Adiabatic dynamics in a V-type quantum system by oppositely chirped counterrotating circularly polarized laser pulses

Abstract: Shaped free electron vortices (SEVs) have recently been studied using atomic (1+2) resonance-enhanced multiphoton ionization (REMPI) by oppositely chirped counterrotating circularly polarized (OC-CRCP) femtosecond laser pulses. By transitioning from the perturbative to the non-perturbative REMPI regime, we identify an adiabatic excitation mechanism in a resonant V-type three-level system, termed V-RAP due to its similarities to rapid adiabatic passage (RAP). Experimentally, we observe a pronounced change in the shape of the measured three-dimensional photoelectron momentum distribution (3D PMD), which we trace back to this mechanism via analytical calculations and numerical simulations of the bound state and ionization dynamics. In V-RAP, the atom adiabatically follows the OC-CRCP field, with the two excited states driven in anti-phase, leading to an adiabatic cancellation of specific ionization pathways and explaining the observed changes in the PMD. In the experiment, we combine supercontinuum polarization pulse shaping to generate OC-CRCP femtosecond laser pulses with velocity map imaging-based photoelectron tomography to reconstruct the 3D PMD. The reconstructed PMDs are decomposed by 3D Fourier analysis into SEVs of different rotational symmetry, revealing a significant enhancement of the $c_6$-symmetric contribution, which is the signature of the V-RAP.