Spirals Asunder: Transformation by Rabi Oscillation of the Photoelectron Spectra Produced by Oppositely Circularly Polarized Laser Pulses

The advance of laser technology has allowed for experimental investigation of numerous strong-field phenomena and expanded theoretical interest therein. Time-delayed pairs of oppositely circularly polarized pulses have previously been employed in 2015 to interrogate, in the perturbative regime, any S-state atom by examining the Einstein photoelectric effect. For helium atom, a distinct spiral pattern was discovered in the momentum distribution for the ionized electron in the polarization plane¹. A year later, the same electron phenomenon with the spiral arms multiplied by multiphoton transitions was reported² and has been confirmed experimentally in 2017 by the Wollenhaupt group^3,4 in the perturbative and non-perturbative regimes. The phenomenon persisting into the non-perturbative regime suggests that it could coexist with other strong-field effects involving population depletion. The Wollenhaupt group^3,4 inverted the population from the ground state to an excited state using a π pulse -- which is essentially one half of a Rabi cycle -- and the population inversion was reflected in the number of spiral arms. Here, we extend the treatment of resonantly driving an atom for sufficient periods of time where Rabi oscillations occur using the QProp package⁵. We find that the spiral formed in the photoelectron momentum distribution (PMD) in the polarization plane is bifurcated (in essence, the Autler-Townes doublet) by the dipole energy associated with the bound states transited. We also find that generally the previously observed control of the pattern is inherited.

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