demonstrated under similar circumstances in narrow band gap
semiconductors
Ultrasmall divergence of laser-driven ion beams from nanometer thick foils
We report on experimental studies of divergence of proton beams from
nanometer thick diamond-like carbon (DLC) foils irradiated by an intense laser
with high contrast. Proton beams with extremely small divergence (half angle)
of 2 degree are observed in addition with a remarkably well-collimated feature
over the whole energy range, showing one order of magnitude reduction of the
divergence angle in comparison to the results from micrometer thick targets. We
demonstrate that this reduction arises from a steep longitudinal electron
density gradient and an exponentially decaying transverse profile at the rear
side of the ultrathin foils. Agreements are found both in an analytical model
and in particle-in-cell simulations. Those novel features make nm foils an
attractive alternative for high flux experiments relevant for fundamental
research in nuclear and warm dense matter physics.Comment: 11 pages, 5 figure
Selectively excited luminescence and magnetic circular dichroism of Cr4+-doped YAG and YGG
Site selective luminescence and magnetic circular dichroism experiments on Cr4+-doped yttrium aluminum garnet and yttrium gallium garnet have been made at low temperature. The spectral assignments for these near-IR lasing materials have been made using experimental data and ligand field calculations guided by the known geometry of the lattices. [S0163-1829(99)07003-4]
The direct evaluation of attosecond chirp from a streaking measurement
We derive an analytical expression, from classical electron trajectories in a
laser field, that relates the breadth of a streaked photoelectron spectrum to
the group-delay dispersion of an isolated attosecond pulse. Based on this
analytical expression, we introduce a simple, efficient and robust procedure to
instantly extract the attosecond pulse's chirp from the streaking measurement.Comment: 4 figure
Theory of high harmonic generation in relativistic laser interaction with overdense plasma
High harmonic generation due to the interaction of a short ultra relativistic
laser pulse with overdense plasma is studied analytically and numerically. On
the basis of the ultra relativistic similarity theory we show that the high
harmonic spectrum is universal, i.e. it does not depend on the interaction
details. The spectrum includes the power law part for
, followed by exponential decay. Here
is the largest relativistic -factor of the plasma
surface and is the second derivative of the surface velocity at this
moment. The high harmonic cutoff at is parametrically
larger than the predicted by the ``oscillating mirror''
model based on the Doppler effect. The cornerstone of our theory is the new
physical phenomenon: spikes in the relativistic -factor of the plasma
surface. These spikes define the high harmonic spectrum and lead to attosecond
pulses in the reflected radiation.Comment: 12 pages, 9 figure
The highly resolved electronic spectrum of the square planar CuClâÂČâ» ion
The low temperature magnetic circular dichroism(MCD) and electron paramagnetic resonance(EPR)spectra of Cu(II) dopedCsâZrClâ are reported. The Cu(II) ion is incorporated as the square planar copper tetrachloride ion, CuClâÂČâ», which substitutes at the Zr(IV) site in the CsâZrClâ lattice, with a complete absence of axial coordination. Both the EPR and MCD show highly resolved spectra from which it is possible to determine the superhyperfine coupling constants and excited state geometries respectively. The FranckâCondon intensity patterns suggest that there is a substantial relaxation of the host lattice about the impurity ion. For the lowest energy ÂČB1g(xÂČ-yÂČ)âÂČB2g(xy) transition, both the magnetic dipole allowed electronic origin as well as vibronic false origins are observed. The high resolution of the spectra allowed the accurate determination of the odd parity vibrations that are active in the spectra. The opposite sign of the MCD of the two components of the ÂČEg(xz,yz)excited state allows this splitting to be determined for the first time. Accurate and unambiguous spectral parameters for the CuClâÂČâ» ion are important as it has become a benchmark transition metal complex for theoretical electronic structure calculations
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