Probing the energy flow in Bessel light beams using atomic photoionization

Abstract

The two-color above-threshold ionization (ATI) of atoms and ions is investigated for a vortex Bessel beam in the presence of a strong near-infrared (NIR) light field. While the photoionization is caused by the photons from the weak but extreme ultra-violet (XUV) vortex Bessel beam, the energy and angular distribution of the photoelectrons and their sideband structure are affected by the planewave NIR field. We here explore the energy spectra and angular emission of the photoelectrons in such two-color fields as a function of the size and location of the target atoms with regard to the beam axis. In addition, analogue to the circular dichroism in typical two-color ATI experiments with circularly polarized light, we define and discuss seven different dichroism signals for such vortex Bessel beams that arise from the various combinations of the orbital and spin angular momenta of the two light fields. For localized targets, it is found that these dichroism signals strongly depend on the size and position of the atoms relative to the beam. For macroscopically extended targets, in contrast, three of these dichroism signals tend to zero, while the other four just coincide with the standard circular dichroism, similar as for Bessel beams with small opening angle. Detailed computations of the dichroism are performed and discussed for the 4s valence-shell photoionization of Ca + ions.

Nonlinear helical dichroism in chiral and achiral molecules

“…This additional degree of freedom influences the characteristics of light-matter interactions. The angular distribution 14 , time delay 15 , and dynamics of photo-electrons 16,17 can be modified during the photo-ionization of atoms and molecules. Also, the OAM of light can be transferred to matter either externally by exerting a torque (as demonstrated in optical tweezers 18 , atomic clouds and Bose-Einstein condensates 19,20 ) or internally by rotating the electron distribution resulting in modified selection rules for transitions between two quantum states that depend on topological charge l 21,22 .…”

Selection rules for atomic excitation by twisted light

“…Up to now vortex beams have been routinely generated in a wide frequency range from terahertz to extreme ultraviolet [2][3][4][5][6] and applied in various fields, such as microscopy, quantum information and manipulation of microparticles and ultracold atoms [7][8][9][10][11][12]. Studies have also been performed to explore the fundamental processes involving twisted light, for example the light scatterings [13][14][15] and photo-ionization of atoms and molecules [16][17][18][19].…”

Biharmonic (ω, 2ω) ionization of atoms by elliptically-polarized light. Carving the photoelectron angular distributions

“…Apart from the simultaneous absorption of two photons from the same beam [7][8][9], the two-color photoionization [10,11] as well as the ultra-fast response of gas targets to intense beams [12] has attracted much recent interest. Moreover, structured light beams have become feasible for photoionization experiments during the last years [13][14][15] and have enlarged our understanding of light-atom interactions [16,17]. With the further advancement of free-electron lasers (FEL), in addition, the two-and multi-photon ionization of inner-shell electrons can now be explored also at extreme ultraviolet (XUV) and x-ray photon energies [18,19], including the associated interplay of sequential, resonant and direct double ionization processes [20][21][22][23].…”