Speaker
Description
The ultra-intense X-ray pulses produced by novel free-electron lasers promise
many applications, e.g. for protein structure-determination or time-resolved
molecular spectroscopy. This requires the pulses to be well-characterised in
terms of focal shape, duration and intensity. Providing tools for calibrating
these properties is however a difficult task, leading to various approaches utilis-
ing different physical effects.
Spatial inhomogeneity in oscillatory electromagnetic fields causes the well-known
ponderomotive force to be exerted on charged particles along the negative field
gradient. As experiments with focused laser pulses on helium atoms have
shown, due to Coulomb attraction, the net drift momentum of bound elec-
trons is transferred to the ionic core, giving rise to a significant center-of-mass
acceleration. From measurements on the deflection of atomic beams or trapped
highly charged ions one may acquire information on the laser’s intensity and
gradient, possibly providing an additional tool for calibration.
A numerical implementation and first results showing this beyond-dipole effect
for a single-electron system will be presented. As the large spatial extent of the
predominantly ionised electronic wave function poses an enormous challenge in
terms of computational resources, the solution of the Schrödinger equation is re-
stricted to a one-dimensional model to ensure a high degree of convergence. The
motion of the ionic core is obtained by a semi-classical description employing
Ehrenfest dynamics via adiabatic coupling. It is found that, depending on the
ionisation potential, at certain intensity thresholds the character of the deflec-
tion changes, with effects due to the gradient-dependence of the ponderomotive
forces being increasingly outweighed by the field’s direction at the instance of
ionisation.
