Streaking of near-threshold photoelectrons produced in laser-assisted attosecond photoionization

A drastic modification of the conventional streaking patterns of double differential cross section (DDCS) for attosecond ionization of infrared (IR) dressed atoms near the ionization threshold is theoretically demonstrated. It is shown that new features can be observed in DDCS for the near-threshold ionization such as interference fringes and strong maxima at small photoelectron energies. They arise when the ejected low-energy electron wave packet is strongly affected by both the ionic core and the IR fields. The computations based on solving the time-dependent Schr"odinger equation (TDSE) show that the most interesting and most informative DDCS pattern can be observed when the ionizing attosecond pulse comes at a maximum value of the IR field. Since attosecond pulses with a carrier frequency of $omega_X sim 40-50$ eV is currently available for experimental studies, investigations of, e.g., subvalence s-shell ionization in noble gases seems to be feasible in the near threshold region. They can provide unique information on low-energy photoelectron dynamics in a strong laser field.