Emergence of light-induced states in the few-photon ionization of atomic helium

In this joint experimental and theoretical work [1], photoelectron emission from excited states of laser-dressed atomic helium is analyzed. The experiment is carried out at DESY in Hamburg using the FLASH2 free-electron laser with analysis at the reaction microscope (REMI) end station [2]. The helium atom is subject to (temporally) overlapping extreme ultraviolet (XUV) and a infrared (IR) pulses, with either parallel or orthogonal relattive polarization. The XUV pulse is scanned over excited states of helium.  Dipole-forbidden transitions to light-induced states (LIS), such as nS and nD states, corresponding to multiphoton (XUV ± nIR) excitation, are observed during temporal overlap of the lasers. Studying photoelectron angular distributions (PADs) in the case where the ionization pathway of a LIS is difficult to resolve energetically allows for an unambiguous determination of the dominant LIS. Relative orientation of the two lasers is employed to study its effect on PADs and to control the suppression of certain ionization pathways. Numerical solutions of the time-dependent Schrdinger equation support the experimental observations. [1] S. Meister et al., Phys. Rev. A 102 (2020) 062809; Phys. Rev. A 103 (2021) in press. [2] S. Meister et al., Applied Sciences 10 (2020) 2953.