The Coulomb effect on a low-energy structure in above-threshold ionization spectra induced by mid-infrared laser pulses

We investigate the low-energy structure (LES) in the above-threshold ionization spectrum at a mid-infrared laser wavelength with a semiclassical model. Using a softened Coulomb potential (CP) and changing the softening parameter, we show that though the very low-energy structure (VLES) and high low-energy structure (HLES) are both due to the interaction between the ionic CP and the electron, the two structures have different physical mechanisms: the VLES can be attributed to the electron - ion Coulomb interaction at a rather small distance and the HLES is more likely to be ascribed to the electron - ion Coulomb interaction at a large distance.