Photoelectron spectra of argon in circularly polarized pulses studied by atom-Volkov strong-field approximation

We studied the above-threshold ionization (ATI) spectra of argon in circularly polarized pulse by the atom-Volkov strong-field approximation (SFA). A dip appears in the spectra due to the presence of a zero in the dipole matrix element, corresponding to a transition from the ground state to the L = 2 continuum at electron kinetic energy similar to 30.6 eV. No such dip appears when the time-dependent Schrodinger equation (TDSE) is used instead, because the transition amplitude in SFA is the first-order term in the S-matrix series while TDSE contains all orders. In contrast, the effect of Cooper minimum shows up in high-order harmonic generations, which is an inverse process of photoionization where the recombination transition is first-order. We examine several sets of pulse parameters and found that when the SFA validity conditions are satisfied, the atom-Volkov SFA generates better ATI spectra than the simple SFA.