Resolving Quantum Interference Black Box through Attosecond Photoionization Spectroscopy.

Multiphoton light-matter interactions invoke a so-called "black box" in which the experimental observations contain the quantum interference between multiple pathways. Here, we employ polarization-controlled attosecond photoelectron metrology with a partial wave manipulator to deduce the pathway interference within this quantum 'black box" for the two-photon ionization of neon atoms. The angle-dependent and attosecond time-resolved photoelectron spectra are measured across a broad energy range. Two-photon phase shifts for each partial wave are reconstructed through the comprehensive analysis of these photoelectron spectra. We resolve the quantum interference between the degenerate p→d→p and p→s→p two-photon ionization pathways, in agreement with our theoretical simulations. Our approach thus provides an attosecond time-resolved microscope to look inside the "black box" of pathway interference in ultrafast dynamics of atoms, molecules, and condensed matter.

Atomic partial wave meter by attosecond coincidence metrology.

Attosecond chronoscopy is central to the understanding of ultrafast electron dynamics in matter from gas to the condensed phase with attosecond temporal resolution. It has, however, not yet been possible to determine the timing of individual partial waves, and steering their contribution has been a substantial challenge. Here, we develop a polarization-skewed attosecond chronoscopy serving as a partial wave meter to reveal the role of each partial wave from the angle-resolved photoionization phase shifts in rare gas atoms. We steer the relative ratio between different partial waves and realize a magnetic-sublevel-resolved atomic phase shift measurement. Our experimental observations are well supported by time-dependent R-matrix numerical simulations and analytical soft-photon approximation analysis. The symmetry-resolved, partial-wave analysis identifies the transition rate and phase shift property in the attosecond photoelectron emission dynamics. Our findings provide critical insights into the ubiquitous attosecond optical timer and the underlying attosecond photoionization dynamics.

Resolving Ultrafast Spin-Orbit Dynamics in Heavy Many-Electron Atoms.

We use R-matrix with time-dependence theory, with spin-orbit effects included, to study krypton irradiated by two time-delayed extreme ultraviolet ultrashort pulses. The first pulse excites the atom to 4s^{2}4p^{5}5s. The second pulse then excites 4s4p^{6}5s autoionizing levels, whose population can be observed through their subsequent decay. By varying the time delay between the two pulses, we are able to control the excitation pathway to the autoionizing states. The use of cross-polarized light pulses allows us to isolate the two-photon pathway, with one photon taken from each pulse.

Two-photon detachment of inner-shell electrons from Li-.

We investigate two-photon detachment of an inner 1s electron from the 1s22s2 ground state of Li- using R-matrix Floquet theory. This detachment rate shows a noticeable structure due to shape resonances just above the lowest three doubly excited thresholds of Li. Two-photon detachment of one 1s electron is about twice as likely as two-photon detachment of (at least) one 2s electron. Two-photon detachment of one 1s electron may be observable experimentally through the detection of electrons with kinetic energies between 45 and 65 eV.