a Key Laboratory for Laser Plasmas (Ministry of Education) and School of Physics and Astronomy,Collaborative Innovation Center for IFSA (CICIFSA), Shanghai Jiao Tong University, Shanghai 200240,China.
b Instituto Universitario de Física Fundamental y Matemáticas, Universidad de Salamanca, Plaza de la Merced s/n, 37008 Salamanca, Spain. 3CAS Center for Excellence in Ultra-intense Laser Science,
Shanghai 201800, China.
A universal mechanism of ultrafast 2-electron orbital swap is discovered through 2-photon sequential double ionization of Li. After a 1s electron in Li is ionized by absorbing an extreme ultraviolet photon, the other 2 bound electrons located on 2 different shells have either parallel or antiparallel spin orientations.In the latter case, these 2 electrons are in the superposition of the singlet and triplet states with different energies, forming a quantum beat and giving rise to the 2-electron orbital swap with a period of several hundred attoseconds. The orbital swap mechanism can be used to manipulate the spin polarization of photoelectron pairs by conceiving the attosecond-pump attosecond-probe strategy and thus serves as a knob to control spin-resolved multielectron ultrafast dynamics.