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We present calculations for attosecond atomic delays in photoionization ofnoble gas atoms based on full two-color two-photon Random-Phase Approximationwith Exchange in both length and velocity gauge. Gauge invariant atomic delaysare demonstrated for the complete set of diagrams. The results are used toinvestigate the validity of the common assumption that the measured atomicdelays can be interpreted as a one-photon Wigner delay and a universalcontinuum--continuum contribution that depends only on the kinetic energy ofthe photoelectron, the laser frequency and the charge of the remaining ion, butnot on the specific atom or the orbital from which the electron is ionized.Here we find that although effects beyond the universal IR--photoelectroncontinuum--continuum transitions are rare, they do occur in special cases suchas around the $3s$ Cooper minimum in argon. We conclude also that in generalthe convergence in terms of many-body diagrams is considerably faster in lengthgauge than in velocity gauge.

Many-body calculations of two-photon, two color matrix elements for attosecond delays