46 research outputs found
High-Harmonic Generation in the Water Window from mid-IR Laser Sources
We investigate the harmonic response of neon atoms to mid-IR laser fields
(2000-3000nm) using a single-active electron (SAE) model and the fully ab
initio all-electron R-Matrix with Time-dependence (RMT) method. The laser peak
intensity and wavelength are varied to find suitable parameters for
high-harmonic imaging in the water window. Comparison of the SAE and RMT
results shows qualitative agreement between them as well as parameters such as
the cutoff frequency predicted by the classical three-step model. However,
there are significant differences in the details, particularly in the predicted
conversion efficiency. These details indicate the possible importance of
multi-electron effects, as well as a strong sensitivity of quantitative
predictions on specific aspects of the numerical model
An experimental and theoretical study of transient negative ions in Mg, Zn, Cd and Hg
A range of experimental and theoretical techniques have been applied to the study of transient negative ions (resonances) formed in electron scattering from the Group II metals Mg, Zn, Cd, and Hg at incident electron energies below the first ionization potential. A wealth of resonance structures have been observed and from the experimental observations and theoretical information, classifications are proposed for some of these negative ion states
Polarization correlations for electron-impact excitation of the resonant transitions of Ne and Ar at low incident energies
The electron-polarized-photon coincidence method is used to determine linear and circular polarization correlations in vacuum ultraviolet (VUV) for the differential electron-impact excitation of neon and argon resonance transitions at impact energies of 25 and 30 eV at small scattering angles up to 40. The circular polarization correlation is found to be positive in the case of Ne at 25 eV and supports the prediction of the present B-spline R-matrix theory concerning the violation of a long-established propensity rule regarding angular momentum transfer in electron-impact excitation of S→P transitions. Comparisons with the results from the present relativistic distorted-wave approximation and an earlier semirelativistic distorted-wave Born model are also made. For the case of Ar, at 25 and 30 eV, the circular polarization measurements remain in agreement with theory, but provide limited evidence as to whether or not the circular polarization at small scattering angles is also positive. For the linear polarizations, much better agreement with theory is obtained than in earlier measurements carried out by S. H. Zheng and K. Becker
Electron-collision cross sections for iodine
We present results from a joint experimental and theoretical study of elastic electron scattering from atomic iodine. The experimental results were obtained by subtracting known cross sections from the measured data obtained with a pyrolyzed mixed beam containing a variety of atomic and molecular species. The calculations were performed using both a fully relativistic Dirac B-spline R-matrix (close-coupling) method and an optical model potential approach. Given the difficulty of the problem, the agreement between the two sets of theoretical predictions and the experimental data for the angle-differential and the angle-integrated elastic cross sections at 40 eV and 50 eV is satisfactory
Attosecond angular streaking and tunnelling time in atomic hydrogen
Tunnelling, one of the key features of quantum mechanics, ignited an ongoing
debate about the value, meaning and interpretation of 'tunnelling time'. Until
recently the debate was purely theoretical, with the process considered to be
instantaneous for all practical purposes. This changed with the development of
ultrafast lasers and in particular, the 'attoclock' technique that is used to
probe the attosecond dynamics of electrons. Although the initial attoclock
measurements hinted at instantaneous tunnelling, later experiments contradicted
those findings, claiming to have measured finite tunnelling times. In each case
these measurements were performed with multi-electron atoms. Atomic hydrogen
(H), the simplest atomic system with a single electron, can be 'exactly'
(subject only to numerical limitations) modelled using numerical solutions of
the 3D-TDSE with measured experimental parameters and acts as a convenient
benchmark for both accurate experimental measurements and calculations. Here we
report the first attoclock experiment performed on H and find that our
experimentally determined offset angles are in excellent agreement with
accurate 3D-TDSE simulations performed using our experimental pulse parameters.
The same simulations with a short-range Yukawa potential result in zero offset
angles for all intensities. We conclude that the offset angle measured in the
attoclock experiments originates entirely from electron scattering by the
long-range Coulomb potential with no contribution from tunnelling time delay.
That conclusion is supported by empirical observation that the electron offset
angles follow closely the simple formula for the deflection angle of electrons
undergoing classical Rutherford scattering by the Coulomb potential. Thus we
confirm that, in H, tunnelling is instantaneous (with an upperbound of 1.8 as)
within our experimental and numerical uncertainty.Comment: 7 figure
Measurement of laser intensities approaching 10 15 W/cm 2 with an accuracy of 1%
Accurate knowledge of the intensity of focused ultrashort laser pulses is crucial to the correct interpretation of experimental results in strong-field physics. We have developed a technique to measure laser intensities approaching 1015W/cm2 with an accu