RABBITT (Reconstruction of Attosecond Beating By Interference of Two-photon Transitions) is a commonly employed technique to access photoionization time delays . Typical RABBITT schemes use time-delayed extreme ultraviolet (XUV) and infrared (IR) pulses , where the XUV photons are all odd multiples of the IR frequency and are energetic enough to ionize the target system. Absorption and emission of IR photons in the continuum creates two separate pathways resulting in the same final photoelectron kinetic energy, thereby leading to the creation of oscillatory sidebands (SBs) in between the harmonic lines in the photoelectron spectra. In this seminar I will present both theoretical and experimental results concerning two different variations of the RABBITT technique to measure photoionization delays. In under-threshold RABBITT (uRABBITT) , the lowest harmonic in the attosecond pulse train is no longer energetic enough to ionize the target system, and so the first RABBITT sideband will have contributions from a bound-continuum pathway. By comparing experimental data with theoretical predictions from the non-perturbative all-electron R-Matrix with Time Dependence (RMT)  approach and perturbation theory (PT) , we hope to determine if signatures of the underlying bound-state dynamics are visible in the uRABBITT spectrum. In multi-sideband RABBITT  the APT harmonics are now separated by four times the IR probe frequency, leading to the creation of three sidebands in each harmonic group instead of one. Comparison between the phase of the central sideband in multi-sideband RABBITT and the single sideband in a traditional RABBITT set-up may enable the direct measurement of the time-delay associated with absorbing a single IR photon in the continuum. This is the subject of recent experimental efforts and RMT calculations. Time permitting, the operation of the RMT code will be demonstrated via the AMOS Gateway , an international effort seeking to make atomic, molecular, and optical science codes more accessible.
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