A multicolor interferometric method for extracting phase information on continuum-continuum couplings.

The reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) [1] method is a widely employed technique to measure attosecond time delays of photoionization processes. One consequence of this technique is the introduction of an additional component to the time delay, the continuum-continuum (CC) time delay~[2], caused by the interaction of the probe field with the photo\-electron. While well studied theoretically, this CC time delay is difficult to observe experimentally~[3]. Following up on the method outlined in~[4] for atomic hydrogen, we describe an approach capable of isolating this CC delay for Ar, which is experimentally easier to access. We show theoretical predictions obtained by the multi\-electron $R$-matrix with time dependence method (RMT)~[5] for two RABBITT measurements of the $3p$ photoionization delay in argon with different orders of CC transitions. The results will be compared with data from experiments currently in progress. [1] P.~Paul et al., Science {\bf 292} (2001) 1689. [2] J.~Dahlstr{\"o}m et al., Chem. Phys. {\bf 414} (2013) 53. [3] J. Fuchs et al., Optica {\bf 7} (2020) 154-161. [4] A.~Harth et al., Phys.\ Rev.\ A~{\bf 99} (2019) 023410. [5] A.~C. Brown et al., Comput.\ Phys.\ Comm.~(2019) 107062.