Attosecond Larmor clock

Summary form only given. We recently developed a new theoretical tool, analytical R-matrix (ARM), adopting the R-matrix approach developed in for collision processes and nuclear resonance reactions, to study strong field ionisation processes in circularly polarised fields. Our method can consistenly include the effects of arbitrary, long-range potentials on the ionisation process and thus focus on non-adiabatic dynamics. One application within this new scheme is the ability to time multiphoton ionisation process on attosecond scale using the spin-orbit interaction of the ionising electron with the core as the clock. We consider multiphoton ionisation from 4p level in Kr atom. Experimentally, first a few femtosecond, infrared (right) circularly polarised field ionises the electron in the spin-orbital split levels, leaving behind a hole in a superposition of ²P_3/2 and ²P_1/2 state. Second, an attosecond, (left) circularly polarised XUV pulse, delayed w.r.t. the the fs-pump pulse, excites the ion into an s-state, at which point the spin-orbital interaction is switched off (1 = 0). This way we also impart a “start” and “stop” mechanism to our Larmor attoclock. Finally, the resulting ion signal can be measured by transient absorption spectroscopy. The interference pattern from the two spin-orbit split levels ²P_3/2 and ²P_1/2 can be mapped into the transient absorption signal, which oscillates as a function of the delay between the pump and probe pulse. The first maximum of this signal yields the analogue of Wigner-Smith (WS) time for multiphoton ionisation. The WS time for multi-photon ionisation can be divided into two parts: 1) A WS time approaching the one-photon ionisation case in the limit n → 1 (red), and 2) A clock-induced delay (green), due to the entanglement of the electron and hole wavepacket, leading to an additional phase dependence - f 1/r³-type which can either compress or stretch the hole wavepacket in the ion. This additional phase delay due to electron-hole interaction is the main difference from one-photon ionisation.