Description
While the influence of the spin angular momentum (SAM) of light on photoelectron angular distributions has been extensively studied [1], the role of orbital angular momentum (OAM) in strong-field ionization remains largely unexplored. In a seminal publication, De Ninno et al. [2] reported that when helium atoms are ionized by a circularly polarized XUV pulse, in the presence of an IR field carrying OAM, the angular distribution of the emitted photoelectrons becomes sensitive to the sign of the OAM, showing photoelectron helicoidal dichroism (PEHD). So far, it was only observed in this direct photoionization regime.
Using two-color multiphoton ionization and by varying the OAM and SAM of both beams, we observed that PEHD also appears in other ionization regimes. First, we used λ=515 nm pulses to excite Rydberg states of ethanol by absorption of 4 photons carrying SAM; and λ=1030 nm pulses carrying both SAM and OAM to ionize the excited molecules by absorption of 1 to 5 circularly polarized photons. We observed asymmetries of the order ∼10% on this 4+n resonantly enhanced multiphoton ionization (Figure 1 - a), reflecting the non-dipolar character of the ionization from Rydberg states [3]. Second, we increased the intensity of the λ=515 nm pulses to produce above-threshold ionization (ATI), and used weak λ=1030 nm pulses with OAM to induce continuum-continuum transitions, creating sidebands. We observed a ∼3% HD at the sideband location, with opposite sign compared to the Rydberg ionization case (Figure 1 - b).
These observations demonstrate the generality of PEHD, offering new insight into elementary light-matter processes beyond electric-dipole approximation, currently under modeling.
References:
[1] S Beaulieu et al., New J. Phys. 18, 102002 (2016)
[2] G De Ninno et al., Nat. Phot. 14, 554 (2020)
[3] F Lépine et al., Phys. Rev. Lett. 93, 233003 (2004)
