Investigations of above-threshold ionization using subpicosecond laser pulses

Abstract

At very high light intensities. the electron energy spectrum in multiphoton iozizatioii ( ~~i j spectroscopy of eve" ;he simpiesi aioms changes irom a singie, we:: defined threshold peak into multiple peaks, separated from one another by the photon energy. This phenomenon is generally referred to as 'above-threshold ionization' (ATI).The original experiments investigating ATI used relatively long laser pulses, with the result that amplitudes, energy widths and angular distributions of the individual photoelectron peaks depended on the laser intensity. I n addition, the widths of the peaks, as well as their absolute energy positions, changed according to the temporal width of the laser pulse. These dependencies were not intrinsic to the ionization process, but rather were all eventually ascribed to ponderomotive forces exerted on free photoelectrons by the laser focus. The ponderomotive effects frustrated comparisons between theoretical calculations and experimental data. More recent studies have shown that a dramatic simplification occurs when M P I is studied with extremely shon laser pulses: both the energies and the momenta of the AT1 electrons become independent of either the laser energy or the pulse duration. Under these cirmmstances, comparisons between theory and experiment can be made in sufficient detail 10 discriminate between competinz models of the high-intensity AT1 process.

Precision calculation of above-threshold multiphoton ionization in intense short-wavelength laser fields: The momentum-space approach and time-dependent generalized pseudospectral method

“…Multiphoton and above-threshold ionization (ATI) of an atom exposed to intense laser fields has been an attractive topic for decades [1,2] since it is first discovered in 1979 [3]. Owing to the advance of the intense and short pulse laser technology, the study of ATI phenomenon continues to attract much attention [4,5].…”

Simple soluble molecular ionization model

“…The development of intense lasers has permitted such probing in regimes where simple perturbative treatments are not valid and one must use a nonperturbative semiclassical approximation or a numerical approach. Experiments with molecules display a much richer range of phenomena than with atoms, due to the additional molecular degrees of freedom [1,2,3]. These include above threshold ionization [4,5], multiple ionization [6,7], alignment effects [8], electron localization [9], non-sequential double ionization [10], direct excitation [11], stabilization [12], dissociative recombination [13], and separation effects [14].…”

Femtosecond Time‐Resolved Photoelectron Spectroscopy

“…45 It is well-known in atomic physics that the dynamic Stark effect causes field-induced changes in the energies of the atomic levels and, hence, leads to field-induced multiphoton resonances known as "sweeping" or "Freeman" resonances and to fine structure in atomic ATI photoelectron spectra. 46 As the dynamic Stark effect is general, similar arguments apply to molecular systems. Beginning in 1996, Baumert and Gerber and their co-workers made extensive studies on the effect of laser intensity on TRPES.…”